CHAPTER II.

The Sub-kingdom Protozoa.

The consideration of the whole special group of organisms forming the subject matter of this chapter, under the heading of Protozoa, were formerly included among Infusoria, which also embraced every kind of microscopical aquatic body, whether belonging to the vegetable or animal series. A more critical survey of the organisation and affinities of Infusoria and the members which constituted the group led to a re-arrangement, which has been very generally accepted as forming a sub-kingdom, Protozoa. This may be defined as embracing all those forms of life, referable to the lowest grade of the animal kingdom, whose members for the most part are represented by organisms possessing a single cell or aggregation of cells (and also included under the general term of unicellular organisms) the whole of which are engaged in feeding, moving, respiring, and reproducing by segmentation or fission much in the same way as that of the unicellular plants described in a previous chapter. Following out this sub-division of the entire series of Protozoa, the several groups range themselves into four readily distinguishable sections. In the first, the most lowly organised and most abundant have no oral orifice in the literal meaning of the word, food being intercepted at any point of the surface of the body. This most simple elementary type of structure of the Protozoa is represented in the Amœba and Actinophrys, the various representatives of the Foraminifera, and certain Flagellata, as Spumella and Anthrophysa. Next in the ascending scale is a group of Protozoa, in which, though differentiation has not proceeded so far as to arrive at the constitution of a distinct oral aperture, the inception of food substance is limited to a discoidal area occupying the anterior extremity of the body and is associated with the special food-arresting apparatus. To this section of the Protozoa are relegated the minuter flagellate, “collar-bearing” animals, and also the entire group of sponges or Porifera.

In the third section the highest degree of organisation is arrived at. Here is represented a single, simple, often highly-differentiated oral aperture or true mouth. Associated with this section are found the majority of those organisms that collectively constitute the class Infusoria in the proper acceptation of the term, and it embraces the majority of the Ciliata, the Cilio-flagellata, as Euglena, Chilomonas, &c., in which the presence of a distinct and circumscribed oral aperture is clearly seen. With the fourth and remaining section of Protozoa, the oral or inceptive apparatus exhibits a highly characteristic structural modification. This is not restricted to a definite area, nor is it associated with the entire surface of the body, but it consists of a number of flexible, retractile, tentacle-like organs radiating from diverse and definite regions of the periphery, each of which subserves as a tubular sucking-mouth, or for the purpose of grasping food. These may be literally described as many-mouthed, and have been appropriately designated Polystomata. The true zoological position of the Spongida or Porifera is not finally settled, the members of this important section having been formerly regarded as a subordinate group of the Rhizopoda or an independent class of the Protozoa; consequently a tendency has been shown to assign to them a position more nearly approximating to that of the Cœlenterata, or zoophytes and corals, or place them among the more highly organised tissue-constructed animals, the Metazoa, these being characterised by groups of cells set apart to perform certain functions for the whole animal. A division of labour is seen to be marked in these lower animals as the organism becomes more specialised, and the number of functions a cell performs becomes more and more limited as the body becomes more complex.

It has been found convenient to adopt the following definition of the Infusoria as one more generally acceptable. The Protozoa in their adult condition are furnished with prehensile or locomotive organs, that take the form of cilia, flagella, or of adhesive or suctorial tentacula, but not of simple pseudopodia; their zooids are essentially unicellular, free swimming or sedentary; they are either naked, loricate, or inhabit a simple, mucilaginous matrix; single or united in aggregations, in which the individual units are distinctly recognisable; not united and forming a single gelatinous plasmodium, as in Mycetozoa, nor immersed within and lining the interior cavities of a complex protoplasmic and mostly spiculiferous skeleton, as in the Spongida, their food substances being intercepted by a single distinct oral aperture, or by several apertures through a limited terminal region or through the entire area of the general surface of the body. They increase by simple longitudinal or transverse fission, by external or internal gemmation or division, preceded mostly by a quiescent or encysted state, into a greater or less number of sporular bodies. Sexual elements, as represented by true ova or spermatozoa, are entirely absent, but two or more zooids frequently coalesce as an antecedent process to the phenomena of open formation.[63]

