CHAPTER II.--THE CENTRAL CAPSULE.

51. _Components of the Central Capsule._--In all Radiolaria without exception, at some period of life or other, the central portion of the soft body is separated from the peripheral portion by an independent, anatomically recognisable membrane; this membrane with all its contents is designated the central capsule, and is the peculiar central organ of the unicellular body, which distinguishes the Radiolaria most clearly from the other Rhizopoda. In the great majority of the Radiolaria the volume of the central capsule is less than that of the surrounding peripheral soft body which we place in opposition to it as "extracapsulum." The "capsule-membrane," which separates these two constituents, arises very early in most Radiolaria, and persists throughout their whole life. In some species, however, the membrane only appears later, immediately before the formation of the spores, and hence is absent for a considerable period. Regarded as a whole, then, the capsule consists of the following parts:--(1) the capsule-membrane; (2) the enclosed endoplasm, or intracapsular protoplasm; (3) the nucleus. But in addition, many other non-essential structures may be enclosed in the central capsule, especially hyaline spheres (vacuoles), fatty spheres, pigment granules, crystals, &c.

The central capsule was first described in my Monograph in 1862 (pp. 69-82) as the most characteristic component of the Radiolarian organism, and distinguished from the whole extracapsular {xxv}soft body. The fact that it has recently been reported as absent by various authors is due to their having observed young or unripe specimens, before the formation of the spores. In some species of #Polycyttaria# and ACANTHARIA the membrane persists only a very short time.

52. _The Primary Form of the Central Capsule._--The form of the central capsule is originally a geometrical sphere; and if in accordance with our monophyletic hypothesis all Radiolaria are to be derived from one common stem-form (_Actissa_, see p. 12), then the central capsule of this common stem-form must be regarded as perfectly spherical (_Procyttarium_, p. 13, Pl. 1, fig. 1). Since, further, the enclosed nucleus and the surrounding calymma of this primitive archaic form must also be spheres, and since the nucleus lies in the centre of the body, and the protoplasm is evenly distributed between it and the membrane, it follows that no axes or excentrically differentiated parts are to be distinguished in this most primitive Radiolarian. Rather in the primary central capsule all parts are concentrically and evenly arranged round its centre. This primary spherical form becomes modified in most Radiolaria into various secondary ground-forms, which are correlated partly with the structure of the capsule itself, and partly also with the development of openings in its membrane. In general the ground-form of the central capsule is polyaxon in the Porulosa (SPUMELLARIA and ACANTHARIA); but in the Osculosa centraxon forms are more frequently observed; in the NASSELLARIA the ovoid (allopolar monaxon) form is predominant, and in the PHÆODARIA the rhomboid or amphithect pyramid. In these latter, the astropyle indicates the basal pole of the vertical main axis, whilst the two parapylæ (right and left) mark the poles of the frontal transverse axis. In the NASSELLARIA the centre of the porochora corresponds with the basal pole of the main axis, whilst no transverse axes are originally present.

53. _The Secondary Forms of the Central Capsule._--The original purely spherical form of the central capsule persists only in the minority of the Radiolaria, namely, the greater part of the SPUMELLARIA and ACANTHARIA; it passes over into various other secondary forms in the majority of the class, in the whole of the NASSELLARIA and PHÆODARIA, and in a considerable portion of the SPUMELLARIA and ACANTHARIA. These secondary or derived forms may be divided into two quite distinct groups, which may be designated endometamorphic and exometamorphic; in the former the cause of the divergence of the secondary form from the sphere lies in the internal structure of the central capsule; in the latter it lies in the external influence exerted by the growth of the skeleton. Obviously the former series of modifications is more significant than the latter.

54. _The Endometamorphic Forms of the Central Capsule._--The secondary forms of the central capsule, which are due to internal causes connected with its growth, are as follows:--

{xxvi}A. _The Ellipsoidal Central Capsule_, with one axis elongated, so that it becomes the vertical main axis of the body.

_a._ Among the SPUMELLARIA, _Actiprunum_ (p. 14), _Colloprunum_ (p. 25, Pl. 3, fig. 9), most #Prunoidea# (p. 288).

_b._ Among the ACANTHARIA, many Amphilonchida (p. 782, Pl. 132, figs. 2, 6), and Belonaspida (p. 861).

_c._ Among the NASSELLARIA, many #Plectoidea# (p. 905, Pl. 91, figs. 5, 9), #Stephoidea# (p. 937, Pl. 81, fig. 16), Monocyrtida (Pl. 51, fig. 3), &c.

B. _The Cylindrical Central Capsule_, with considerable elongation of the vertical main axis, which is several times as long as the horizontal transverse axis.

_a._ Amongst the SPUMELLARIA, _Collophidium_ (p. 26, Pl. 3, figs. 1-3) and many #Prunoidea# (_Spongurus_, &c.).

_b._ Among the ACANTHARIA, some Amphilonchida.

C. _The Discoidal, Spheroidal, or Lenticular Central Capsule_, with one axis shorter than the others, which becomes the vertical main axis.

_a._ Among the SPUMELLARIA, _Actidiscus_ (p. 15), _Collodiscus_ (p. 27), and the large group #Discoidea# (p. 408).

_b._ Among the ACANTHARIA, many Quadrilonchida (p. 768, Pl. 131), and most Hexalaspida (p. 874).

_c._ Among the NASSELLARIA, certain #Stephoidea# and #Cyrtoidea#.

_d._ Among the great legion PHÆODARIA the spheroidal central capsule is almost always more or less flattened in the direction of the main axis (p. 1525, Pls. 101-128).

D. _The Lentelliptical Central Capsule_ (or triaxial ellipsoid), with three unequal but isopolar axes at right angles to each other, the sections in all three dimensions of space being ellipses.

_a._ Among the SPUMELLARIA, _Actilarcus_ and the large group #Larcoidea# (p. 604).

_b._ Among the ACANTHARIA, certain Amphilonchida and Belonaspida.

E. _The Polymorphic, Amoeboid or Irregular Central Capsule._

_a._ Among the SPUMELLARIA, _Collodastrum_ (p. 28, Pl. 3, figs. 4, 5), and some #Larcoidea#.

55. _The Exometamorphic Forms of the Central Capsule._--The secondary forms of the central capsule, which are brought about by external causes, chiefly dependent on the formation of the skeleton, are very various and in many cases devoid of special interest; in other instances, on the contrary, they are of great importance, because of the clear relation of cause and effect which can be traced between the development of the skeleton and of the capsule. The most important phenomena to be recorded in this connection are as follows:--

{xxvii}I. SPUMELLARIA.--(A) In many of the #Sphæroidea#, the central capsule of which is originally enclosed by a simple lattice-sphere, it puts out protrusions through the meshes of the shell, thus forming club-shaped processes, corresponding in number with the meshes of the lattice (Pl. 11, figs. 1, 5; Pl. 20, fig. 1_a_; Pl. 27, fig. 3, &c.). The whole surface of the spherical capsule may thus be covered with numerous independent radial clubs of equal size, but usually they unite again outside the shell to form a simple sphere with smooth surface. (B) In many #Prunoidea# whose originally ellipsoidal body has become cylindrical by the marked prolongation of the main axis, the central capsule is divided by a series of constrictions into segments, which correspond with the annular constrictions of the skeleton (Pls. 39, 40). (C) In most #Discoidea# whose lentiform or discoidal shell develops radial arms at its margin, the central capsule sends out processes into these arms, and adapts itself to the stellate form of the skeleton (p. 409, Pl. 43, fig. 15; Pl. 47, &c.) (D) In many #Larcoidea# whose growth is originally lentelliptical, but later spiral or irregular, the central capsule follows the mode of growth and develops irregular protuberances.

II. ACANTHARIA.--Whilst the central capsule of most ACANTHARIA retains its primitive spherical form, in a minority of the group it passes over into various secondary forms, which are directly determined by the growth of the skeleton; especially common are lappet or club-shaped prominences which follow the larger radial spines. Hence the central capsule may assume the form of a violin, with two lobes corresponding to the two poles of the elongated main axis, as in many Amphilonchida (p. 782, Pl. 132, fig. 10), and the Diploconida (p. 884, Pl. 140). On the other hand the central capsule becomes cruciform, with four lobes disposed at right angles, as in Lithoptera and other Quadrilonchida (p. 768, Pl. 131, fig. 10, &c.).

III. NASSELLARIA.--The primitive ellipsoid or ovoid form of the central capsule persists only in a few NASSELLARIA, such as the simplest and most archaic forms, the Nassellida, many #Plectoidea#, #Stephoidea#, Monocyrtida, &c. In the great majority of the NASSELLARIA, on the contrary, the ellipsoid or ovoid form passes over into a secondary form which is usually characterised by the presence of lobes, and is obviously dependent upon the previous development of the skeleton. In many #Stephoidea# and #Spyroidea# (probably the majority), a bilobed central capsule is formed (with symmetrically equal right and left lobes), since the primary vertical sagittal ring interferes with the growth in the median plane (Pl. 90, figs. 7-10). In other {xxviii}#Spyroidea#, on the contrary, and the majority of the #Cyrtoidea#, the central capsule forms at its basis rounded lobes, which protrude and hang down from the meshes of the cortinar plate; and since this latter has usually three or four large pores, the capsule similarly develops three or four processes (Pl. 53, fig. 19; Pl. 55, figs. 4-11; Pl. 59, figs. 4-13; Pl. 60, figs. 3-7; Pl. 65, fig. 1).

56. _The Membrane of the Central Capsule._--The capsule-membrane or envelope of the central capsule is both morphologically and physiologically one of the most important parts of the Radiolarian body, for it separates its two main constituents, the capsule with its nucleus and endoplasm and the extracapsulum with the calymma and exoplasm. The capsule-membrane is invariably present at some time or other during the life of the organism, even though in a few species it may persist only for a short time. It is characterised in general by its power of resistance to chemical and physical reagents, and appears to be related to the elastic tissues or perhaps even more to the chitinous substances. Its thickness is usually less than 0.0001, though in certain groups it ranges between 0.001 and 0.002, and in many of the larger Radiolaria (such as Collida and PHÆODARIA) it may attain a thickness of 0.003 to 0.006 or more. In the three legions SPUMELLARIA, ACANTHARIA, and NASSELLARIA the capsule-membrane is single, while in the PHÆODARIA it is always double, being composed of a firm outer and a delicate inner membrane, which are in contact at only few points. Usually it is quite structureless, except for its apertures; the thicker membrane showing occasionally a fine concentric lamination. In certain large #Colloidea# (_e.g._, _Thalassicolla_, Pl. 1, fig. 5_b_) the membrane is covered on the inner surface by a network of polygonal ridges, and in some large PHÆODARIA with remarkable small curved rods (Pl. 114, fig. 13). In all Radiolaria the membrane is perforated by definite openings or pores, through which the intracapsular and extracapsular protoplasm are in direct communication. These openings (or "pylae") show very characteristic and constant differences in the four legions, which have given rise to the names--PERIPYLEA, ACTIPYLEA, MONOPYLEA, CANNOPYLEA.

The capsule-membrane was first indicated as the most important and absolutely constant component of all Radiolaria, and as the differential character of the class, in my Monograph (1862, pp. 69-71). The careful investigations of R. Hertwig have confirmed this view and at the same time have yielded the most important conclusions regarding the nature and systematic significance of the openings in the capsule (_op. cit._, 1879, pp. 105-107). On the contrary, Karl Brandt has recently propounded the theory that the capsule-membrane is by no means a constant part of the Radiolarian organism, but is lacking in certain species of _Collozoum_ and _Sphærozoum_ (1881, p. 392). This contradiction is explained by the fact that in some #Collodaria# and #Acanthometra# the formation of the central capsule takes place much later than in the other Radiolaria, in some {xxix}species indeed only just prior to the development of the swarm spores. I have recognised the presence of it in all species which I have investigated (more than a thousand), and even in those in which Brandt denies its existence. It is often very delicate and may easily be overlooked, especially when the contents of the capsule are colourless, but in all cases by the prudent application of staining fluids and other reagents its presence may be demonstrated. Even in those cases in which the contour of the capsule was not visible, and its contents appeared to pass without definite boundary into the matrix of the extracapsulum, it was possible by the use of appropriate stains or reagents, which would not penetrate the capsule, or of those solvents which were capable of dissolving its contents and of causing it to swell up like a distended bladder, to recognise the existence of the membrane. Those Radiolaria in which it is truly absent are young animals of species in which the membrane is only formed immediately before sporification, and persists but for a short time (_e.g._, species of _Collozoum_, _Sphærozoum_, _Acanthometra_, _Acanthochiasma_, &c.).

57. _The Capsule-Openings of the Peripylea (or Spumellaria)._--The capsule-membrane of the PERIPYLEA is generally perforated by extremely fine and numerous pores, which are distributed at equal distances over the whole surface, and are precisely alike in all parts of the capsule. Hence the SPUMELLARIA may be called "Holotrypasta" or "Porulosa"; they agree with the ACTIPYLEA in being devoid of an osculum or operculum; they are distinguished from the latter group mainly in that their pores are equally distributed over the whole surface of the capsule, whilst in the ACTIPYLEA the pores are disposed in definite groups or lines, separated by large imporous areas.

The central capsule of the SPUMELLARIA, with its innumerable fine and evenly distributed pores, must be regarded as the primitive arrangement, from which the different central capsules of the three other legions have been developed. The central capsule of the ACTIPYLEA has been derived from that of the PERIPYLEA by reduction in the number of the pores and their distribution in definite, regularly disposed areas in the membrane. The central capsule of the Osculosa is characterised by the formation of a special main-aperture (osculum) at the basal pole, which is closed in the MONOPYLEA by the porochora, and in the CANNOPYLEA by the astropyle; the remaining pores, with the exception of the accessory openings of many CANNOPYLEA, remain undeveloped in both these legions. In the same way Hertwig regards the central capsule of the PERIPYLEA as the primitive form (1879, L. N. 33, p. 107).

58. _The Capsule-Openings of the Actipylea (or Acantharia)._--The capsule-membrane of the ACTIPYLEA is perforated by very numerous fine pores, which are regularly distributed over the surface of the central capsule, and separated by imporous intervals. Hence the ACANTHARIA belong to the "Holotrypasta" or "Porulosa"; they have neither osculum nor operculum, and agree in this particular with the PERIPYLEA; but they are separated from these latter chiefly by the fact that their pores are much less numerous, and marked off into regularly arranged groups or lines by imporous intervals. In the PERIPYLEA, on the contrary, the pores are much more numerous and are evenly distributed over the whole surface of the capsule.

{xxx}The central capsule of the ACANTHARIA has hitherto been for the most part confounded with that of the SPUMELLARIA, and no clear distinction has been drawn in this respect between the two legions of the Porulosa. Hertwig, who in 1879 first discovered the remarkably different structure of the Osculosa (NASSELLARIA and PHÆODARIA), recognised no distinction between the structure of the capsules in the PERIPYLEA and ACTIPYLEA (his Acanthometrea), and supposed that in both these legions "very fine pores were evenly distributed in large numbers over the capsule-membrane" (_loc. cit._, p. 106). I have, however, during the last few years convinced myself, by the careful comparative investigation of numerous ACANTHARIA, that in this respect they are quite distinct from the SPUMELLARIA (with perhaps the exception of the Astrolophida, which are nearly related to the primitive _Actissa_). The number of pores in the ACTIPYLEA is usually very much smaller than in the PERIPYLEA, and they are regularly arranged in groups.