The infusorial body in its simplest type of development, as in Amœba, exhibits a structural composition substantially corresponding with that of the lowest organised tissue cell. There is no distinct bounding membrane, or cell-wall, and it is throughout, and apart from the nucleus or endopart, one continuous mass of granular matter, but otherwise homogeneous and undifferentiated protoplasm. Professor Greef, who has made a study of the Amœba, describes motor fibrils in the exoplasm which are active and large in _A. terricola_. These are readily seen by staining with osmic acid, and, after washing this out with water, immersing in a weak alcoholic solution. In Amœba so prepared and examined with a high power, the whole body will be seen to be surrounded by a distinct double integumentary layer. Highly refractive bodies may also be seen in the interior, connected together by extremely fine filaments. Professor Greef concludes that here we have to do with muscular fibrillæ, which traverse the contractile outer zone in a radial direction and there terminate for the time being. By a similar method, axial filaments can be demonstrated in Heliozoa; these, it is believed, are the true motors of their pseudopodia, and also the axial structures of the Acineta, a marine animal related to ciliate infusoria.

In the Amœba, at one time well known as the _Proteus animalcule_, Fig. 325, the marvellous body creeps onward in a flowing manner, occasionally and languidly emitting a single pseudopod first on one side, then on the other. More commonly it puts on a dendroid or palmate form; then again it assumes more or less grotesque shapes in which almost any conceivable image may be imagined. The body, as will be seen in this highly-magnified figure, is full of granules (with the exception of a thin clear outer hyaline zone), and near the centre is a globular or discoid body known as the nucleus, composed of slightly denser material than that which surrounds it. The division of the body into two is preceded by a division of this nucleus. Near the latter is a clear spherical space--the contractile vacuole--which gradually expands, and then rather suddenly collapses and reappears at the same spot, the systole and diastole being slow and continuous. The contractile vacuole contains a clear liquid which is expelled on the collapse of the vacuole. This organ probably serves the double function of respiration and excretion. The Amœba is omnivorous, chiefly a vegetarian, and, therefore, found on the ooze of ponds or on the under surface of the leaves of aquatic plants, especially among Confervæ. It can be readily produced by placing a few fibres of fresh meat in an infusion of hay.

The Gregarinæ consist of a remarkable group of organisms, but these, although unicellular, are, for the most part, confined to the intestinal tract of worms and of the higher animals, and will therefore be described among internal parasites.

Tho fungus-animals, Mycetozoa, have already been referred to in a previous chapter. The best known species, however, is found in tan yards in the form of creeping masses of naked protoplasm, termed Plasmodia. Cakes of protoplasm become segregated from the main mass, and break up into Amœba-like spores, which unite again to form Plasmodia.

The Rhizopoda, or root-footed class of animals, are among the most interesting simple organisms with which the microscope has made us acquainted. In the living state they have the power of protruding pseudopodia from the body, by which they creep about, or cling to plants when in search of food. This group, in fact, includes Amœba, Foraminifera, Sun-animalcules, and Radiolarians. In the first the pseudopodia are simple and lobose; in the second they are slender, confluent and reticulate; while in the two last they are simple, radiating and somewhat stiff, and partake of a calcareous formation.

Of the Lobosa, we may take a well-known representative of the group, the Protomyxa, found at the bottom of fresh-water pools, especially those near bog-moss, where its minute orange-coloured particles of jelly-like substance are seen creeping over stones or shells. If quietly watched the pseudopodia, some of which are broad and others slender, become quiescent spheres, which break up into numerous portions, each of which becomes a new animal.

This group is divided into the shell-less (Nuda) and shell-formed (Testacea). The brown, horny covering is often finely faceted, and is either shaped like a dome, semi-circular, or flat as a box, through which they protrude their few or many pseudopodia (seen in Fig. 326).

In the Difflugia the lorica or shell is strengthened by the addition of silicious particles; in Euglypta it is sac-shaped, with a jagged free margin, the surface being covered by overlapping scales; while Arcella are capable of secreting vesicles of air in their interior, whereby they are enabled to rise to the surface. On some parts of our coast, if the sea sand be carefully looked over with a pocket lens, there will often be found minute grains of a porcelain oval kind, belonging to the Miliolina, segmented or strung together not quite in the same plane.

The Foraminifera are rhizopods, whose simple protoplasmic bodies send forth, through perforations in the membrane or outer covering of calcium carbonate and silica, branching rays of pseudopodia. The order is divided into two groups, the Imperforata and the Perforata; in the former the shell or harder structure possesses only one or more apertures, whereas in the latter, in addition to the main opening, the shell has its walls perforated throughout, which admits of minute pseudopodia or fine threads being protruded (Fig. 328). (See also Plate III., Nos. 75-85.) The vast majority of Perforata form their shells, or rather skeletons, of calcium carbonate and silica, which renders them almost indestructible. Consequently the form is preserved through ages, and they present objects of the greatest interest to the microscopist.

A curious and interesting feature of the Foraminifera--often an element of difficulty to the student--is the tendency of modifications of types comprising the larger groups to run into parallel isomorphous series. Thus, if the entire class be roughly divided, as it sometimes has been, into three orders, comprising respectively the forms characterised by porcellaneous, arenaceous, and hyaline “tests,” the same general conformation and arrangement of chambers will be found in each of the three series. The most remarkable example, even among the smaller groups, is the Rotaliidæ, of which three or four genera may be arranged in parallel lines, and in more or less closely isomorphous series. In the report appended to the “Challenger” scheme of classification many examples are enumerated. In Arenacea we have a small family of Foraminifera, the external surfaces of which present a ridge and furrow arrangement, and the incrustations are entirely of a sandy nature held together by a cement secreted by the animal. (Plate XV., No. 1, _Astrorhiza limicola_.)

_Gromia._--Among the more remarkable of the Perforata group the Gromia have a foremost place. They are very minute globular or oval-shaped bodies, about one-twenty-fourth of an inch in length, found in fresh, brackish, and salt water. The forms brought up in Dr. Wallich’s deep sea soundings of 1860 were taken attached to pieces of corallines, or found loose among Globigerina ooze. At first there appears to be nothing peculiar about these tiny specks of matter resembling the ova of a zoophyte, but presently, at the smaller end, a very fine thread is protruded, and then another, dividing into finer branches, and, ultimately, a complete network of filaments extends on all sides, and become attached to the side of the glass jar that contains them. Now, on employing magnifying power, every thread exhibits a circulatory motion, an up and down stream or cyclosis of granules suspended in a fluid mass. It is by means of these pseudopodia, as the threads are termed, that the Gromia moves its body along and clings to the glass. We may surmise, then, that these pseudopodia are either gelatinous, glutinous, or terminate in sucker-like processes. Increase in the “test,” integument, is brought about, as in Difflugia, by the secretion of calcareous matter or by cementing fine silicious particles to the outer wall, as the protoplasm is seen to flow over the test, so that when it comes in contact with a diatom it is thereby drawn towards the oral opening and slowly digested.

Some considerable time elapsed between the discovery of Gromia by Mr. W. Archer, F.R.S., and the demonstration of a nucleus and contractile vesicle by Dr. Wallich. It was thought that in the whole of the Monozoa the nucleus was absent, but it is now known that this important body is embedded in the protoplasmic substance, and the reproduction of these curious animals is thereby secured. Among the better known species of Gromia is _G. Dujardinii_, chiefly distinguishable by the darker colour of the “test,” by the greater quantity of silica that enters into the formation of its pseudopodia, and by the formation of isogamous zoospores, two of which are seen in conjugation in Plate XV., No. 2. An excess of protoplasm must also be secreted to admit of so large a protrusion outside the testa.

_G. Lieberkühnia_ (of Claparède and Lachman), No. 5, differs in formation. Its shape is pyriform, and the opening whence the pseudopodia streams out is situated in a lateral depression about midway in the testa, _c, o_. Hence a trunk branch is seen to issue forth, and from this a ramification of threads, _psdp_, extends to a considerable distance in all directions.