59. _The Capsule-Openings of the Monopylea (or Nassellaria.)_--The capsule-membrane of the MONOPYLEA always possesses a single large main-opening, an osculum, which lies at the basal pole of the main axis, and is closed by a circular perforated lid (operculum porosum). When seen from the surface this lid appears as a clearly defined porous area (porochora or area porosa), and forms the horizontal base of a peculiar cone, which stands vertically in the interior of the capsule and may be designated the "thread-cone" (podoconus). The NASSELLARIA may hence be termed "Merotrypasta" or "Osculosa," like the CANNOPYLEA; the structure and significance of the circular lid (operculum), which closes the main-opening (osculum) is, however, quite different in the two legions. Whilst the lid of the CANNOPYLEA (astropyle) is solid, traversed by radial ribs, and only perforated in its centre by a short tube (proboscis), in the MONOPYLEA the operculum (porochora) is always perforated by numerous vertical fine pores, and is in connection with the peculiar internal "pseudopodial cone" (podoconus, Pl. 51, figs. 5, 13; Pl. 81, fig. 16; Pl. 91, fig. 5; Pl. 98, fig. 13). The pores are separated by small vertical, highly refractive rods (opercular rhabdillæ); these become intensely stained by carmine, and are either evenly distributed over the surface of the porochora or arranged in definite groups. The outer or distal end of each rod is rounded, sometimes thickened like a club or split into lobes; the inner or proximal end is usually pointed, and stands in connection with a myophane thread of the podoconus (see § 79). The primary circular form of the porochora, in which the opercular rhabdillæ are evenly distributed in a horizontal plane, undergoes various secondary modifications in many NASSELLARIA. The triradial structure of the skeleton, which characterises the majority of the legion, causes a splitting of the base of the central capsule into three or four lobes; this division also affects the porochora, which lies in the centre of the base, so that the rhabdillæ become arranged in three or four equal circles. If, however, the lobes of the central capsule become larger and protrude through the three or four collar pores of the cortinar septum, the central porochora may separate entirely into three or four elongated tracts, which lie on the axial side of the magnified lobes; the rhabdillæ are then arranged over the whole surface of {xxxi}these tracts, on the outer aspect of which run the longitudinal myophane fibrillæ of the podoconus (compare §§ 79 and 99).

The porous area of the MONOPYLEA was first described by Hertwig in 1879, and shown to be the characteristic main-opening of the central capsule in various families belonging to this legion (L. N. 33, pp. 71, 73, 83, 106, Taf. vii., viii.). According to his view "the capsule-membrane in the porous area becomes thickened around each pore into a rod, perforated by a canal," and the intracapsular protoplasm passes outwards through these fine canals (_loc. cit._, p. 106). I am not able to share this interpretation, but think rather that I have convinced myself by the examination of some living NASSELLARIA, and of many well-stained and preserved preparations in the Challenger collection, that the rods are _solid_, specially modified portions of the capsular wall, and that the protoplasm does not pass through them but through pores which lie between them.

60. _The Capsule-Openings of the Cannopylea (or Phæodaria)._--The capsule-membrane of the CANNOPYLEA always possesses only a single large main-opening or osculum, which lies at the basal pole of the vertical main axis, and is closed by a circular radiated lid (operculum radiatum). This operculum appears, when seen from the surface, as a sharply defined stellate area (astropyle), from the middle of which arises a shorter or longer cylindrical tube, the proboscis. Hence the PHÆODARIA, like the MONOPYLEA, belong to the "Merotrypasta" or "Osculosa"; the structure and significance of the circular operculum, which closes the main-opening (osculum), are, however, quite different in the two legions. Whilst the operculum of the MONOPYLEA (porochora) is perforated by numerous fine vertical pores, and connected with the peculiar internal pseudopodial cone (podoconus), this structure is entirely wanting in the CANNOPYLEA, and instead of it there is a solid operculum, with radial ribs which originate at the base of its central tubular mouth; this tube (proboscis) is cylindrical, often conical at the base, of very variable length and with a round aperture at either end. In spite of the great difference which the various families of CANNOPYLEA exhibit in the formation of their skeleton and its appendages, the constitution of this characteristic stellate main-opening (astropyle) is always essentially the same; both the stellate operculum itself, and the proboscis which rises from its centre, show only slight differences in the various groups. In addition to this large main-opening most PHÆODARIA possess several small accessory openings (parapylæ); and usually two of these are present, placed symmetrically right and left of the aboral pole of the main axis and in the frontal plane (Pl. 101, figs. 2, 6, 10; Pl. 104, figs. 1, 2_a_). Sometimes there are more numerous accessory openings (three to six or more) regularly arranged, as in the two peculiar families, Circoporida and Tuscarorida; occasionally also there is only a single parapyle, at the aboral pole of the main axis (_e.g._, in _Tuscaridium_). The parapylæ seem to be quite absent in the families Challengerida, Medusettida, Castanellida, and perhaps also in other PHÆODARIA. The form and structure of the small accessory openings appear to be always the same. The {xxxii}outer capsule-membrane is elevated in the form of a short cylindrical tube or "apertural ring" (collare paraboscidis), the external margin of which bends inwards, and at the base of the ring passes over into the delicate internal capsule membrane. Upon this apertural ring is situated a longer or shorter "apertural cone" (paraboscis), which is a tubular, cylindrical or conical, prolongation of the membrane, open externally.

The peculiar capsule-openings of the PHÆODARIA were first discovered and carefully described by Hertwig in 1879 (L. N. 33, pp. 95, 107). He found in all the six genera which he examined _three_ openings, a main-opening at the basal pole of the main axis and two accessory openings, one on either side of the apical pole; hence he named the whole group "TRIPYLEA." This name, however, is not applicable to the numerous PHÆODARIA mentioned above, which have only a main opening without any accessory openings, nor to those genera in which the number of the latter is variable. I have, therefore, replaced Hertwig's designation by the term "CANNOPYLEA," which has reference to the peculiar tubular form of the opening. This I find much more developed in many PHÆODARIA than Hertwig has represented, and I must also, in certain particulars, dissent from his delineation of the minute structure, although this is in the main remarkably accurate.

61. _The Nucleus._--The nucleus, enclosed in the central capsule of all Radiolaria, behaves in every respect like a true cell-nucleus, and thus lies at the base of the now universal opinion, that the whole Radiolarian organism, in spite of its varied development and remarkable variations, is unicellular and remains throughout life a true individual cell. This important theory is not invalidated by the fact that the nucleus undergoes peculiar modifications in many groups, and in certain groups presents appearances seldom or never seen elsewhere.

62. _Uninuclear and Multinuclear Radiolaria (Monocaryotic and Polycaryotic)._--All Radiolaria present two different conditions in respect of the behaviour of the nucleus, since in their young stages they are uninuclear (_monocaryotic_), and in later stages multinuclear (_polycaryotic_). This is readily explained by the fact that each individual Radiolarian is developed from a simple unicellular swarm-spore, and that afterwards, before the formation of swarm-spores, the single nucleus divides into many small nuclei. Thus in the Radiolaria the nucleus is pre-eminently the _organ of reproduction and inheritance_. The division of the originally single nucleus into many small nuclei may take place, however, at very different periods, so that the Radiolaria may be divided in this respect into precocious and serotinous.

63. _Serotinous and Precocious Radiolaria._--In the great majority of the Radiolaria the division of the nucleus takes place only at a late period, a short time or even immediately before the process of spore formation; it then breaks up rapidly into numerous small nuclei (always more than one hundred, sometimes many thousands), and each of these {xxxiii}either becomes itself the nucleus of a swarm-spore, or by repeated division gives rise to a group of spore-nuclei. All those Radiolaria which are uninuclear during the greater part of their existence, and in which the process of division is late, and takes place rapidly, are called "serotinous" or late-dividing forms. To this category belong all PHÆODARIA and NASSELLARIA, as well as all the solitary or monozoic SPUMELLARIA and some ACANTHARIA. On the other hand, the name "precocious," or early dividing, is applied to those Radiolaria in which the division of the nucleus takes place very early, and in which, therefore, the cell is multinuclear during the greater part of its existence. This is the case in all the social or polyzootic Radiolaria (Polycyttaria, Pls. 3-8), and also in the great majority of the ACANTHARIA, both #Acanthometra# and #Acanthophracta#. In the last two groups, however, there are numerous exceptions, and these are seen in remarkably large species, characterised by the great size of the central capsule. From a phylogenetic point of view, the conclusion is allowable that the precocious forms are secondary, and have arisen by adaptive modification from the primitive serotinous stem. In the Polycyttaria (or social SPUMELLARIA, _i.e._, the three families Collozoida, Sphærozoida, and Collosphærida), the cause of the adaptation lies most probably in the formation of the colony itself, for all these three families are so closely related to three corresponding families of serotinous, monozootic Radiolaria (Thalassicollida, Thalassosphærida, Ethmosphærida), that certain species of the latter are hardly to be distinguished from isolated individuals of the former. Perhaps the remarkable formation of the large central oil-globule, which particularly characterises the Polycyttaria, is the prime cause of their early nuclear division. In the ACANTHARIA the cause is most likely to be found in the characteristic _centrogenous development_ of their acanthin skeleton, whose radial bars first of all appear in the centre of the capsule. Hence arises directly the excentric position of the nucleus, which in the archaic stem of ACANTHARIA (_Actissa?_) was probably central. In any case, but little weight is to be laid upon the precocious division of the nucleus in the ACANTHARIA in general, inasmuch as in certain species (both #Acanthometra# and #Acanthophracta#) the more usual serotinous division persists.

64. _Central and Excentric Nuclei._--The position of the nucleus in the interior of the central capsule was no doubt primitively central, and this situation in the geometrical centre of the original spherical central capsule has been accurately retained in all monozootic SPUMELLARIA; in the polyzootic families of this legion (Polycyttaria), on the contrary, it is obscured by the precocious division of the nucleus. In the other three legions, which may be phylogenetically derived from the SPUMELLARIA, the position of the nucleus is rarely central, but usually excentric, or at most subcentral. In the ACANTHARIA (both #Acanthometra# and #Acanthophracta#) the central position of the nucleus is at once excluded by the constantly centrogenous development of the skeleton; the nucleus is therefore always excentric, and may lie at either side; it usually {xxxiv}divides very early into numerous separate nuclei, which are usually distributed in the peripheral portions of the central capsule. In the NASSELLARIA the development of the porochora, and of the podoconus which stands upon it, brings about the formation of a vertical axis, and in consequence the central capsule assumes a monaxon form (usually ovoid or conical); the nucleus then lies in the main axis, but excentrically between the apex of the podoconus and the aboral pole. In many NASSELLARIA, however, especially when the podoconus is so large that its apex approaches the aboral pole of the central capsule, the nucleus is pressed to one side and lies quite excentrically. The PHÆODARIA exhibit a different arrangement; the large spheroidal nucleus is always subcentral, so that its main axis corresponds with that of the concentric spheroidal central capsule; but since the astropyle always occupies the oral pole of the latter, and since the distance of the nucleus from this pole is always somewhat different from its distance from the other, it follows that, strictly speaking, the nucleus never lies accurately in the geometrical centre.

65. _Homogeneous and Allogeneous Nuclei._--The nucleus of the Radiolaria not only exhibits a similar structure and composition, and suffers similar modifications to those which are found to occur in the case of other cell-nuclei, but also to some extent shows very peculiar developmental forms, which are seldom or never found in other cells. In the first place the nuclei may be divided into homogeneous and allogeneous, the former are structureless and consist of a uniform mass of nuclein, whilst the latter are composed of different substances and show various structural relations. _Homogeneous_ nuclei, whose whole mass is uniform and exhibits no structural differentiation, are probably always to be found in the swarm-spores; in the fully developed Radiolarian body they are found only in the first legion, SPUMELLARIA, and that both in many Monozoa (especially small #Sphæroidea# and #Prunoidea#) and in the Polyzoa (or Polycyttaria). The whole mass of these homogeneous nuclei, which are usually spherical or ellipsoidal, consists of uniform, perfectly clear and transparent nuclein, and becomes evenly stained by carmine, hæmatoxyline, &c. They may be readily distinguished by these means from the clear vacuoles or "hyaline vesicles," which are evenly distributed in the endoplasm of many Radiolaria, and may be confused with the former. _Allogeneous_ nuclei, which are always composed of different parts and often show complicated structural relations, are found developed in the great majority of Radiolaria. The most important differentiation exhibited by these secondary forms is the separation of the nuclear mass into a firm nuclear substance (caryoplasm) and a fluid nuclear juice (caryolymph). In addition in each nucleus a nucleolus is visible, and often several or many may be seen (see §§ 67 to 70).

66. _The Form of the Nucleus._--The nucleus of the Radiolaria shows greater variations in form and structure than are to be found in the majority of cell-nuclei; {xxxv}exception must, however, be made in the case of many animal ovicells, which, in their peculiar form and composition, often recall large Radiolarian nuclei. With respect to the external shape two main forms may be distinguished, as primary and secondary. The _primary form_ of the Radiolarian nucleus is the sphere; it occurs not only in most swarm-spores, but also in most adult forms belonging to the legion SPUMELLARIA, and in individual instances in other groups; indeed the nuclei of most SPUMELLARIA, as also the concentric central capsules in which they lie, are true geometrical spheres. The _secondary forms_ of the nucleus are found in the majority of adult Radiolaria, and arise from the primary spherical forms in various ways, either by the elongation or contraction of one axis, or by the formation of apophyses or processes. The most important of these secondary forms are as follows:--

1. _Ellipsoidal nuclei_, arising by elongation of one principal axis; very common among the NASSELLARIA, as well as in many #Prunoidea# and #Larcoidea# among the SPUMELLARIA; also in several ACANTHARIA.

2. _Discoidal nuclei_, arising by contraction of one principal axis, sometimes lenticular or spheroidal, biconvex, sometimes shaped like a disc or coin; especially common in the #Discoidea# among the SPUMELLARIA, also in some ACANTHARIA; the large nucleus of the PHÆODARIA is always spheroidal or almost spherical, with a slightly shortened main axis.

3. _Stellate nuclei_, spherical, and armed with evenly distributed radial club-shaped or conical processes; rare but very characteristic, especially in the two large Thalassicollida _Thalassopila_ (Pl. 1, fig. 3), and _Thalassophysa_ (Monogr. d. Radiol., Taf. i.); also in some #Sphærellaria# (Pl. 11, fig. 5).

4. _Amoeboid nuclei_, with unequal processes irregularly arranged, in certain irregular forms of SPUMELLARIA and ACANTHARIA.

5. _Lobate nuclei_, with several (usually two or three) large ovoid or pyriform lobes, which protrude into corresponding larger lobes of the central capsule, in many NASSELLARIA, especially the multiarticulate #Cyrtoidea# (Pl. 59, figs. 12, 13). The budding nucleus of the ACANTHARIA is also lobate (Pl. 129, figs. 6-11).