The Micro-gromia of Hertwig, No. 4, is the minutest form of the genus yet discovered, and differs from those already described in the mode of reproduction. The individual takes the shape of a water bottle with a short neck, whence issue forth a limited number of very slender threads. The test is quite transparent, and it was in this species that the nucleus and contractile vesicle, which lie embedded near the mouth, were first clearly made out.

The zoospores of Micro-gromia have a curious habit of uniting with their neighbours to form a colony, No. 4. Their colonisation is apparently intended to facilitate multiplication. Reproduction is carried on somewhat after the manner of Volvox. The globular bodies formed sink to the bottom of the glass vessel, and there remain for a time in a quiescent state. In the course of a day or two the mass assumes a motive appearance, increases in bulk, becomes more ovoid in shape, and ultimately the nucleus shows the first sign of division. Vertical segmentation takes place, as at A, into two equal parts; each half is seen to possess its fair share of the nucleus and contractile vesicle. It then turns in the horizontal direction, and now there appears to be an upper and a lower division, the uppermost having a neck-like attachment, and this is making its way to the narrow oral opening in the parent testa, as at B. Here it is seen pressing forward, and at C the neck is protruding some distance, and the second half assumes a bottle shape; at D the greater part of the animal is nearly set free, and after a short rest it fully launches forth. It finally pulls itself together, as at E, and either develops a pair of flagella and swims off, or assumes the form of an Actinophrys. In either case, and in a very short space of time, the separated young animal is quite ready to re-unite, as at F, and assist in forming a new colony of the species.

The Polymorphina belong to a low genus of the Foraminifera. They consist of a number of forms and exhibit a rather extensive series of variations, although consisting of a few simple types, and showing transitions between forms which at first seem to be distinct. The majority of species keep to the sea bottom; some few are pelagic, and occur in abundance on the surface of the ocean. Among the latter are the Globigerina: its shell is about one-fortieth of an inch in diameter, and usually composed of seven globular chambers arranged spirally in such a manner that all are visible from above, each chamber opening by a crescentic-shaped orifice into a depression in the middle of the next. Perfect specimens bristle with long slender spines, the pores affording passage to pseudopodia, which stream out along the spines. The more carefully-conducted deep-sea investigations have brought to light the fact that the floor of the ocean, at great depths, and over a vast area, is formed of these white or pinkish coloured bodies, all containing on an average about 60 per cent. of calcium carbonate. It is a question whether the Globigerinidæ which make up the bulk of the ooze actually live at the bottom as well as the surface of the sea. This question has given rise to much discussion. Dr. Murray came to the conclusion that pelagic species do not live near the ocean floor. This opinion is partly based on the fact that the area of the Globigerina ooze coincides with the area of surface of temperature at which these bodies are found to exist. When the surface water is too cold for them, they are not to be found, neither are they found below. Major S. R. J. Owen, while dredging the surface of mid-ocean--the Indian, and the warmer portion of the Atlantic--found attached to his nets a number of these interesting bodies, and which always made their appearance just about sunset. In Plate III., Nos. 43-52, a number of these interesting and variously-formed bodies are given, and an attempt is also made to show the richly-tinted colour appearances presented by the sarcode or protoplasm of the Globigerina.

“Many of the forms,” writes Major Owen,[64] “have hitherto been claimed by the geologist, but I have found them enjoying life in this their true home, the silicious shells filled with coloured sarcode, and sometimes this sarcode in a state of distension somewhat similar to that found projecting from the Foraminifera, but not in such slender threads. There are no objects in nature more brilliant in their colouring or more exquisitely delicate in their forms and structure. Some are of but one colour, crimson, yellow, or blue; sometimes two colours are found on the same individual, but always separate, and rarely if ever mixed to form green or purple. In a globular species, whose shell is made up of the most delicate fretwork, the brilliant colours of the sarcode shine through the little perforations very prettily. In specimens of the triangular and square forms (Plate III., Nos. 43, 44, 45 and 46), the respective tints of yellow and crimson are vivid and delicately shaded; in one the pink lines are concentric; while another is of a stellate form, the points and uncoloured parts being bright clear crystal, while a beautiful crimson ring surrounds the central portion. A globular form resembles a specimen of the Chinese ball-cutting--one sphere within another; this, however, appears to belong to a distinct species.

“The shells of some of the globular forms of these Polycystina, whose conjugation I believe I have witnessed, are composed of a fine fretwork, with one or more large circular holes; and I suspect the junction to take place by the union of two such apertures. That the figures of these shells become elongated, lose their globular form after death, and present a disturbed surface is seen in some of the figures represented in Plate III., Nos. 82-85.” Those without internal chambers have been described as _Orbulina universa_, Plate III., Fig. 78, while Nos. 75 and 76, although members of the same family, have been separated, but all should certainly be united under Globigerina.

“The minute silicious shells of Polycystina present wonderful beauty and variety of form; all are more or less perforated, and often prolonged into spines or other projections, through which the sarcode body extends itself into pseudopodial prolongations resembling those of Actinophrys. When seen disporting themselves in all their living splendour, their brilliancy of colouring renders them objects of unusual attraction. It will appear that they wish to avoid the light, as they are rarely found on the surface of the sea in the daytime; it is after sunset and during the twilight that they make their appearance.”

Many forms of Globigerina and Foraminifera are represented in Figs. 329 and 330. These varied and beautiful forms were dredged up with soundings made in 1856 for the purpose of ascertaining the depth of the Atlantic, prior to the laying down of the electric telegraph wire from England to America, and taken at a depth of 2,070 fathoms.

_Heliozoa._--_Actinophrys-Sol_, “sun-animalcules,” belong to this group; most of them inhabit fresh water (Plate III., No. 66). The chief characteristic, and the one to which they owe their name, is the possession of long, slender, somewhat stiff pseudopodia; these radiate from all parts of the body. The living animal usually contains green-coloured particles within a minute translucent spherical globule of about 1/250th of an inch in diameter. It is, therefore, variously designated the green sun-animalcule, Acanthocystis, or Actinophrys-Sol. It is commonly found amongst the weeds in clear pools of water, where desmids abound. The pseudopodia appear to be stiff; they are, however, quite flexible, and the body contains more than one clear vesicle with a nucleus; reproduction is secured by the simple division commencing in the nucleus. The little animal can move over a hard surface by the alternate relaxation and stiffening of its pseudopodia; when one of these touches a small organism, it is believed to paralyse it, then envelop, and deliberately digest it. In another species, the lattice-animalcule (Cathrulina), the pseudopodia or silicious threads are arranged tangentially. It grows on a long flexible stalk, attached to an aquatic plant, the total length of which is about 1/200th an inch. The globular body is perforated in all directions, through which the fine stiff pseudopodia are thrust out; it is often known to form colonies.

In this order may well be placed the Radiolaria; they are, however, usually separated. But Radiolarians, whether seen alive or in their skeleton form, are surpassingly beautiful. By the favour of Messrs. Warne, I am enabled to append a frontispiece plate to this volume taken from their “Royal Natural History.” These bodies are all marine, and live in zones of several thousand fathoms, and like their congeners, the Globigerina, they avoid a strong light, and only appear after sunset. Their bodies are supposed to emit a phosphorescent glow, but more is known of their silicious skeletons than of their living forms; yet it is not this feature that separates them from other orders of rhizopods, but the possession of a membranous central capsule enclosing the nucleus. The body substance outside this capsule is highly vacuolated in some species, especially in surface forms. A few are without a skeleton, and these consist of oval masses of protoplasm, with slender pseudopodia. In a few species the skeleton is formed of a glassy horny substance, termed acanthin, arranged in the form of radiating spines.