67. _The Nucleus of the Peripylea._--The nucleus of the SPUMELLARIA or PERIPYLEA shows in certain groups a very primitive arrangement, indeed the archaic structure from which the various forms of nuclei of other Radiolaria may be derived; but on the other hand, in other groups it exhibits very peculiar and remarkable differentiations. In the first place it may be noted that the monozootic or solitary SPUMELLARIA usually possess a single serotinous nucleus, which only divides into numerous swarm-spores at a late period; {xxxvi}whilst, on the contrary, the polyzootic colonial SPUMELLARIA (or Polycyttaria) are uninuclear only in the young state (Pl. 3, fig. 12), and speedily present numerous small homogeneous nuclei, which have arisen by precocious division of a single nucleus; these are usually spherical and 0.008 to 0.012 mm. in diameter. The serotinous nucleus of the monozootic SPUMELLARIA, in many divisions of this large legion, and especially in the simply constituted #Sphæroidea#, is a homogeneous sphere of nuclein, lying in the middle of the central capsule. In many other cases it assumes the form of a spherical vesicle ("Binnen-Bläschen"), whose fluid or semi-fluid contents are enclosed by a more or less firm membrane. This vesicle often contains a single central spherical _nucleolus_ (Pl. 1, figs. 1_l_, 4_l_), but sometimes a variable number of small excentric nucleoli (Pl. 1, figs. 1_a_, 2_a_). The nuclear membrane is often somewhat thick, presenting a double contour, and in such cases may even exhibit a fine radial striation, the expression of minute pores (Pl. 1, fig. 2_a_). In the colossal nuclei (as much as 1 to 2 mm. in diameter) of certain large Thalassicollida the nucleolus presents a very remarkable form, becoming stellate by the protrusion of processes, which may again branch in a dendritic fashion (as in the common _Thalassicolla nucleata_), or it may develop into a very long cylindrical thread, which is disposed in serpentine coils, and in _Thalassophysa pelagica_ passes into the different cæcal processes of the stellate nucleus. In many #Sphæroidea#, whose skeleton is composed of numerous concentric lattice spheres, the small central spherical nucleus lies at first within the innermost of these (the medullary shell); but afterwards it grows through the meshes of the lattice-work, and the radiating club-shaped processes thus formed (Pl. 11, fig. 5) unite with each other outside the medullary shell, and form an external nuclear sphere which completely encloses the latter. In the Polysphærida (with several concentric lattice-shells) and in the Spongosphærida (with spongy lattice-spheres), this process may be several times repeated, so that eventually the central spherical nucleus attains considerable dimensions, and encloses two or more concentric lattice-shells with their radial connecting rods. The nuclear membrane is in these cases usually penetrated by radial bars, which connect the outermost of the enclosed shells with the remaining cortical shells which surround the central capsule. The same remarkable arrangement is also very common among the #Discoidea#. The small spherical primary nucleus is in such instances immediately surrounded by the innermost earliest developed lattice-shell, around which the concentric rings are subsequently deposited; it then grows out through the meshes, and the processes fuse outside the ring to form a homogeneous lentiform nucleus (Pl. 43, fig. 15). The same process recurs in certain #Prunoidea# and #Larcoidea#, whilst in other SPUMELLARIA of these groups (_e.g._, Pylonida) the lobate processes of the nucleus remain free.

Both the simple serotinous nucleus of the monozootic SPUMELLARIA, and the numerous precocious nuclei of the Polycyttaria, were first described in my Monograph in 1862, the former as the "endocyst" ("Binnen-Bläschen"), the latter as "spherical transparent vesicles" ("Kugelige {xxxvii}wasserhelle Bläschen"). I was in error, however, in regarding the latter as identical with the so-called "hyaline spherules" in the central capsule of many Monozoa, which rather belong to the category of intracapsular vacuoles (see § 72). The credit of recognising, by the aid of the modern methods of staining, the distinctness of these two structures, which may readily be mistaken for each other, and of demonstrating the true nature both of the serotinous and precocious nuclei, belongs to Richard Hertwig (1879, L. N. 33).

68. _The Nucleus of the Actipylea._--The nucleus of the ACANTHARIA or ACTIPYLEA shows very peculiar relations in respect of structure and division, particularly special forms of lobular budding, which belong to the characteristic peculiarities of this singular legion, and are not found among other Radiolaria. The position of the nucleus is _always excentric_, even in the youngest ACANTHARIA, for the centrogeneous formation of the skeleton, the constant development of the earliest radial portions of it in the middle of the central capsule, forces the nucleus from its normal central position. The majority of the ACANTHARIA, like most Polycyttaria, are precocious, the primary nucleus early dividing into numerous small nuclei (see note A below). Nevertheless there are many exceptions to this rule in different families, _e.g._, _Stauracantha_, _Xiphacantha_, _Phatnacantha_, and _Pristacantha_ among the #Acanthometra#, and _Stauraspis_, _Echinaspis_, _Dodecaspis_, and _Phatnaspis_ among the #Acanthophracta#. In these instances the primary nucleus remains for a long time as a simple excentric ellipsoidal or irregularly round body, even in the fully developed stage, and only at a very late period (sometimes just before the formation of the spores) divides into many small nuclei. Since this serotinous division of the nucleus takes place in different genera of very various groups, it can only be decided by further investigations how widely it is spread among the ACANTHARIA, and upon what circumstances it is dependent (see note B). The division of the nucleus appears to be precocious in the majority of this legion, and a number of small nuclei appear to be early formed by a peculiar process of budding; in most fully developed ACANTHARIA these are disposed in one or two layers under the surface of the central capsule, but if their numbers increase to any considerable extent, the whole space between the skeletal rods becomes filled with small nuclei; sometimes these are homogeneous, sometimes vesicular, 0.002 to 0.012 mm. in diameter; usually they are spherical and have a small nucleolus (compare Pl. 129, figs. 6-11, and note C).

A. The numerous nuclei, which are to be found in the central capsule of most mature ACANTHARIA, were first described in my Monograph (1862) as "spherical, transparent vesicles, provided with a small dark granule" (p. 374, Taf. xv. figs. 2, 5; Taf. xvi. figs. 2, 4; Taf. xxi. fig. 7, &c.). Their more minute constitution and peculiar origin were first accurately delineated by R. Hertwig (1879, _loc. cit._, pp. 11-24, Taf. i-iii.).

B. The fact that in a number of ACANTHARIA the nucleus does not divide early as in the majority of the legion, but only at a later period, was first observed by R. Hertwig in a species of #Acanthometra# (_Xiphacantha serrata_), and a species of #Acanthophracta# (_Phatnaspis {xxxviii}mülleri_ = _Haliommatidium mülleri_) (_loc. cit._, pp. 11 and 27). This serotinous division of the nucleus seems, however, to be rather widely spread in both sublegions of the ACANTHARIA; I have found, not only in the forms above mentioned, but also in several others belonging to different genera, a single large excentric nucleus, even in those individuals in which the skeleton was fully developed.

C. The peculiar mode of nuclear budding, by which these small nuclei arise, appears to proceed in the following manner (Pl. 129). The vesicular primary nucleus, which, in consequence of the centrogeneous development of the skeleton protrudes as it grows into irregular lobes (Pl. 129, fig. 9), assumes a peculiar concavo-convex form, sometimes that of a hood or dish, sometimes that of a kidney or sausage. The convex surface is apposed to the capsule-membrane, while the concave is turned towards the central star of the skeleton (fig. 6). There is now formed at the centre of the convex surface of the strong, doubly-contoured, nuclear membrane, a flask-shaped invagination with a narrow neck and expanded base; the membrane now becomes disposed in peculiar folds, which at the narrow aperture of invagination appear as folds, but on the expanded body of the flask take the form of concentric rings, laid closely side by side (Pl. 129, fig. 10). The convex bottom of the flask, which is directed towards the concave proximal side of the nucleus, becomes again invaginated by a central conical apophysis of the enlarged nucleolus, which is situated between them. Usually the nucleolus has already become flattened into a lentiform shape, and upon its distal face a conical apophysis has been developed, which is divisible into a darker proximal and clearer distal portion. The tip of the latter appears to be in direct connection with the nuclear membrane at the centre of the base of the flask-shaped invagination (figs. 6, 10). At this stage of development the nucleus of the ACANTHARIA generally presents the characteristic form of a hood-shaped, concavo-convex vesicle, whose radial axis is also the axis of the flask-shaped distal invagination, and of the depressed conical nucleolus, which lies between the latter and the concave side of the nucleus. After this peculiar invagination has persisted for some time in connection with the enlarged nucleolus, both disappear, and then a remarkable growth of lobular processes takes place on the concave proximal side of the hood or kidney-shaped nucleus; from four to eight knobs of unequal size usually appear, and their thickened wall encloses a variable number of small of nucleoli; these are at first few but afterwards more numerous (fig. 7). Subsequently these knobs or lobes become completely separated by constriction from the original central mass of the nucleus, and appear as so many separate independent "sausage-shaped bodies" in the hollow central capsule (fig. 8). Each of the bodies now appears, and at first on its convex aspect, to form a large number of small nucleoli, which either separate by constriction from it or become free by its breaking up and lie in numbers in the central capsule. Finally the buds or lobes of the nucleus break up entirely into such nucleoli, which are evenly distributed in the central capsule, and become the nuclei of the swarm-spores (fig. 11). Compare R. Hertwig, L. N. 33, Taf. i.-iii. pp. 19-25.

69. _The Nucleus of the Monopylea._--The nucleus of the mature forms of the NASSELLARIA or MONOPYLEA is generally simple or lobate, homogeneous or vesicular and _excentric_, and appears only to divide into numerous small nuclei just before the formation of the spores. Nevertheless I have sometimes, though not often, seen in representatives of very various families of the MONOPYLEA, the central capsule filled with many small spherical homogeneous nuclei (Pl. 53, fig. 19). Hence all the families of this legion appear to be serotinous, their simple primitive nucleus persisting for a long period. It {xxxix}is commonly placed excentrically, and most usually in the apical or aboral portion of the central capsule, either between its apex and the podoconus, or quite excentrically on the dorsal aspect. The simple nucleus of the NASSELLARIA usually appears to be vesicular and to possess a somewhat firm membrane, clear contents, and a rather large, dark coloured nucleolus. In many NASSELLARIA the nucleus is spherical or ellipsoidal (Pl. 53, fig. 11); whilst in many #Stephoidea# and #Spyroidea#, where the central capsule is constricted by the sagittal ring and divided into two symmetrical lateral lobes, the nucleus partakes of the same mode of growth and appears in the middle of the capsule as a transversely placed ellipsoid or even as a short cylinder (Pl. 90, figs. 7, 9). The most remarkable modification in the form of the nucleus is to be found in the multi-articulate #Cyrtoidea#. Here it is usually enclosed in the cephalis and is spherical, ellipsoidal or spheroidal, often flattened almost into a disc. If now the central capsule increase greatly in size and put forth three or four clavate lobes which hang down through the pores of the cortinar septum into the thorax (or even into the succeeding joints), the nucleus usually undergoes similar modification, and three or four finger-like apophyses are developed from its base, which project into the corresponding lobes of the central capsule (Pl. 59, figs. 4, 12, 13).

The numerous small, spherical, homogeneous nuclei which are to be found in the central capsules of those NASSELLARIA, which are ripe and about to develop spores, were described in 1862 in my Monograph, as "numerous, small, transparent, spherical cells" in the case of various #Cyrtoidea# (_Arachnocorys_, _Lithomelissa_, _Eucecryphalus_, _Eucyrtidium_, &c.) (_loc. cit._, pp. 302, 305, 309, 321, &c.), and I find them of the same form and dimensions, but deeply stained with carmine in many preparations in the Challenger collection. R. Hertwig has delineated them very accurately in the case of _Tridictyopus_ (1879, _loc. cit._, p. 84, Taf. vii. fig. 3). He was also the first to recognise the uninucleate condition of the NASSELLARIA, which is much more frequently observed than the serotinous multinucleate condition, and he described very clearly the peculiar lobed nuclei which arise in #Cyrtoidea#, owing to the protrusion of the nucleus through the cortinar septum (_loc. cit._, p. 85, Taf. viii. figs. 3-8).

70. _The Nucleus of the Cannopylea._--The nucleus presents the same remarkable structures in all species of the PHÆODARIA or CANNOPYLEA which have been examined, and closely resembles the germinal vesicle of an amphibian ovum, being a large spherical or spheroidal vesicle with numerous nucleoli. Its diameter usually amounts to half or two-thirds, sometimes even three-quarters, that of the central capsule. The vertical main axis of the latter is also that of the nucleus, which usually lies somewhat nearer to the aboral pole. The nucleus is generally rather more strongly compressed in the direction of the main axis than the capsule itself. The membrane of the vesicular nucleus is thin, but firm, and encloses a clear or finely granular mass of nuclein. The number and size of the contained nucleoli are variable even in one and the same species, and stand in inverse ratio to each other, an obvious result of the gradual process of division. Commonly {xl}from twenty to fifty roundish or spherical, strongly refracting nucleoli, are present; more rarely there are several hundred very small ones. Sometimes the nucleus is penetrated by fine trabeculæ, in whose meshes lie the nucleoli (Pl. 101, fig. 2). In certain nuclei, which contained a few large nucleoli, these were of irregular form, probably the result of amoeboid movements (Pl. 101, fig. 1). In the formation of spores in the CANNOPYLEA, the nucleus apparently becomes dissolved, and its numerous nucleoli develop directly into the nuclei or mother-nuclei, which produce the nuclei of the flagellate spores. Furthermore, many PHÆODARIA seem to multiply by simple cell-division, since very commonly (especially in the #Phæocystina# and #Phæoconchia#) two large nuclei (right and left), may be met with in one central capsule; sometimes also a single large nucleus, in which a sagittal constriction marks the commencing division of the capsule (Pl. 101, figs. 2, 36; Pl. 104, fig. 3; Pl. 124, fig. 6, &c.).

The large nucleus of the PHÆODARIA was first described in my Monograph in 1862, in the case of _Aulacantha_ (p. 263), _Aulosphæra_ (p. 359), and _Coelodendrum_ (p. 361), as a "large, spherical, thin-walled endocyst," from 0.1 to 0.2 mm. in diameter. More detailed descriptions, especially with respect to the behaviour of the nucleoli were given by R. Hertwig in 1879 (L. N. 33, p. 97).

71. _The Endoplasm or Intracapsular Protoplasm._--In all Radiolaria the intracapsular protoplasm, which, for the sake of brevity, may be termed "endoplasm," constitutes originally, and especially in the earliest stages, the only important content of the central capsule, except the nucleus. In certain SPUMELLARIA and NASSELLARIA, of simple structure and of small dimensions, this condition persists for a long period, and the endoplasm then appears as a homogeneous, colourless, turbid or finely granular, mucous, semi-solid mass, which cannot be distinguished from the ordinary undifferentiated protoplasm of young cells; no definite structure, and in particular, no fibrillar network, can be discovered in it even by the use of the customary reagents. In the great majority of the Radiolaria, however, this primitive homogeneous condition of the endoplasm is very transient, and it soon undergoes definite modifications, becoming differentiated into separate parts or producing new constituent contents. Such products of the internal protoplasm are in particular hyaline spheres (vacuoles and alveoles), oil-globules, pigment-bodies, crystals, &c. The most important of the differentiations which take place in the endoplasm is that into an internal, granular, _medullary_ substance and an external, fibrillar, _cortical_ substance; although the various legions behave somewhat differently in this respect (§§ 77-80).