Radiolarians secrete a silicious skeleton, which assumes a variety of forms, as trellis-work, boxes joined by radiating spines, helmets, baskets, bee-hives, discs, rings, and numerous other forms. Haeckel has described upwards of four thousand species, and possibly as many more could be added to this number. Radiolaria are divided into two groups. In the one there is either no skeleton or one of silex; in the other the skeleton is formed of radiating spines of a horny nature. These are again subdivided according to the characters of the central capsule. In those forms with a silicious skeleton the geometrical pattern conforms more or less to the shape of the central capsule, being either spherical or conical. The central capsule is regarded as being homologous with the calcareous shell of Globigerina. Reproduction takes place by simple division into two, or by the body breaking up into spores, each provided with a flagellum, or two spores may fuse together, and the result will be an adult Radiolarian. Certain yellow corpuscles present in the outer part of their body-surface change into unicellular parasitic algals; these can be separated and cultivated independently of their host. The Radiolarians live floating at all depths from 1,000 to 2,500 fathoms, and are distributed over areas in the central Pacific and the south-eastern part of the Indian Ocean, the ooze forming the ocean bed being made up of their skeletons to an extent of 80 per cent. of the deposit; hence it has become known as Radiolarian ooze. The chalky-looking Barbadoes earth, a Tertiary formation, is composed almost entirely of their skeletons. Somewhat similar deposits exist in the Nicobar Islands, in Greece, and in Sicily.

It will have been noticed that by far the greater number of Foraminifera are of marine origin, and these occur in such widespread profusion that the finest calcareous particles which constitute the seashore in some places consist almost wholly of their microscopic remains. At former periods of the earth’s history they appear to have existed even in greater profusion than at the present time. This is evidenced by their remains forming the principal constituent of our largest geological formations.

Moreover, during the Canadian Geological Survey large masses of what appeared to be a fossil organism were discovered in rocks situated near the base of the Laurentian series of North America. Sir William Dawson, of Montreal, referred these remains to an animal of the foraminiferal type; and specimens were sent by Sir W. Logan to the late Dr. Carpenter, requesting him to subject them to a careful examination. As far back as 1858 Sir W. Logan had suspected the existence of organic remains in specimens from the Grand Calumet limestone, on the Ottawa River, but a casual examination of the specimens was insufficient to determine the point. Similar forms being seen by Sir W. Logan in blocks from the Grenville bed of the Laurentian limestone were in their turn tried, and ultimately revealed their true structure to Sir William Dawson and Dr. Sterry Hunt, who named the structure _Eozoon Canadense_.

The masses of which these fossils consist are composed of layers of serpentine alternating with calc spar. It was found by these observers that the calcareous layers represented the original shell, and the silicious layers the softer parts of the once living Foraminifera. The results were arrived at through comparison of the appearance presented by the Eozoon with the microscopic structure which Dr. Carpenter had previously shown to characterise certain members of the Foraminifera. The Eozoon not only exceeded other known Foraminifera in size to an extent that might have easily led observers astray, but, from its apparently very irregular mode of growth and general external form, no help was derived in its identification, and it was only by microscopical examination of its minute structure that its true character was ascertained. Dr. Carpenter wrote:--“The minute structure of Eozoon may be determined by the microscopic examination either of thin transparent sections, or of portions which have been subjected to the action of dilute acids, so as to remove the calcareous portion, leaving only the internal casts, or models, in silex, of the chambers and other cavities originally occupied by the substance of one animal.” Subsequently he found portions of minute structure so perfect that he was able to say that “delicate pseudopodial threads were originally put forth through openings in the shell wall of less than 1/10000th of an inch in diameter” (Plate III., Nos. 64, 65). In a paper read at the meeting of the Geological Society he stated that he had since detected Eozoon in a specimen of ophicalcite from Bohemia, in a specimen of gneiss from near Moldau, and in specimens of serpentine limestone sent to Sir C. Lyell by Dr. Gümbel, of Bavaria. These also were found to be parts of the great formation of the “fundamental” gneiss, considered by Sir Roderick Murchison as the equivalent of the Laurentian rocks of Canada.[65]

If the remains of Foraminifera be dissolved in dilute hydrochloric acid, an organic basis is left, after the removal of the calcareous matter, accurately retaining the form of the shell with all its openings and pores. The earthy constituent is mainly calcium carbonate; but there is also a small amount of phosphate of lime in the shells of many of them.

Infusoria.

We are now brought face to face with animals which possess considerable variation of structure, _Infusorial animalcules_, as they are termed. It was Ehrenberg who attributed to them a highly complex organisation, but later observations negatived these views and showed them to be animals formed of one or more cells, or colonies of so-called individuals. It is true that this cell or united protoplasm may show a wonderful amount of differentiation, what with its nucleus and vacuole, mouth and gullet, its variously-arranged cilia or flagella, its contractile fibres, its separation into an outer denser and a more fluid inner protoplasm, and its horny cup and stalks.

In these few lines we have a condensed summary of the special qualities of minute forms of life that afford much interesting work for the microscope.

Among those widespread, and in some respects heterogeneous, forms of life associated under the comprehensive title of Infusoria, we encounter types that not only differ very widely from one another, but which occupy a different rank or position, so to speak, with regard to the relation they bear to each other, and also to the outlying representatives of the series--differences that permeate throughout the ranks of this extensive group. Furthermore, a considerable number of Infusorial animalcules foreshadow or typify, in a corresponding degree, the separate or associated cell elements out of which higher tissue structures--metazoic organisms--are built up. We may take the well-known example _Euglena viridis_ (Plate III., No. 67), or Paramecium (No. 74), and their allies; these would appear to be the prototypes of Turbellaria. Another more lowly organised group of the Ciliata exhibits a distinct and highly-interesting affinity to the Opalinidæ. There are many other species (Acineta, Plate III., No. 68, for instance), which at first sight would seem to stand by themselves and present no marked agreement with any metazoic type. Indeed, the function of these and other polypites consists simply in seizing food and conveying it through perforations at the extremity of each separate tentaculum to its interior. In Acineta certain of the tentacles only are suctorial, and these, being the inner ones, fulfil the ingestive function, while the peripheral series are prehensile. This stalked club-shaped body (Fig. 332), which fixes itself to seaweeds or Bryozoa, is seen to have a nucleus, and also clear vesicles in the body-substance; its embryos are ciliated. It is an object of considerable interest even among curious marine animalcules; one or two species inhabit fresh water. The spiral-mouthed Spirostomum are among the largest of the class, and in sunlight are visible to the naked eye as slender golden threads of about 1/10th of an inch in length. The mouth slit, extending half the length of the body, is bordered on one side by cilia. The body is cylindrical and the surface covered with rows of cilia. Its multiplication takes place by transverse fission through the middle.

_Flagellate Infusoria._--The characteristic of this group, as its name implies, is the possession of one or more flagella or whip-like appendages, at the base of which is an opening in the denser surface layer of protoplasm, and in the interior a nucleus and one or more contractile vacuoles, and not infrequently a brilliant red spot of pigment known to microscopists as the eye-spot. The Monads, which constitute the simplest members of the group, are commonly found in fresh-water pools and vegetable infusions. The typical form consists simply of a spherical or oval cell provided with a flagellum. The Volvox was formerly placed in this group, but as it contains chlorophyll it is properly claimed by the botanist. The collared group possesses cup-like collars, and these frequently secrete horny receptacles or cups, and form elegant tree-like colonies.

The mail-coated group are of very varied form, the body being often prolonged into spiny processes. They have two long flagella which fit into grooves purposely provided. But the most interesting and remarkable are the phosphorescent animalcules (Noctiluca), whose beautiful bluish-green luminosity on the surface of the sea has attracted attention from very early periods. It was, however, not until the first half of the present century that the luminosity was discovered to be due to the presence of multitudes of these minute jelly-like spheres.

The body of the Noctiluca (Fig. 333) is a nearly globular-shaped cyst, enclosed in a tough membranous wall, from a grooved opening in which a striated muscular flagellum or proboscis is projected forth, and it is by means of this the animal swims away even in rough seas. A fine whip-like flagellum is also located in the same groove. At the apex of the funnel there is a mass of protoplasm which extends itself as a widely-meshed, highly-vacuolated network to the inner wall of the cyst, whence it is believed the phosphorescent light emanates. It multiplies by self-division, first becoming encysted after withdrawing its flagellum, and then breaking up into numerous ciliated helmet-shaped swarm spores. Frequently two organisms fuse into one and then divide into spores.