72. _Intracapsular Hyaline Spheres._--The central capsule of very many Radiolaria contains in its endoplasm numerous spherical bodies of varying size, which consist of watery or albuminous fluid, and have previously been regarded as nuclei, or described as products of the internal protoplasm, under various names, such as "spherical transparent {xli}vesicles" (see note A, below), "albumen spheres" (see B), "gelatinous spheres" (see C), "alveolar cells" (see D), &c. Some of these spheres are perfectly transparent, structureless and of varying refractive power, producing the impression of drops of fluid; others contain various formed constituents, such as oil-globules, fat-granules, pigment-granules, concretions, crystals, &c. From a morphological point of view they may all be divided into two categories, membraneless vacuoles and vesicular alveoles. The _vacuoles_ are simple spherical drops of fluid or of gelatinous material, devoid of a special envelope, but immediately surrounded by the endoplasm. The _alveoles_, on the other hand, are true vesicles with a thin spherical envelope, enclosing a drop of fluid or jelly. This envelope is commonly very thin, homogeneous, and often scarcely discernible, so that in practice a sharp line of demarcation cannot be drawn between alveoles and vacuoles; the former are usually somewhat larger than the latter. The fact is, nevertheless, certain that the hyaline spheres, which may be isolated on rupturing the central capsule of many Radiolaria, in certain cases, particularly in large species, possess a clear, anatomically demonstrable membrane, whilst in others no such appearance is presented. It may be assumed that the vesicular alveoles are developed from the drop-like vacuoles by increase in size, and by the precipitation of a delicate envelope from the endoplasm. The character common to all these hyaline spheres, whether vacuoles or alveoles, is found in their aqueous, not adipose, constitution, and in their clear transparent appearance, which allows of no structure (the above-mentioned contained bodies excepted) being recognised. Their refractive power and consistency vary somewhat, and probably their chemical constitution still more. Sometimes they are strongly refractive and shining, and sometimes feebly refractive and pale; their consistency shows all intermediate stages between a thin fluid, which readily disappears in water, and a firm, insoluble jelly. As regards their chemical composition (which is probably very variable), the hyaline spheres may be best divided into two groups, the organic and inorganic. The _inorganic hyaline spheres_ are simple drops of saline solution without any carbonaceous constituent; the _organic_, on the other hand, contain a small quantity of organic matter dissolved in the watery fluid, and may be either albuminous or gelatinous spheres. The formed contents which are commonly present are of very various natures, usually small fat-granules, more rarely larger fat-granules or pigment-granules, sometimes concretions or crystals. In many groups, especially among the large PHÆODARIA and #Collodaria#, the numerous hyaline spheres are remarkable for their equal size and even distribution throughout the endoplasm (Pl. 1, figs. 1, 4; Pl. 104, fig. 2, &c.). In some genera belonging to the Thalassicollida the alveoles are of enormous size (Pl. 1, figs. 2, 3); they then become flattened by mutual pressure into polyhedra and distend the central capsule to unusual dimensions (in _Physematium_ and _Thalassolampe_ 8 to 12 mm.).

A. The "_spherical hyaline vesicles_," which I described in my Monograph (1862, p. 71) as among the most important and constant contents of the central capsule, are partly vacuoles, {xlii}partly homogeneous nuclei. Most recent investigators, Bütschli in particular (1882, L. N. 41), have pointed out and rightly criticised this confusion. The criticism might, however, have been more justly expressed by stating that, in the preparation of my Monograph (1859-1862), I did not make use of modern methods of demonstrating the nucleus by staining fluids, which were quite unknown at the time, and only discovered a decade later. In fact, without the aid of such reagents, it is quite impossible to distinguish between the various "spherical transparent vesicles," of which those found in the central capsule of the PHÆODARIA and many monozootic #Collodaria# are simple vacuoles lying in the endoplasm, whilst, on the other hand, those of the Polycyttaria and many other Radiolaria are true homogeneous nuclei. For not only are the general appearance of the small clear spheres, their refractive power, and regular distribution in the endoplasm quite similar, but they are also of much the same size, for the diameter ranges from 0.005 to 0.015 mm., being generally between 0.008 and 0.012 mm. In addition to this there is generally in each hyaline sphere a dark brightly shining granule, which, in the case of the vacuole, is simply a fat-granule, whilst in the case of the nucleus, it is a true nucleolus. The small hyaline spheres in the young uninucleate capsules of the Polycyttaria are simple vacuoles (Pl. 3, fig. 12), whilst in the ripe multinucleate capsules they are true nuclei (Pl. 3, figs. 3, 8, 9), and it is quite impossible to discriminate between these two conditions without the use of reagents. This has been expressly recognised by R. Hertwig, who has the merit of having been the first to clearly distinguish, by the aid of staining fluids, between these two different constituents (1879, L. N. 33, p. 108).

B. The "_albumen spheres_," which were first observed by A. Schneider in 1858 in the common cosmopolitan _Thalassicolla nucleata_ (L. N. 13, p. 40), and which appear to occur in only a few other Thalassicollida, are distinguished from the ordinary hyaline spheres of about the same size by their higher refractive power and by certain albuminoid reactions, especially the coagulation of a membranous envelope under the influence of certain reagents (see my Monograph, p. 250, and Hertwig, L. N. 26, 1876, p. 46). They often enclose various formed contents, and require further investigation.

C. The _gelatinous spheres_ of various sizes, found in the endoplasm of the Radiolaria, agree in their reactions (especially in staining by certain reagents) with the common extracapsular jelly of the calymma, and are hence distinguishable both from the true (coagulable) "albumen sphere," and from the ordinary watery vacuoles.

D. The _alveoles_, which are only accurately known in the case of certain large monozootic #Collodaria#, but which also seem to occur in the central capsule of other remarkably large Radiolaria, were described in my Monograph in the case of _Thalassolampe margarodes_ and _Physematium mülleri_, under the name "intracapsular alveolar cells" (1862, pp. 77, 254, 257). They are not, however, true nucleated cells, and the body described as a nucleus is not such in reality. Nevertheless these large hyaline spheres do possess a special envelope, as I have recently convinced myself by the examination of ruptured central capsules of _Thalassolampe maxima_, _Thalassopila cladococcus_, and _Physematium atlanticum_ (Pl. 1, figs. 2, 3). The central capsule of these #Collodaria# becomes distended to most unusual dimensions (2 to 12 mm. in diameter) by the great development of these large hyaline vesicles, each of which measure from 0.1 to 0.5 mm. in diameter.

73. _The Intracapsular Fat-Globules._--Fat is present in the central capsule of all Radiolaria in larger or smaller quantities, and generally appears in the form of very {xliii}numerous, small, spherical granules, which are either distributed evenly in the endoplasm (as an emulsion) or enclosed in the vacuoles; the latter, in particular, is the case in most PHÆODARIA, perhaps generally. In this group each vacuole contains as a rule a single dark, shining fat-granule, and sometimes also an irregular bunch composed of from two to five or more granules. In addition to these small fat-granules (_granula adiposa_) which are always present, the central capsule of many Radiolaria contains also larger fat-globules (_globuli adiposi_). These appear to be generally wanting in the PHÆODARIA, and are on the whole rare in the ACANTHARIA; whilst, on the contrary, they are very common in the NASSELLARIA and SPUMELLARIA. The Polycyttaria or social Radiolaria are as a rule distinguished by the possession of a single large central oil-globule, which lies in the centre of the central capsule, and is on an average about one-third of it in diameter (Pl. 3, figs. 4, 5). This is absent, however, in those young capsules of the Polycyttaria in which the primary nucleus is centrally situated (Pl. 3, fig. 12). Those species of Polycyttaria whose central capsule reaches a considerable size, often enclose numerous oil-globules, and in _Collophidium_ (species of _Collozoum_ with an elongated cylindrical capsule, Pl. 3, figs. 1, 3) the axis of each capsule is occupied by a row of numerous oil-globules. In the monozootic SPUMELLARIA, in which the nucleus is always centrally situated, the large oil-globules are, of course, excentric, being in apposition to the inner surface of the capsule-membrane (Pl. 1, fig. 3; Pl. 2, figs. 2, 5). In the #Discoidea# the oil-globules, which are often present in large numbers, form elegant concentric rings around the central nucleus, and in those species with segmented arms, there are one or more transverse rows in each segment (Pl. 43, fig. 15). In the NASSELLARIA the number and distribution of the oil-globules are dependent upon the form of the central capsule. When this is simple, without lobes, and ovoid or conical, they generally lie in its aboral half above the podoconus (Pl. 51, figs. 5, 13; Pl. 97, fig. 1). When, on the contrary, the basal portion of the capsule sends out three or four dependent processes (as in the majority of the #Cyrtoidea#), a large globule may generally be seen in the swollen distal part of each conical or ovoid lobe (Pl. 53, fig. 19; Pl. 60, figs. 4-7). In many #Stephoidea# and #Spyroidea#, whose central capsule is separated into two lateral portions by the constriction corresponding to the sagittal ring, each of these contains either a single large globule or a group of small ones (Pl. 90, figs. 7, 10). These oil-globules are usually colourless and highly refractive; rarely they are yellow or brown, sometimes rose-coloured, or an intense blood-red (_e.g._, in _Thalassophysa sanguinolenta_) or even orange (in _Physematium mülleri_). In many SPUMELLARIA, and particularly in the Polycyttaria, an albuminous substratum may be recognised in them, which is sometimes disposed in layers, and after extraction of the fat presents the appearance of a laminated sphere. The physiological significance of the oil-globules is twofold; in the first place they tend to diminish the specific gravity of the organism; in the second they may be utilised as a reserve store {xliv}of nutriment. In the latter respect they are of special importance in the process of spore-formation, each flagellate spore usually containing a fat-granule.

74. _The Intracapsular Pigment-Bodies._--In the majority of Radiolaria when observed alive, the central capsule is coloured, only in the minority is it colourless. The colour is never diffuse, but always due to the formation of definite pigment granules or vesicles, which are sometimes distributed evenly throughout the endoplasm, sometimes aggregated in the central or peripheral regions. Their form may be either spherical, irregularly rounded, or polyhedral. They vary much in dimensions, but in most cases are immeasurably small, and appear under a high magnifying power as fine dust; occasionally, however, their diameter may amount to from 0.001 to 0.005 or more. The chemical constitution of the intracapsular pigment is unknown in most Radiolaria, and is probably very various. In many instances the pigment-granules consist of fat, in others not. The commonest colours are yellow, red, and brown; violet and blue are rare, and green still rarer. Sometimes a definite tone of colour prevails throughout a whole group, and may then be attributed to inheritance, _e.g._, red is found in most #Sphæroidea#, and blue in the Polycyttaria (see note A). One colour is almost always constant in the members of the same species. True pigment-cells, belonging to the Radiolarian organism, do not occur within the central capsule. The peculiar yellow cells which are found in the central capsule of many ACANTHARIA are symbiotic xanthellæ (see § 76).

A. The number of Radiolaria whose pigment has been examined in the living state, is too small to allow of any general conclusions being drawn. Regarding the different colours known, see my Monograph, L. N. 16, p. 76.

75. _The Intracapsular Crystals._--The crystals found in the central capsule of many Radiolaria may be divided into two groups, of very different significance; small crystals, which are very widely distributed, and large crystals, which occur in only a few genera. The _small crystals_ may also be termed "spore-crystals," since each swarm-spore often contains such a crystal. They are rod-like or spindle-shaped, and consist of an organic substance which probably serves as a reserve of nutriment for the developing spores. Such spore-crystals have been observed in numerous SPUMELLARIA and ACANTHARIA belonging to various families, and are probably present throughout the two legions which make up the Porulosa. On the other hand, they have not been noticed in the Osculosa (NASSELLARIA and PHÆODARIA), the few swarm-spores belonging to these groups which have been observed not exhibiting any crystals. The _large crystals_, which occur in small numbers in the endoplasm, have hitherto only been observed in a few species of SPUMELLARIA, belonging to the Polycyttaria. They were first noticed in the common _Collosphæra huxleyi_, and regarded as coelestin. They are also found in the central capsule of many other Collosphærida, _e.g._, _Buccinosphæra_ (Pl. 5, figs. 11, 12). Crystal-masses, crystal-sheaves, or spherical masses of radiating acicular crystals are enclosed in {xlv}the vacuoles or "albumen globules" of _Thalassicola nucleata_ and other Thalassicollida, as well as in the central capsule of _Coelographis_ and some other PHÆODARIA (Pl. 127, figs. 4-7). All these large crystals are probably to be regarded as excretory products.

75A. _The Intracapsular Concrements._--Concretions, either mineral or organic, of varying form and constitution, are to be found in the endoplasm of Radiolaria belonging to very different families. They are most abundant and multiform in _Thalassicolla nucleata_, being usually circular or elliptical discs, which are concentrically laminated and highly refractive, resembling starch-grains. Among them twin forms may frequently be observed, as though the concrements were in process of division (see note A). Similar amyloid concretions are to be seen in the central capsule of different SPUMELLARIA and NASSELLARIA, _e.g._, in _Cephalospyris triangulata_ (Pl. 96, fig. 28). Violin-shaped, highly refractive concrements have been observed in the central capsule of numerous SPUMELLARIA, NASSELLARIA, and ACANTHARIA, _e.g._, _Thalassosphæra_, _Spongosphæra_, _Plegmosphæra_, _Cyrtocalpis_, _Peripyramis_, _Botryocella_, &c. (see note B). The chemical constitution of these concrements is insufficiently known.

A. The amyloid concretions of _Thalassicolla nucleata_ have been described in detail in my Monograph (pp. 80, 250, Taf. iii. figs. 2, 3), and by R. Hertwig in the Histologie der Radiolarien (1876, p. 47, Taf. iii. figs. 9-13).

B. The violin-shaped concretions of _Thalassosphæra bifurca_ have been figured in my Monograph (pp. 80, 261, Taf. xii. fig. 1).

76. _The Intracapsular Xanthellæ._--The xanthellæ, zooxanthellæ, or symbiotic "yellow cells" are found within the central capsule only in the ACANTHARIA, whilst in other Radiolaria they only occur in the extracapsulum. They are most frequent in the #Acanthometra#, rarer in the #Acanthophracta#, but even in the former they are often wanting. Their number is very variable, but usually small, from ten to thirty in one capsule. They lie for the most part immediately below the capsule membrane, in the cortical layer of the endoplasm. The form of the yellow cells is either spherical or ellipsoidal, often also spheroidal or even lentiform. The diameter varies from 0.01 to 0.03 mm. They possess a distinct membrane and an excentric nucleus, and contain numerous yellow pigment-granules in the endoplasm. This yellow pigment dissolves in mineral acids to form a green fluid, and in other respects also behaves somewhat differently from the yellow pigment in the extracapsular yellow cells of the SPUMELLARIA and NASSELLARIA. In both cases, however, the xanthellæ are not integral portions of the organism, but unicellular algae, living as parasites or symbiontes in the body.

A. The yellow cells in the central capsule of the ACANTHARIA were first observed by Joh. Müller (L. N. 12, pp. 14, 47). In my Monograph I described them at greater length, and indicated their differences from the extracapsular yellow cells of other Radiolaria (L. N. 16, pp. 77, 86). Since then, R. Hertwig has demonstrated their cellular nature (L. N. 33, pp. 12, 113), and still more recently {xlvi}Brandt has given further accurate information regarding their occurrence, constitution, and physiological significance (L. N. 39, ii. Art., p. 235, figs. 62-73).

77. _The Endoplasm of the Peripylea._--The intracapsular protoplasm of the SPUMELLARIA or PERIPYLEA is usually distinguished by a more or less complete radial arrangement, which does not occur in the same form in other Radiolaria; it may be regarded as characteristic of this legion, for it probably occurs in all the species at some period of life or other, and stands in a direct causal relationship with the typical structure of the capsule-membrane in all the "PERIPYLEA" (see note A). For as this is commonly perforated by very numerous pores distributed at equal intervals over the whole surface of the capsule, and since a communication between the intra- and extracapsular sarcode takes place through these, the radiate structure of the endoplasm may be readily explained as due to the influence of radial currents which take place continuously or intermittently in the endoplasm. This radiate structure is most obvious when the endoplasm contains no secondary products or only an insignificant amount of these, and thus appears colourless and almost homogeneous, or only finely granular. Under these circumstances, an optical section of the central capsule usually reveals a distinct radial striation; numerous narrow, straight, dark streaks alternating regularly with still narrower clear ones; the latter consist of homogeneous, the former of more or less granular protoplasm (Pl. 20, fig. 1_a_). Often there may be distinguished in each darker streak a single straight row of strongly refracting (fat?) granules, sometimes several such rows. Occasionally the whole endoplasm becomes divided up into a number of large "radial wedges," club-shaped, conical or pyramidal masses of granular protoplasm, separated by clear divisions of hyaline plasma (_e.g._, in _Actissa radiata_, p. 14, where in the optical section of the central capsule, between the membrane and the nucleus, twenty-five dark radial wedges of equal size were separated by thick clear partitions of hyaline protoplasm). In the majority of the SPUMELLARIA this radial striation is partially or entirely concealed by the formation of pigment or of other products. Very often it is only visible in the cortical layer, which lies immediately below the capsule-membrane (Pl. 1, figs. 1, 3). The remarkable "centripetal cones" which characterise the Thalassicollid genus _Physematium_, and were formerly described as "centripetal cell-groups," are probably a special development of these cortical radial wedges; they are conical cortical bodies, regularly distributed on the inner surface of the membrane of the central capsule, and disposed with the apex turned towards the centre (see note B). More rarely than in the cortical layer, a similar radial structure is to be found in the innermost medullary layer immediately surrounding the nucleus. Here the endoplasm sometimes breaks up into fine radial threads, which are anatomically separable and hang down from the free nucleus as thin processes (see note C). In some cases it is also possible to isolate radial rods from the cortical layer of teased out central capsules.

{xlvii}A. The radial structure of the endoplasm was first described in my Monograph (1862, p. 74), though R. Hertwig (1879, p. 112) was the first to indicate its typical significance in the case of the PERIPYLEA, and to demonstrate its causal relation with the radial currents in the central capsule of this legion. More recent investigations have led me to the conviction that this phenomenon is more widespread, and often more strongly developed, than was formerly imagined, and that it is probably one of the typical characters of all SPUMELLARIA (at least of the Monozoa).

B. The centripetal cones of _Physematium_, which have hitherto been known only in these colossal Thalassosphærida, were fully described in my Monograph under the name "conical centripetal cell-groups"; by their first discoverer, A. Schneider (L. N. 13), they were termed "nests," and compared with the "nests" (central capsules) of the Polycyttaria. In the _Physematium mülleri_ of the Mediterranean (hitherto only observed by Schneider and myself at Messina) it appeared as though each centripetal cone were composed of a group of from three to nine (usually four or five) slender wedge-shaped cells, whose common centripetal apex was produced into a radial thread of sarcode (L. N. 16, p. 258, Taf. iii. fig. 7). Since then (1866) I have observed at Lanzerote, in the Canary Islands, a nearly related form, which I take to be _Physematium atlanticum_, Meyen. In this, however, the "centripetal cell-groups" were wanting, and the whole cortical layer of the endoplasm was cleft into numerous radial portions, each enclosing a nucleus (probably the mother-cells of flagellate spores, see p. 35).

C. The radial fibres of the medullary endoplasm which cling to an extracted nucleus have been observed by Hertwig in certain #Sphæroidea# (_Diplosphæra_, _Arachnosphæra_) (L. N. 33, p. 40).

78. _The Endoplasm of the Actipylea._--The intracapsular protoplasm of the ACANTHARIA or ACTIPYLEA is often distinguished by a partial or complete radial arrangement like that of the PERIPYLEA, but differing in the number, size, form, and distribution of the radial portions into which the endoplasm is differentiated. For since the pores of the capsule membrane are distributed at equal distances all over the surface in the SPUMELLARIA, whilst in the ACANTHARIA they are arranged in definite groups, and since the number and arrangement of the pores has a direct influence upon the internal currents of the endoplasm, it follows that the radial structure in the latter legion must be very different from that in the former. In addition to this there must not be forgotten the important influence which the early centrogenous formation of the skeletal rods exercises upon the disposition and growth of the intracapsular structures. Hence the endoplasm of the ACANTHARIA does not separate into innumerable thin, closely packed radial wedges or cortical radial rods, but into a small number of large pyramidal portions between which run the radially disposed heterogeneous portions of the contents of the capsule, viz., the radial bars of acanthin and the peculiar intracapsular "axial threads." As a direct consequence of the regular disposition of these heterogeneous radial portions, which is often characteristic of the various families of the ACANTHARIA, a corresponding differentiation of the endoplasm is brought about; it divides into a number of conical or pyramidal portions (radial pyramids), whose bases rest upon the capsule-membrane and whose apices are directed towards the centre of {xlviii}the capsule (the central star of the skeleton). These radial pyramids are, however, but rarely visible, being usually more or less concealed by a dark pigment.

The differentiations of the endoplasm in the central capsule of the ACTIPYLEA have been but little investigated, but they appear to vary somewhat in the different groups of this legion. In all ACANTHARIA in which the twenty radial bars are regularly arranged according to the Müllerian law (see p. 717) and in which axial threads constant in number and disposition run between them from the central star to the capsule-membrane, it obviously follows that the endoplasm must be divided into more or less distinct radial pyramids, and this must the case whether these take the form of continuous tracts or of actually separable portions. The regular polygonal figures, often seen on the surface of the central capsule (with special distinctness in _Acanthometron elasticum_ and _Acanthometron pellucidum_) separated by a network of granular threads, are the bases of such radial pyramids (see Hertwig, L. N. 43, p. 12, Taf. i. figs. 1-7).

79. _The Endoplasm of the Monopylea._--The intracapsular protoplasm of the NASSELLARIA or MONOPYLEA is distinguished from that of any of the other three legions by the development of a quite peculiar fibrillar structure, the axial "pseudopodial cone," which may shortly be termed the "podoconus" (foot-cone). Since this is in direct correlation with the peculiar structure of the capsular opening, the large "porochora," which is situated at the basal pole of the main axis, it is quite as characteristic of the legion as the latter itself (see note A). The podoconus is primitively a vertical regular cone whose circular base occupies the horizontal porochora or "basal porous area" of the central capsule, while its vertical axis coincides with that of the latter. The apex of the cone, usually somewhat rounded off, is therefore directed towards the aboral or apical pole of the central capsule and separated from it by a larger or smaller interval. In this interval the nucleus originally lies (as in Pl. 51, fig. 13; Pl. 98, fig. 13); but it is usually displaced subsequently and lies excentrically. The cone is of very variable height; on an average its vertical height is about equal to the diameter of its horizontal base; these dimensions are, however, dependent upon the form of the central capsule; the height being greater in slender ovoid or conical capsules, and less in depressed sphæroidal or discoidal ones, than the diameter of the base. The podoconus consists of differentiated endoplasm, which becomes more deeply stained by carmine and offers greater resistance to solvents than the surrounding finely granular protoplasm. The apex, especially, becomes very intensely stained. It always exhibits a very characteristic fine but distinct striation, numerous straight radial lines diverging from the apex of the cone towards the base. The number of these striæ appears to correspond with that of the vertical rods in the porochora, and each of these latter stands apparently in direct communication with the basal end of an apical stria (§ 59). These threads are probably differentiated constant contractile threads of endoplasm, or even myophanes, comparable with the contractile cortical threads of the CANNOPYLEA and the permanent axial threads of the ACTIPYLEA. The numerous modifications, {xlix}undergone by the form and contents of the central capsule in the different groups of MONOPYLEA, especially those due to the formation of the skeleton, are not without influence upon the podoconus. The most important divergencies from the above described primary form are the following:--(1) The vertical axial cone becomes oblique, its axis inclining in the sagittal plane and approaching either the dorsal or the ventral wall of the capsule; the cause of this appears to be usually the excentric development of the growing nucleus or the formation of a large oil-globule. (2) The smooth mantle of the podoconus becomes divided by three longitudinal furrows into three equal prominent ridges, which correspond to three circular lobes in the porochora; the cause of this basal triradial lobular formation lies probably in the triradial development of the skeleton in many NASSELLARIA or in the cortinar structure of the collar septum. (3) The simple podoconus splits into three or four elongated lobes, which eventually become almost completely separated and correspond to the lobes of the central capsule, in the axial wall of which they lie as longitudinally striated bands. The behaviour of these bands justifies the hypothesis that the podoconus is a muscular differentiated portion of the endoplasm and is composed of myophane fibrillæ, whose contraction determines the opening of the central capsule.

A. The podoconus of the MONOPYLEA was first described by R. Hertwig in 1879, and recognised as a characteristic component of the central capsule in the most various groups of this legion (in #Plectoidea#, #Stephoidea#, #Spyroidea#, and #Cyrtoidea#; see his figures, _loc. cit._, Taf. vii., viii., and the description, pp. 71, 73, 83, 106). Hertwig called it the "pseudopodial cone," and regarded it as a conical process of the capsule-membrane, which is developed from this latter and projects from the porous area into the interior of the central capsule; "it is penetrated by fine canals which arise at the apex of the cone, diverge towards the base, and terminate there in the rods of the pseudopodial area. The intracapsular protoplasm penetrates at the apex of the pseudopodial cone into its fine canals, runs along them and emerges from the rods of the porous area in the form of slender threads" (_loc. cit._, p. 19). I cannot agree with this view of Hertwig, although I have been able to confirm the accuracy of his description by my own observations upon numerous excellently stained and preserved preparations in the Challenger collection. As I have proved by numerous teased out preparations, and as Hertwig himself correctly states, "the cone is more readily detached from the membrane than from the protoplasm, when the capsule is teased" (_loc. cit._, p. 73). Hence I regard the podoconus not as a differentiated portion of the capsule-membrane but as endoplasm, and believe that it is composed of myophanes or "contractile muscular fibrils" in the same manner as the cortical layer of the CANNOPYLEA. Probably the contraction of these fibrils serves to raise the opercular rods and hence to allow the exit of the endoplasm through the pores which lie between these opercular rhabdillae (compare § 59).

80. _The Endoplasm of the Cannopylea._--The intracapsular protoplasm of the PHÆODARIA or CANNOPYLEA is distinguished from that of the other three legions by several characteristic peculiarities, which are very important, since they stand in causal relation to the typical structure of the capsule-membrane and in particular of its {l}remarkable aperture. In the case of many and perhaps of all PHÆODARIA the endoplasm is differentiated into a granular medullary and a thin fibrillar cortical layer, the former of which usually encloses numerous small vacuoles, while the latter contains muscular fibrillæ. In the voluminous central capsule of large PHÆODARIA the whole cortical layer of the endoplasm, which lies immediately below the delicate inner capsule-membrane, sometimes appears delicately and regularly striated, and most distinctly so under the apertures, towards the centre of each of which the dark striæ are radially directed (see note A, below). These striæ are probably contractile muscular fibrillæ; or "myophanes," by whose contraction the openings are voluntarily widened. In the Tripylea this fibrillar star is much more strongly developed under the astropyle (the main opening) than under the parapylæ (or accessory openings); and probably the peculiar radial structure of the operculum of the former is due to the stronger development of these radial fibrils (being their impression). In many PHÆODARIA, indeed, the fine myophane fibrils are only visible under the apertures, whilst in others they form a continuous fibrillar cortical layer on the whole inner surface of the inner capsule-membrane; the fine fibrillæ run meridionally from one pole of the main axis to the other; perhaps the whole central capsule may change its form in consequence of their contractions. The medullary portion of the endoplasm, which lies below this thin cortical layer, is usually finely granular in the PHÆODARIA, and permeated by numerous spherical vacuoles, which are noteworthy from their equal size and regular distribution. Each clear vacuole usually contains a dark shining fat-granule, more rarely a group of such granules (see note B). Compare § 60, and Pl. 101, figs. 1-3; Pl. 104, figs. 1, 2; Pl. 111, fig. 2; Pl. 128, fig. 2, &c.

A. The fine fibrillæ in the cortical layer of the endoplasm were first described by Hertwig in 1879 (L. N. 33, p. 98, Taf x. figs. 6-10). He found them, however, only below the three openings in the capsule of the Tripylea, where they form three stellate groups of fibrils. I find them very clearly shown, and with especial distinctness, under the astropyle in most PHÆODARIA of which I have had the opportunity of examining well-stained and preserved central capsules. In many cases, also, the striation is not confined to the apertures, but spreads over the whole cortical layer. Perhaps this constitutes in all PHÆODARIA a thin myophane-sheet, whose contractile fibrils run from one pole of the main axis to the other and cause by their contraction changes in the form of the spheroidal central capsule.

B. The granular medullary portion of the endoplasm of the PHÆODARIA, with its numerous clear spherical vacuoles, was first described in my Monograph (1862), in the case of _Aulacantha_ (p. 263), _Aulosphæra_ (p. 359), and _Coelodendrum_ (p. 361) as a "finely granular, mucous substance (intracapsular sarcode), packed more or less closely with clear spherical vesicles from 0.005 to 0.015 mm. in diameter, each of which contains one or two, rarely three, dark shining granules." That these clear spheres are true vacuoles was first clearly proved by Hertwig (L. N. 33, p. 98). As a rule all the vacuoles of the same central capsule are of equal size (generally from 0.008 to 0.012 mm. in diameter), and are distributed at equal intervals throughout the finely granular endoplasm.

{li}CHAPTER III.--THE EXTRACAPSULUM.

(§§ 81-100).

81. _The Components of the Extracapsulum._--The extracapsulum or extracapsular malacoma, under which name are included all those parts of the soft body which lie outside the central capsule, consists of the following constant, and important constituents:--(1) The _calymma_ or extracapsular jelly-veil; (2) the _sarcomatrix_ or layer of exoplasm immediately surrounding the membrane of the central capsule; (3) the _sarcodictyum_ or network of exoplasm, covering the surface of the calymma; (4) the _pseudopodia_ or radial fibres of exoplasm, which may again be subdivided into intracalymmar pseudopodia, uniting the sarcomatrix and sarcodictyum, and extracalymmar pseudopodia, radiating freely into the water outside the calymma.

82. _The Calymma._--The calymma or extracapsular jelly-veil of the Radiolaria is always the most voluminous portion of the extracapsulum, and in spite of its simple structureless constitution is of great morphological and physiological importance. In all Radiolaria this gelatinous mantle completely surrounds the central capsule, but is separated from its outer surface by a continuous, though thin, layer of exoplasm, the sarcomatrix. The pseudopodia radiating from the latter pierce the calymma, form the sarcodictyum at its surface, and radiate from its nodal points freely into the surrounding water. The calymma is rarely visible in living freshly captured Radiolaria, examined in sea-water, for its gelatinous substance is perfectly hyaline, colourless and pellucid, and possesses the same refractive index as sea-water; but when the object is removed from this fluid and transferred to carmine solution or some other colouring matter, the extent and figure of the calymma become apparent, for the staining fluid does not at first penetrate into the gelatinous material. When this has taken place, however (after a longer or shorter time), and the gelatinous material has become coloured, its form and size may be observed by the converse experiment; the object is transferred once more to water and the outlines of the calymma become as clear as those of the central capsule. The same is the case with dead specimens in which the sticky surface of the calymma has become covered with dust.

The jelly-veil of the Radiolaria was recognised even by the earliest observers of the group, Meyen (1834), and Huxley (1851), and compared with that of the Palmellaria; the former noticed it in _Physematium_ and _Sphærozoum_ (L. N. 1, p. 283), and the latter in _Thalassicolla_ and _Collosphæra_ (L. N. 5, p. 433). In all these SPUMELLARIA, both in the monozootic _Thalassicolla_ and in the polyzootic _Sphærozoum_ and _Collosphæra_, the calymma is very voluminous and filled with large alveoli. Meyen called them "muco-gelatinous masses, in the interior of which are contained small equal-sized vesicles"; Huxley likewise found clear vesicles in the jelly and compared them with Dujardin's vacuoles. Johannes Müller observed the jelly-veil in many different Radiolaria, in particular in the #Acanthometra#, first discovered by him, but erroneously believed that it only originated {lii}after death by liquefaction of the sarcode (L. N. 12, p. 6). This mistake is, however, easy to understand, since in living Radiolaria the calymma is usually invisible on account of its perfect transparency, whilst in dead specimens it is usually quite distinct on account of the dust clinging to its adhesive surface. I myself believed that the formation of the voluminous hyaline jelly-veil was only partially due to liquefaction after death, but that it was to some extent present in the living organism and that it might vanish and subsequently reappear by means of imbibition (L. N. 16, pp. 109, 110). R. Hertwig was the first to demonstrate, in 1879, that the jelly-veil is constantly present in living Radiolaria, that it forms the basis of the extracapsular malacoma and surrounds the central capsule as a second protective sheath (L. N. 33, p. 114).

83. _The Structure of the Calymma._--The extracapsular jelly-veil appears structureless in most Radiolaria, inasmuch as it represents a homogeneous pellucid excretion of the exoplasm and contains neither fibres nor other formed structures. In some groups, however, definite structural characters become secondarily developed. The most common and striking of these is the formation of alveoles, which takes place in the extracapsulum (see § 86). In consequence of this the calymma assumes a remarkable frothy consistency and appears to be composed of large, clear, thin-walled vesicles; this is especially the case in the #Collodaria# (#Colloidea#, Pls. 1, 3, and #Beloidea#, Pls. 2, 4), and in many large PHÆODARIA, especially among the #Phæocystina# (Phæodinida and Cannorrhaphida, Pl. 101, and Aulacanthida, Pls. 102-104). More rarely the calymma is not permeated by vacuoles, but there appear in it fine striæ parallel to the surface as though it were composed of thin concentric laminæ like an onion; perhaps these are the expressions of a different quantity of water in the various layers. In the calymma of many Radiolaria thin, straight, radial lines are to be seen, which are probably pseudopodia, and not to be attributed to any structural modification, or they may be slender canals which serve for the exit of the pseudopodia. On the outer surface of the calymma of different Radiolaria, and especially in the ACANTHARIA, a peculiar network of fibres is to be found, composed of polygonal meshes, like elastic fibres, probably due to a local thickening of the jelly. These polygonal meshes are often very regularly distributed between the radial spines of the #Acanthometra#, and stand in a definite relation to them. The fibres which form the meshes are often rather strong, resembling elastic fibres, as above-mentioned, and either simple or composed of bundles of very fine fibrillæ (L. N. 33, p. 15, Taf. i. fig. 1, Taf. ii. fig. 4).

84. _The Consistency of the Calymma._--The gelatinous material of which the calymma of the Radiolaria consists is a pellucid mass, rich in water and usually quite hyaline and structureless; its consistency is very variable. In the majority of the Radiolaria it may perhaps be about equal to that of the jelly which composes the umbrella of most Medusæ; but as in these latter it may vary between very wide extremes, constituting on the one hand a very soft jelly-mantle, offering but little {liii}resistance to mechanical influences and almost disintegrating under the eyes of the observer, and on the other hand forming a firm gelatinous shell, comparable to cartilage in hardness, elasticity, and power of mechanical resistance. In many Radiolaria of large dimensions with an alveolar calymma (especially in numerous #Collodaria# and PHÆODARIA) this may be split by means of dissecting needles and the central capsule extracted like the stone from a cherry, and then it is easy to ascertain that the firmness and elasticity of this jelly-veil are not less than those of a cherry. The different degrees of consistency in the various Radiolaria may be dependent either upon the relative amount of water which they contain, or upon qualitative or quantitative variations in the organic substance of which the jelly consists. Great importance is to be attached to the considerable consistency of the calymma, because it furnishes the indispensable groundwork for the deposition of many parts of the skeleton and particularly of the lattice-shells.

85. _The Primary and Secondary Calymma._--In most Radiolaria the external form and volume of the calymma are different at different stages of growth, and this difference is mainly dependent upon the development of the skeleton. Hence it is advisable to distinguish in general the primary from the secondary calymma. The _primary calymma_ is in the great majority of Radiolaria a perfect sphere, in the middle of which lies the concentric central capsule; on the surface of this gelatinous plate the primary spherical lattice-shell is secreted in most SPUMELLARIA and #Acanthophracta#, as well as in those PHÆODARIA which possess a spherical shell; in the remaining PHÆODARIA also and in the NASSELLARIA, where the lattice-shell is not spherical but monaxon, it is secreted on the surface of the primary calymma. This takes place at a definite time, very important in the development of the Radiolarian, which for the sake of brevity we shall term the "_lorication-period_." Since the firm surface of the primary calymma furnishes the necessary foundation for the deposition of the primary lattice-shell, it is of the greatest mechanical significance in all shell-bearing Radiolaria. The _secondary calymma_ arises only after the lorication-period by further growth of the primitive jelly-mantle and in the fully developed Radiolarian usually encloses wholly or partially the external parts of the skeleton, in consequence of which it assumes the most various forms. Very often the secondary calymma is polyhedral, being stretched between the radial spines of the skeleton, the distal ends of the latter then forming the fixed points of the gelatinous polyhedron.

86. _The Extracapsular Vacuoles and Alveoles._--The calymma of the Radiolaria usually appears completely homogeneous and hyaline without any structure; sometimes it encloses numerous clear vesicles, vacuoles or alveoles, and then assumes a frothy appearance, the expression of a more or less distinct alveolar structure. {liv}The clear vesicles to which this is due are either spherical, or polyhedral from mutual pressure, and like the similar ones in the central capsule may be divided into membraneless vacuoles and vesicular alveoles. The _vacuoles_ are simple drops of fluid, without a special envelope, and immediately surrounded by the gelatinous substance of the calymma, in which they appear as simple cavities. The _alveoles_ on the contrary are true vesicles, with a thin envelope, which encloses a drop of fluid or a globule of jelly; in the latter case its contents are different in refracting power and amount of contained water from the substance of the surrounding calymma. A sharp boundary between the membraneless vacuoles and the vesicular alveoles cannot be drawn in the case of the extracapsular hyaline spheres any more than in the intracapsular; the envelope of the alveoles is sometimes very distinct and even anatomically separable, whilst at other times it is very thin and scarcely recognisable; it may occasionally arise and disappear within a very short time (see note A). There is no doubt that in the calymma as in the central capsule the vesicular alveoles are secondary products, which have arisen from the vacuoles by the secretion of an enveloping membrane. This membrane is either a delicate sheath of exoplasm, or a firmer and more resistant skin, distinct from the exoplasm, and probably an excretion from it (_e.g._, Pl. 4, figs. 2, 3). In many cases the outer surface even of the vacuoles is covered by a network of pseudopodia, which form a sarcoplegma similar to a fenestrated alveolar membrane. The colourless pellucid fluid in the vacuoles and alveoles is usually simple sea-water, more rarely it contains a small quantity of albumen ("albumen-spheres") or jelly ("gelatinous spheres"). The size of these spheres is very variable. Quite small vacuoles may be found in the calymma of many Radiolaria. Large vacuoles, on the other hand, producing the appearance of an alveolar structure, are confined to but few groups, to a part of the SPUMELLARIA (#Colloidea#, #Beloidea#, and a few #Sphæroidea#), and to the #Phæocystina# (PHÆODARIA with incomplete skeleton); besides they occur only rarely in individual genera, _e.g._, _Nassella_ among the skeletonless NASSELLARIA. Since the volume of the calymma is much increased by the development of vacuoles, and the power of mechanical resistance is at the same time much increased, the fact is explained that the vacuoles occur mainly in Radiolaria which have no skeleton or only an incomplete one (see note B). Among the monozootic #Collodaria# the alveolar structure is especially well developed in the following genera; _Thalassicolla_ (Pl. 1, figs. 4, 5), _Thalassophysa_, _Thalassoplancta_, _Lampoxanthium_ (Pl. 2, figs. 1, 2); among the PHÆODARIA in most genera of the Phæodinida, Cannorrhaphida and Aulacanthida (Pls. 101-104), and probably also in other voluminous PHÆODARIA (_e.g._, #Phæosphæria#). The alveoles or vacuoles in the calymma of these large Radiolaria lie usually in several layers, one above another, and increase in size from within outwards. The Polycyttaria or social Radiolaria (the three families Collozoida, Sphærozoida and Collosphærida) without exception have an alveolar structure, and the special form of {lv}their colonies or coenobia is to a great extent determined by the development, number, size and arrangement of the alveoles in their calymma (compare Pls. 3-8). In these cases there is not unfrequently developed a large central alveole (see note C) whose thickened wall encloses a globe of jelly and serves as the central support of the whole colony (Pl. 5, fig. 1). Still more striking, however, is the arrangement of certain Polycyttaria, where each individual of the colony (or each central capsule with its calymma) is enclosed in a large alveole, whose firm wall often attains considerable thickness (Pl. 4, figs. 2, 3). The whole colony then appears as an aggregate of numerous cells, each of which possesses two envelopes, the inner central capsule and the outer alveolar membrane; between these lies in the Collosphærida the siliceous lattice-shell (Pl. 6, fig. 2). These pericapsular alveoles may be regarded as an outer cell-wall more correctly than the membrane of the central capsule itself, but the arrangement may also be compared to the temporary encystation of other Protista (see note D).

A. The extracapsular vacuoles in the calymma were first observed in 1851 by Huxley, in _Thalassicolla_ and _Sphærozoum_, and compared with Dujardin's sarcode vacuoles (L. N. 5). Afterwards J. Müller noticed that generally these "large clear vesicles are covered by a fine membrane," and hence he called them "alveoles" (L. N. 12, pp. 3, 7, &c.). In my Monograph I have described them more in detail as "extracapsular alveoles" (1862, p. 88, Tafs. i.-iii. xxxii.-xxxv.). Ever since then the point has been debated whether these clear spaces are simple vacuoles in the sense of Huxley or vesicular alveoles as stated by J. Müller. This contention is unnecessary, for both varieties are present, and often no sharp line can be drawn between them. R. Hertwig has recently come to the conclusion that they are as a rule "membraneless vacuoles," but that they "sometimes become surrounded by a special envelope" (L. N. 33, p. 31). He even succeeded "in extracting from a _Collosphæra_ the large vesicle which lies in the centre of many colonies and removing its covering of central capsules and jelly."

B. The _mechanical importance_ of the alveolar structure, which certainly increases the elasticity and mechanical resistance of the voluminous calymma, has not yet been sufficiently realised; in the case of those Radiolaria which have no skeleton, or at all events no lattice-shell, it may take the place of this as a protective envelope. Furthermore, by taking in and giving out water it may discharge a hydrostatic function, causing the organism to rise or sink in the water.

C. The _large central alveole_ found in the colonies of many Polycyttaria (especially Collosphærida) and first described in my Monograph (Taf. xxxiv. fig. 1), has since then been observed by Hertwig, Bütschli, and other investigators, and recognised as the "central support of the whole colony, surrounded by a delicate membrane" (compare L. N. 33, p. 31, and L. N. 41, p. 436). In a colony of _Trypanosphæra transformata_ (Pl. 5, fig. 1), which I observed living while in Ceylon in 1881, the membrane of the large central alveole was surrounded by a firm network of sarcoplegma, and could be mechanically isolated from the central jelly-sphere which it enclosed.

D. The _pericapsular alveoles_, figured in Pl. 4, figs. 2, 3, from a _Sphærozoum_, and in Pl. 6, fig. 2, from a _Siphonosphæra_, were very well preserved in some preparations in the Challenger collection; perhaps their development coincides with the formation of spores, and may be regarded as an encystation.

{lvi}87. _The Extracapsular Fat-Globules._--Fat is probably as widely distributed in the exoplasm as in the endoplasm of the Radiolaria; a considerable proportion of the small, dark, highly refractive granules appear to consist of fat; most likely they are for the most part direct products of metastasis. These widely-spread granules, which are sometimes coloured, and which by their passive motion produce the phenomenon of granular circulation in the exoplasm, are not the only fatty structures in the extracapsulum; larger globules sometimes occur. In certain large #Collodaria# (_e.g._, _Thalassicolla melacapsa_, Pl. 1, fig. 5; _Thalassophysa sanguinolenta_, &c.) radial series of oil-globules are found in the calymma, especially in its proximal portion; in others the central capsule is surrounded by a layer of oil-globules (situated in the sarcomatrix). In the PHÆODARIA a part of the phæodium appears to consist of fat-globules.

88. _The Extracapsular Pigment._--The formation of colouring matters in the extracapsulum is on the whole rare in the Radiolaria, apart from the "yellow cells" (see § 91) and from the peculiar phæodium of the PHÆODARIA, which will be separately treated of in the next paragraph. Considerable masses of extracapsular pigment, usually black or blue, rarely brown or red, are found only in a few Radiolaria belonging to the first three legions; most often in the SPUMELLARIA. Some large #Collodaria#, _e.g._, the common _Thalassicolla nucleata_ and a few other species of this genus (Pl. 1, fig. 4), are characterised by a rich deposit of black or blue pigment in the sarcomatrix and in the proximal portion of the calymma. Brown pigment is deposited in the calymma of many #Sphæroidea# and #Discoidea#, as well as of some NASSELLARIA (_Cystidium_, _Tridictyopus_, &c.). In a part of the ACANTHARIA red pigment granules are thickly strewn in the sarcoplegma and pass along the free pseudopodia, as for example in _Actinelius purpureus_ and _Acanthostaurus purpurascens_. The composition and significance of these extracapsular pigments are not completely known.

On the extracapsular pigment of _Thalassicolla nucleata_, compare my Monograph, pp. 87, 251. On the red extracapsular pigment-granules of the ACANTHARIA, see L. N. 19, pp. 345, 364, &c.

89. _The Phæodium of the Phæodaria._--The PHÆODARIA, which are distinguished from the other three legions of Radiolaria by the double membrane of the central capsule, and the peculiar structure of the main-opening (astropyle), differ also in other points, the most important of which is the constant presence of a voluminous mass of extracapsular pigment. This possesses a peculiar constitution and special significance, and is not to be confounded with the extracapsular pigment-granules of other Radiolaria (_e.g._, _Thalassicolla_), and hence it has been distinguished by the name "Phæodium," and the individual granules which compose it as "Phæodella" (see note A). The phæodium is always excentric in position relatively to the central capsule, of which it {lvii}surrounds the oral half in the form of a voluminous concavo-convex cap, hiding the astropyle at its basal pole so completely that the latter is rarely visible until the phæodium has been removed (Pls. 99-104; Pl. 115, fig. 8; Pl. 123, &c.). The central capsule is generally almost completely embedded in the phæodium, so that only its aboral pole (with the two parapylæ in the TRIPYLEA) projects. In the #Phæogromia#, in which the lattice-shell possesses a special opening and the central capsule lies excentrically in the aboral position of its interior, the phæodium occupies the oral aspect, between the capsule and the aperture (Pls. 99, 100, 118-120, &c.). In the peculiar family Coelographida (Pls. 126-128) a special receptacle (galea with its rhinocanna) for the phæodium is developed outside the bivalve shell, within which the central capsule lies. The proboscis, which in all PHÆODARIA arises from the centre of the astropyle, lies in the vertical axis of the phæodium and is entirely surrounded by it. The volume of the phæodium in the majority of the PHÆODARIA may be said to be about as great as that of the central capsule, although in some species it is considerably larger. Its colour is always dark, usually between green and brown, commonly olive-green or blackish-brown, rarely reddish-brown or black. The phæodellæ or pigment-granules which make up the greater part of the phæodium (see note B) are irregular in form and unequal in size and show no definite structure; usually they are spherical or ellipsoidal, and exhibit fine parallel striæ which run transversely or obliquely (Pl. 101, fig. 3, 6, 10; Pl. 103, fig. 1, &c.). Between the larger granules is usually found a thick dust-like mass of innumerable very small grains. The physiological significance of this peculiar phæodium is still unknown, but is probably considerable, judging from its large size and especially from its constant topographical relation to the astropyle; the latter consideration would lead to the supposition that it plays an important part in the nutrition and metastasis of the PHÆODARIA (see note C).

A. The phæodium of _Aulacantha_, _Thalassoplancta_, and _Coelodendrum_ was first described in 1862, in my Monograph, as an excentric extracapsular mass of pigment of blackish-brown or olive-green colour (pp. 87, 262, 264, 361, Taf. ii. iii. xxxii.). Since then John Murray, who investigated many living PHÆODARIA during the Challenger expedition, has shown its general distribution in this legion (Proc. Roy. Soc. Lond., vol. xxiv. p. 536, 1876). From the constancy of its presence I gave the legion the name PHÆODARIA in 1879 (L. N. 34).

B. With regard to the special composition of the phæodium and the constitution of the phæodellæ, see the general description of the PHÆODARIA, pp. 1533-1537.

C. Perhaps the phæodellæ are to some extent symbiontes with the PHÆODARIA; the xanthellæ present in most other Radiolaria are absent in this legion.

90. _The Extracapsular Xanthellæ._--Xanthellæ or Zooxanthellæ, symbiotic "yellow cells," are very commonly found in the extracapsulum of the Radiolaria, especially in many SPUMELLARIA and NASSELLARIA; whilst in the ACANTHARIA similar yellow cells usually only occur within the central capsule, and in the PHÆODARIA their {lviii}presence has not been certainly demonstrated. The extracapsular Xanthellæ are found most abundantly in the #Collodaria#, both in the monozootic Thalassicollida and in the polyzootic Sphærozoida. They occur in smaller numbers in the #Sphærellaria#, and in many divisions of the latter they seem to be entirely absent. Also it sometimes happens that, though present in large numbers in some SPUMELLARIA, they are entirely absent in others nearly related to them; indeed, this has also been observed in the case of different individuals of the same species. This fact alone is sufficient to show that the Xanthellæ are not an integral part of the Radiolarian organism (as was formerly believed) but parasites or more correctly symbiontes, which live as inhabitants of the calymma. More recent investigations have shown, that besides the yellow pigment-grains they contain starch or an amyloid substance, that is to say, vegetable reserve materials, that their thin envelope contains cellulose, and that their yellow colouring-matter resembles chlorophyll and is related to that of the Diatomaceæ ("Diatomin"). Hence they are now generally regarded as unicellular Algæ, nearly related to those which occur as symbiontes in other marine animals (_Exuviella_, &c.). The starch, which they develop with the formation of oxygen, may serve as nutriment to the Radiolaria, while the carbonic acid yielded by the latter is also beneficial to the Xanthellæ. The form of the Xanthellæ is usually spherical and elliptical, often also sphæroidal or discoidal. Their diameter is usually between 0.008 and 0.012 mm., rarely more or less. The differences exhibited by Xanthellæ which live in different groups of Radiolaria demand further investigation, which will perhaps lead to the establishment of several species of the genus _Zooxanthella_. At present _Zooxanthella extracapsularis_, in the calymma of SPUMELLARIA and NASSELLARIA, may be clearly distinguished from _Zooxanthella intracapsularis_, in the central capsule of the ACANTHARIA.

The "yellow cells" were first described in 1851 by Huxley, in the #Collodaria#, and afterwards by J. Müller (1858) in many SPUMELLARIA and NASSELLARIA. In my Monograph (1862, pp. 84-87) I gave a detailed account of their structure and increase by division, and laid special emphasis on the fact that they are the only elements in the Radiolarian organism which "are _undoubtedly cells_ in the strict histological sense of the word." Afterwards, in my Beiträge zur Plastiden-Theorie, I showed the constant presence of "starch in the yellow cells of the Radiolaria" (1870, L. N. 21). Shortly afterwards Cienkowski observed that the yellow cells live independently and reproduce themselves after the death of the Radiolaria, and in consequence first put forth the hypothesis that they do not belong to the Radiolarian organism, but that they are unicellular Algæ parasitic upon it (1871, L. N. 22). This view was ten years later more fully established by Karl Brandt, and elucidated by comparison with the symbiosis of the gonidia of Algæ, and the hyphæ of Fungi in the formation of Lichens, which had in the meantime become known (1881, L. N. 38). Brandt gave this unicellular yellow Alga the name _Zooxanthella nutricola_, and afterwards gave fuller details regarding its remarkable vital relations (L. N. 39). Patrick Geddes, who named it _Philozoon_, supplemented this account and showed experimentally that it gives off oxygen under the influence of sun-light (1882, L. N. 42, 43). In consequence {lix}of this there is no doubt that all Xanthellæ (the _Zooxanthella extracapsularis_ of SPUMELLARIA and NASSELLARIA, and the _Zooxanthella intracapsularis_ of the ACANTHARIA, and possibly also the _Zooxanthella phæodaris_ of the PHÆODARIA) do not originally belong to the Radiolarian organism, as was believed up to the time of Cienkowski, but penetrate actively into it from without, or are taken in passively by means of the pseudopodia. In any case their symbiosis, when they are associated with the Radiolarian cell in large numbers, may be of great advantage to both parties, since the metastasis of the Xanthella is vegetable, that of the Radiolarian animal in character. In any case their symbiosis is to a large extent accidental, by no means as necessary as in the case of the Lichens. See on these points in addition to Brandt and Geddes (_loc. cit._) also Geza Enz, Das Consortial-Verhältniss von Algen und Thieren, Biol. Centralbl., Bd. ii. No. 15, 1883, Oskar Hertwig, Die Symbiose oder das Genossenschaftsleben im Thierreich, Jena, 1883, and Bütschli, Die Radiolarien, in Bronn's Klass. u. Ord. d. Thierreichs, 1882 (L. N. 41, pp. 456-462).

91. _The Exoplasm or Extracapsular Protoplasm._--The extracapsular protoplasm, which may be shortly termed the "exoplasm" (or ectosarc), is primitively in all Radiolaria (and especially in their earliest development stages) the only important constituent of the extracapsulum, besides the calymma. Although the extracapsular and intracapsular protoplasm of the Radiolaria are everywhere in direct communication, and although the openings in the membrane of the central capsule bring about an interchange between them, still the two portions of sarcode show certain constant and characteristic differences, which are due to the physiological division of labour between the central and peripheral parts of the body and their corresponding morphological differentiation. The extracapsular, like the intracapsular, protoplasm is originally homogeneous, but may afterwards become differentiated in various ways, producing the special constituents of the extracapsulum. Such "external protoplasmic products" are vacuoles, pigment-bodies, &c. More important, however, are the topographically different sections into which the exoplasm may be divided according to its relations to the central capsule and the calymma. In this respect the following parts may be generally distinguished--(1) the _Sarcomatrix_, or fundamental layer of the exoplasm, which surrounds the central capsule as a continuous sheath of sarcode and separates it from the calymma; (2) the _Sarcoplegma_, an irregular network of the exoplasm, which spreads throughout the gelatinous material of the calymma; (3) the _Sarcodictyum_ or network of sarcode on the outer surface of the calymma; and (4) the _Pseudopodia_, which project outwards from the latter and radiate into the water.

92. _The Sarcomatrix._--The sarcomatrix, being "the fundamental layer of the pseudopodia" (or "matrix of the exoplasm"), constitutes the proximal innermost section of the extracapsular sarcode, and in all Radiolaria forms a thin continuous mucous layer, which covers the whole outer surface of the central capsule and separates it from the surrounding calymma (see note A, below). The sarcomatrix communicates internally {lx}through the openings of the central capsule with the endoplasm, whilst externally the pseudopodia or mucous threads arise from it, which by their union form the sarcoplegma. The sarcomatrix is only interrupted in the SPUMELLARIA and ACANTHARIA by those parts of the skeleton which perforate the membrane of the central capsule. In all NASSELLARIA and PHÆODARIA, as in the #Collodaria#, it appears as a perfectly continuous sarcode-envelope of the central capsule. Its thickness is variable; in general it is most strongly developed in the SPUMELLARIA and PHÆODARIA, less so in the NASSELLARIA, and is thinnest in the ACANTHARIA. The thickness seems, however, to vary even in one and the same individual, the difference depending partly upon the different stages of development and partly upon nutritional conditions. After abundant inception of nutriment the thin protoplasmic layer of the matrix is thickened and turbid, rich in granules and irregular masses, which are probably due to enclosed but only half-digested food; xanthellæ also, as well as foreign bodies taken up with the nutriment, such as frustules of Diatoms and shells of smaller Radiolaria, and of pelagic infusoria, larvæ, &c., are often, especially in large individuals, aggregated in considerable quantities in the matrix. After long fasting, on the contrary, this is poor in these enclosed bodies and in granules; it then forms a thin colourless more or less hyaline mucous coating to the central capsule. From a physiological standpoint the sarcomatrix is to be regarded as the _central organ of the extracapsulum_, and as of pre-eminent significance. Probably it is not only the most important organ for the nutrition of the Radiolaria (especially for digestion and assimilation in particular), but perhaps is also the central organ of perception. On the other hand the sarcomatrix belongs to those components of the Radiolarian organism which take no part in the formation of the skeleton.

A. The sarcomatrix was first described in my Monograph in 1862 (p. 110) as the "Mutterboden der Pseudopodien," possessing a pre-eminent physiological importance. Compare also my paper on the sarcode elements of the Rhizopoda (Zeitschr. f. wiss. Zool., Bd. xv. p. 342, 1865).

93. _The Sarcoplegma._--By the name sarcoplegma, as distinguished from the remaining extracapsular sarcode, is understood the intracalymmar web of exoplasm or "ectosarcode network," which ramifies within the gelatinous mass of the calymma. Internally it is in direct connection with the continuous sheath (sarcomatrix), which encloses the central capsule, whilst externally it is in contact with the superficial sarcode network (sarcodictyum) which surrounds the calymma. The configuration of this exoplasmic web, which penetrates the jelly-veil in all directions, is exceedingly variable; in most Radiolaria it is extremely irregular in form, like the protoplasmic network in the ground-substance of many kinds of connective tissue. In some groups, however, it assumes a rather regular shape which it appears to retain (_e.g._, in many ACANTHARIA). It must be assumed also that in those instances where the consistency {lxi}of the calymma approaches that of cartilage, the tracks of the exoplasmic threads remain constant, but accurate observations are wanting as to how far the configuration of the sarcoplegma is constant or variable in the different groups, as well as regarding its peculiar behaviour in those Radiolaria whose calymma is characterised by the formation of vacuoles or alveoles (see § 86). Usually it envelops the larger alveoles in the form of a reticulate veil. In many #Collodaria# the exoplasm is aggregated at certain points of the intracalymmar web, so that large balls or amoeboid bodies appear to be distributed between the alveoles, _e.g._, in _Thalassophysa pelagica_ and _Thalassicolla melacapsa_ (Pl. 1, figs. 4, 5). The sarcoplegma is metamorphosed directly into silex in the Radiolaria spongiosa, or those genera which possess a spongy cortical skeleton, and were formerly known as Spongurida; to this category belong the Spongosphærida (Pl. 18) and Spongodiscida (Pl. 47) as well as certain NASSELLARIA and PHÆODARIA. The single siliceous spicules, which are irregularly interwoven to form the spongy web, are to be regarded as the silicified threads of the intracalymmar sarcode network. From a physiological point of view the sarcoplegma is of importance both for the nutrition and motion of the Radiolaria, since it brings the sarcomatrix and the sarcodictyum, with the pseudopodia which radiate from it, into direct communication.

94. _The Sarcodictyum._--The sarcodictyum may be defined as the extracalymmar network of exoplasm, and is a reticular covering which lies upon the outer surface of the gelatinous calymma. Internally, the sarcodictyum is in direct communication with the sarcoplegma, or the web of exoplasmic threads which ramifies in the gelatinous substance of the calymma; externally, on the other hand, the pseudopodia radiate freely from it; thus its relation to these is similar to that which the sarcomatrix bears to the roots of the sarcoplegma. Relations similar to those which have led to the separation of the primary from the secondary calymma, induce us to distinguish also a primary and secondary sarcodictyum. The original or _primary sarcodictyum_ ramifies over the surface of the original or primary calymma, and like this is of pre-eminent importance in the formation of the primary lattice-shell; if we regard the surface of the primary calymma as the indispensable foundation for the deposition of this latter, then the primary sarcodictyum furnishes the material from which it is developed: silex in the SPUMELLARIA and NASSELLARIA, a silicate of carbon in the PHÆODARIA, and acanthin in the ACANTHARIA. It may indeed be said that the primary lattice-shell of the Radiolaria arises by a direct chemical metamorphosis of the primary sarcodictyum, by a chemical precipitation of the dissolved skeletal material (silex, silicate, or acanthin), which was stored up in the exoplasm of the sarcodictyum. Hence a deduction from the special conformation of the former to that of the latter is permissible. The particular form of the primary lattice-sphere with its regular or irregular meshes is due to the corresponding form of the primary sarcodictyum; both regular and irregular forms of this {lxii}commonly occurring. The form of the _regular sarcodictyum_ with circular or regular polygonal, usually hexagonal, meshes is constantly maintained during the formation of the regular lattice-shells (_e.g._, Pl. 12, figs. 5-10; Pl. 52, figs. 8-20; Pl. 96, figs. 2-6; Pl. 113, figs. 1-6). The form of the _irregular sarcodictyum_, on the other hand, with irregular polygonal or roundish meshes, persists during the development of the irregular lattice-shells (_e.g._, Pls. 29, 70, 97, 106). All this is true also of the _secondary sarcodictyum_, or the exoplasmic network which ramifies over the surface of the secondary calymma. The secondary lattice-shells, which are deposited on the surface of the latter, retain the configuration of the secondary sarcodictyum, by the chemical metamorphosis of which they have originated; this is the case in many SPUMELLARIA which develop several concentric lattice-shells (Pl. 29), in some NASSELLARIA (Pl. 54, fig. 5), in the Phractopeltida among the ACANTHARIA (Pl. 133), and in the double-shelled PHÆODARIA, Cannosphærida, and part of the Coelodendrida and Coelographida (Pls. 112, 121, 128). In those Radiolaria which form no lattice-shell whatever, the conformation of the sarcodictyum is usually irregular, with meshes of irregular form and unequal size; sometimes, however, they seem to be very regular, as in many #Acanthometra# (Pl. 129, fig. 4).

95. _The Pseudopodia._--On the whole the pseudopodia or thread-like processes of the exoplasm exhibit in the Radiolaria the same characteristic peculiarities as in all true Rhizopoda; they are usually very numerous, long and thin, flexible and sensitive filaments of sarcode, which show the peculiar phenomena of granular movement. Their physiological significance is in several respects very great, for they serve as active organs for the inception of nutriment, for locomotion, sensation, and the formation of the skeleton (see note A, below). The presence of a calymma, however, which distinguishes the Radiolaria from the other Rhizopoda, brings about certain modifications in the behaviour of the pseudopodia. If in general all the threads, which arise from the sarcomatrix or fundamental layer and radiate outwards, be called "pseudopodia," then that part of them which is included in the gelatinous substance of the calymma and forms the sarcoplegma may be termed the "collopodia" (or intracalymmar pseudopodia), and the remaining portion, which passes outwards from the sarcodictyum freely into the water, may be described as "astropodia" (or extracalymmar pseudopodia). In many Radiolaria these two portions present some differences in morphological and physiological respects, and certain distinctions are probably generally present (see note B). Apart from this universal differentiation in the different groups of the Radiolaria, specially modified forms of pseudopodia may be recognised as the axopodia and myxopodia of the ACANTHARIA (see § 95, A), and the sarcode-flagellum of certain SPUMELLARIA (see note C).

A. The pseudopodia of the Radiolaria have been so fully described in my Monograph, in 1862, both morphologically and physiologically, that I need only refer to the account there given {lxiii}(pp. 89-127); for supplementary observations see R. Hertwig (1879, L. N. 33, p. 117) and Bütschli (1882, L. N. 41, pp. 437-445).

B. The _Astropodia_, or free radiating pseudopodia, are in many Radiolaria more or less clearly distinguishable from the collopodia, which form the sarcoplegma within the calymma; how far these distinctions depend upon a permanent differentiation (especially in the ACANTHARIA and PHÆODARIA) needs further investigation.

C. The _sarcode-flagellum_ (perhaps better termed _axoflagellum_) was first described in my Monograph (1862, p. 115) in the case of various #Discoidea# (Taf. xxviii. figs. 5, 8; Taf. xxx. fig. 1). Hertwig has given a substantially similar account of the organ in some other #Discoidea# (L. N. 33, p. 67, Taf. vi. figs. 10, 11); probably this peculiar structure is confined to the order #Discoidea# among the SPUMELLARIA, but is widely distributed within its limits. The axoflagellum is a thick cylindrical thread of sarcode, finely striated and pointed towards its free end. It always lies in the equatorial plane of the discoidal body, and always unpaired in one of its axes; in the triradiate #Discoidea# it is in the axis of the unpaired principal arm and opposite to it (Pl. 43, fig. 15). In the Ommatodiscida (p. 500, Pl. 48, figs. 8, 19, 20) the axoflagellum probably passes out through the peculiar marginal ostium of the shell. Perhaps it is always connected with the central nucleus by intracapsular axial fibres, and is to be regarded as a specially differentiated bundle of pseudopodia (or axopodia?).

95A. _The Myxopodia and Axopodia._--The two forms of pseudopodia which we distinguish as myxopodia and axopodia differ markedly from each other both morphologically and physiologically. The _myxopodia_, or ordinary free pseudopodia, which are found in large numbers in all Radiolaria, and constitute their most important peripheral organs, are simple homogeneous exoplasmic threads, which arise from the sarcodictyum or extracalymmar sarcode network, and radiate freely into the water; here they may branch and combine by anastomosis to form a changeable network, but they never contain an axial thread. The _axopodia_, on the other hand, are differentiated pseudopodia, which consist of a firm radial thread, and a soft covering of exoplasm; they penetrate the whole calymma in a radial direction and project freely from its surface, and generally (if not always) they are produced inwards to the middle of the central capsule, perforating its membrane; their proximal end is lost in a dark central heap of granules. Such axopodia are at present known with certainty only in the ACANTHARIA, where they are widely, and perhaps universally, distributed. Their development in this legion probably stands in direct causal relation to the peculiar structure of the central capsule and the centrogenous formation of the skeleton. Since the radial skeletal rods of the #Acanthometra# possess originally a thin coating of protoplasm, it may be said that the centrogenous axopodia of this group became differentiated in two ways, the firm axial threads of one section remaining very thin and covered by protoplasm, whilst those of the other section became metamorphosed into radial bars of acanthin. This hypothesis acquires more probability from the regular distribution and arrangement of the axopodia in the ACANTHARIA; they usually stand at fixed intervals {lxiv}between the radial bars, singly or in groups; sometimes their number seems to be not greater than that of the bars, whilst in other cases a circlet or group of axopodia corresponds to each radial bar. Perhaps their fine axial thread consists of acanthin. At all events the axopodia are constant organs (probably sensory, like the "palpocils") and not retractile like the movable myxopodia.

The axial threads in the pseudopodia of the #Acanthometra# were first discovered by R. Hertwig, who accurately described their peculiar structure and arrangement (L. N. 33, pp. 16, 117).

96. _The Myophriscs of the Acanthometra._--The #Acanthometra# are characterised by a very peculiar differentiation of the exoplasm, namely, by the formation of myophriscs or contractile threads from the sarcodictyum. In most (and perhaps in all) ACANTHARIA of this order each radial bar is surrounded by a circlet of such contractile threads, which was first described as a "ciliary corona" (see note A, below). The number of contractile threads in each circlet usually amounts to from ten to twenty, rarely being more than thirty and less than eight; it often appears to be constant in the individual species (see note B). In the living state the myophriscs are long, thin filaments, the pointed distal end of which is inserted into the radial bar, whilst the thicker proximal end is attached to the surface of the calymma, which is elevated round the base of each rod into the form of a gelatinous cone or skeletal sheath (see note C). Probably the myophriscs lie on the outer surface of the apical portion of this gelatinous cone, and are hence to be regarded as exoplasmic threads differentiated from the sarcodictyum. Sometimes, however (as in _Acanthochiasma_), they fuse into a contractile membrane and form the envelope of a cone, whose interior is occupied by a gelatinous papilla of the calymma. On mechanical irritation the myophriscs contract rapidly and suddenly, like muscle-fibrillæ, becoming at the same time thicker, and hence are very different from pseudopodia. Their distal point of insertion being fixed to the firm acanthin rod, they raise by their contraction the skeletal sheath, to which their bases are attached or in the surface of which they lie. The result of their contraction is therefore a distention and increase in volume of the calymma, with which is no doubt connected an inception of water into the gelatinous mass, and hence a diminution in its specific gravity. Probably the #Acanthometra# contract their myophriscs voluntarily when they wish to rise in the water; when these relax the calymma collapses owing to its elasticity, water is then expelled and the specific gravity increases. From a physiological point of view, then, the myophriscs are to be regarded as a hydrostatic apparatus, morphologically as myophanes or muscular fibrillæ, such as also occur in the intracapsular protoplasm (see §§ 77-80). On more violent irritation and after the death of the #Acanthometra# the myophriscs separate from the radial bars and remain attached to the distal ends of the conical gelatinous sheaths as free "ciliary coronas." At the same time, {lxv}they melt into short, thick, hyaline rods, the so-called "gelatinous cilia." The myophriscs are found only in the order #Acanthometra#, and are wanting in the #Acanthophracta#, as well as in the other three legions of Radiolaria.

A. The "_ciliary coronas_" on the skeletal rods of dead #Acanthometra# were first described by the discoverer of this order, Johannes Müller, and referred to as "the stumps of the contracted, thickened threads" (L. N. 12, p. 11, Taf. xi.).

B. The "_number of the gelatinous cilia_" I found constant in certain species of #Acanthometra#, and stated in my Monograph (L. N. 16, p. 115) "that here is to be found the first differentiation of the diffuse sarcode into definite organs of regular definite number, size, and position, which deserve the name tentacles rather than pseudopodia."

C. The nature of the myophriscs as fibrillæ allied to muscles was first discovered by R. Hertwig, who described them as "structures of peculiar nature," under the name of "contractile threads," and pointed out in detail their histological and physiological peculiarities (L. N. 33, pp. 16-19, Taf. i.).

97. _The Exoplasm of the Peripylea._--The extracapsular protoplasm of the SPUMELLARIA or PERIPYLEA is in communication with the intracapsular sarcode by the innumerable fine pores of the capsule-membrane, and like these pores is evenly distributed over the whole surface. The sarcomatrix which immediately surrounds the central capsule is moderately strong, and sends out innumerable long, thin pseudopodia, which probably correspond to the pores of the membrane. Their number is markedly greater in the SPUMELLARIA than in the other three legions. The ramifications and communications which the radiating fibres of the sarcomatrix undergo within the calymma, apparently present the most manifold variations, so that the sarcoplegma or intracalymmar network thus formed has very diverse forms. On the surface of the calymma the exoplasmic threads constitute a variously disposed sarcodictyum, a regular or irregular exoplasmic network, by the silicification of which a primary lattice-shell arises in the majority of the SPUMELLARIA. The free ends of the pseudopodia, which arise from this extracalymmar network and radiate out into the water, appear in most SPUMELLARIA to be relatively short, but exceedingly numerous. Specially modified pseudopodia and axial threads in particular do not seem to occur in this legion. Perhaps, however, among the latter may be reckoned the remarkable pseudopodia which combine to form the sarcode flagellum in many #Discoidea# (and perhaps in other SPUMELLARIA). This axoflagellum is a particularly strong thread of sarcode, arising from a definite point in the central capsule; it is cylindrical or slenderly conical in form, much longer, stronger, and more contractile than the ordinary pseudopodia; it contracts in a serpentine fashion on mechanical irritation and seems to originate by the fusion of a bundle of pseudopodia (compare § 95, C).

98. _The Exoplasm of the Actipylea._--The extracapsular protoplasm of the ACANTHARIA or ACTIPYLEA differs in several important respects from that of other {lxvi}Radiolaria, and appears to undergo more significant differentiations than that of the three other legions. Since the pores in the wall of the central capsule are not distributed evenly and at equal intervals over its whole surface (as in the PERIPYLEA), but rather exhibit a regular disposition in groups at unequal intervals, the number of projecting pseudopodia is much less and the law of their arrangement different from that which obtains in the PERIPYLEA (§ 58). In many and probably in all ACANTHARIA they are divided into two groups, those which arise from the centre of the capsule and possess firm axial threads, and those which have not these characters (compare § 95, A). The axopodia, or stiff pseudopodia with axial threads, arise from the centre of the capsule, are present in much smaller numbers than the soft and flexible myxopodia, and are regularly disposed between the radial bars of acanthin, usually so that they are as far removed from them as possible, _i.e._, in the centre between each three or four bars; these latter may indeed be regarded as strongly developed axial threads, which have become changed into acanthin (§ 95, A). The soft myxopodia, or pseudopodia without axial threads, are much more numerous than the others, and arise from the sarcodictyum or exoplasmic network which ramifies over the surface of the calymma. Their number and arrangement seem, however, in many (if not in all) ACANTHARIA to be regular and not to possess the extraordinary variability seen in the other three legions. In many #Acanthometra# the sarcodictyum exhibits a symmetrical conformation, with regular or subregular, polygonal (mostly hexagonal) meshes, and generally the stronger threads of the sarcodictyum secrete a firm, homogeneous or fibrillar, striated substance, which forms a network of ridges on the surface of the calymma. In the #Acanthophracta# the place of this is taken by the acanthin network of the primary lattice-shell. The axopodia of the #Acanthometra# are usually about as long as the radial spines between which they stand; their stiff axial thread is surrounded by a soft sheath of protoplasm, communicating with the thin sarcomatrix which surrounds the central capsule. Numerous branches pass into the calymma from the exoplasmic sheath of the axial threads, and form by their interweaving a loose sarcoplegma. The most peculiar differentiated products of the exoplasm of the ACANTHARIA, however, are the myophane fibrillæ of the #Acanthometra#, which have already been described under the name of myophriscs (§ 96).

99. _The Exoplasm of the Monopylea._--The extracapsular protoplasm of the NASSELLARIA or MONOPYLEA arises only from the porochora, or the intracapsular podoconus, the oral base of which is formed by this porous area. The pseudopodia or protoplasmic threads which pass through the pores of the latter, united into a bundle, are not very numerous (in most NASSELLARIA probably between thirty and ninety), and unite just outside it to form a thick discoid sarcomatrix; this covers the porochora completely below, and spreads out in the form of a thin envelope of exoplasm over the whole {lxvii}surface of the central capsule; at the apical portion of the latter the sarcomatrix is often so thin that it can only be recognised by the aid of reagents; it separates the membrane of the central capsule from the surrounding calymma. The pseudopodia, which penetrate the latter and by loose anastomoses from a wide-meshed sarcoplegma within it, are usually not very numerous. The greater part of them radiate in a bunch downwards from the basal disc of the sarcomatrix, and a smaller number arise from the thinner envelope which covers the remainder of the central capsule (Pl. 51, fig. 13; Pl. 65, fig. 1; Pl. 81, fig. 16). On the outer surface of the calymma the collopodia, which have passed through it, unite to form the sarcodictyum, and through the silicification of this the primary lattice-shell arises in the great majority of the NASSELLARIA. From the surface of the sarcodictyum arise the astropodia, or free pseudopodia which radiate outwards into the water. Their number in most MONOPYLEA is relatively small, but their length appears to be very great.

100. _The Exoplasm of the Cannopylea._--The extracapsular protoplasm of the PHÆODARIA or CANNOPYLEA is much better developed as regards volume than in the other three legions, and is connected with the intracapsular sarcode by only a few apertures in the capsule-membrane. In most PHÆODARIA three of these are present, the astropyle or main-opening at the oral pole of the main axis, and the two lateral parapylæ or accessory openings on either side of the aboral pole (§ 60). In several families the latter appear to be wanting, whilst in others their number is increased; these families have not yet, however, been observed during life. The protoplasm projects both from the oral main-opening and from the two aboral accessory openings in the form of a thick cylindrical rod; the tube into which each opening is produced in many PHÆODARIA (longer in the case of the astropyle, shorter in the parapylæ) being regarded as an excretion from this protoplasmic cylinder. The sarcode threads within the tube appear like a bundle of fibrils, either quite hyaline or finely striated. After issuing from the mouth of the aperture they pass over into a thick sarcomatrix, which surrounds the central capsule entirely and separates it from the enclosing calymma. In the neighbourhood of the basal astropyle the sarcomatrix is usually swollen into a thick lenticular disc, which is in direct contact with the peculiar phæodium of this legion (§ 89). The pseudopodia, which radiate from the sarcomatrix, and form by anastomosis a wide-meshed sarcoplegma within the calymma, are usually not very numerous in the PHÆODARIA, but are very strong. Sometimes two stronger bundles of collopodia may be distinguished at the two poles of the main axis, an oral bundle (in the direction of the proboscis of the astropyle) and an aboral bundle (at the opposite pole between the parapylæ). The collopodia of the sarcoplegma unite at the surface of the calymma into a regular or irregular sarcodictyum, which, in most PHÆODARIA produces by the secretion of a peculiar silicate the primary lattice-shell. {lxviii}The free astropodia, which pass outwards from the sarcodictyum into the water, are in most PHÆODARIA very numerous (Pl. 101, fig. 10). Since, however, only a few species of this great legion have been observed in a living state, their pseudopodia require further accurate examination.