Noctiluca mainly confines itself to the shallower seas, but there are related forms met with in the warmer open seas; these belong to the genus Pyrocystis (Fig. 334). In one variety the body is perfectly spherical and without the big flagellum or proboscis. Professor Butschli, however, regards this species as an encysted or resting phase of the commoner and better-known form.

The late Mr. Philip Gosse, F.R.S., was the first microscopist to describe the Noctiluca. After careful observation, he wrote in his “Naturalist’s Rambles” as follows:--“I had an opportunity of becoming acquainted with the minute animals to which a great portion of the luminousness of the sea is attributed. One of my large glass vases of sea-water I had observed to become suddenly at night, when tapped with the finger, studded with minute but brilliant sparks at various points on the surface of the water. I set the jar in the window, and was not long in discovering, without the aid of a lens, a goodly number of the tiny jelly-like globules of _Noctiluca miliaris_ swimming about in various directions. They swam with an even gliding motion, much resembling that of the _Volvox globator_ of our fresh-water pools. They congregated in little groups, and a shake of the vessel sent them darting down from the surface. It was not easy to keep them in view when seen, owing rather to their extreme delicacy and colourless transparency than to their minuteness. They were, in fact, distinctly appreciable by the naked eye, measuring from 1/50th to 1/30th of an inch in diameter.”

Among the numerous fresh-water members of the flagellate infusoria, there is one which especially calls for notice, Codosiga, discovered by the late Professor H. J. Clark. This minute body bears a delicate funnel-shaped protoplasmic expansion or collar, common to the several members of this organic series. The flagellum is placed at the base of the oral opening, and within the circumscribed area of the collar, which is of such extreme tenuity that its true form and nature can only be determined by a very careful adjustment of the achromatic condenser and accessory apparatus employed, together with a wide-angled objective. It is seen to greater advantage by supplying the animal with very fine particles of colouring matter. In this way it is found that the infundibuliform cup consists of protoplasm, through which the flagellum is protruded and withdrawn into the general substance of the Monad’s body (Fig. 335). As many as twenty or more zooids are attached to the extremity of a slender footstalk. The length of the body, exclusive of the collar, is 1/2500th to the 1/1200th of an inch. The habitat of these bodies is fresh water. Mr. Saville Kent in 1869 discovered some of these interesting infusoria in the London Docks.

“The more exact significance of the special organ, the collar, is manifest by the circulatory currents or cyclosis induced, and there can be no room for doubt that this structure finds its precise homologue in the pseudopodia of the foraminiferous group of the Rhizopoda, in which a similar circulation or cyclosis of the constituent sarcode is exhibited. The whole of this highly-interesting flagellate order, a comparatively small one as yet, are remarkable for their pale glaucous green or florescent hue, such colour assisting materially in their recognition, even when the magnifying power employed is insufficient for the detection of the very characteristic collar with its enclosed flagellum.”[66]

_Ciliata._--Types of Ciliata obtained from hay infusions are very numerous. Ehrenberg’s animalcules were mainly of a large size, and of those belonging to the higher order of the Ciliata, pertaining to such genera as Paramecium, Colpoda, Cyclidium, Oxytricha, and Vorticella. These, however, represent but an insignificant minority of the hosts of flagellate forms which abound in our humid climate, and in hay infusions in particular. In such infusions, watched from day to day and produced from hay obtained from different localities, the number of types developed in regular sequence is found to be perfectly marvellous, commencing with the _Monas_ proper, Amphimonas and Heteromita; while Bacteria, in their motile and quiescent forms, are invariably present and furnish an abundant supply of material for the microscope.[67]

Vorticellidæ constitute one of the most numerous families of the ciliate infusoria. All its members are at once recognised by their normal stationary condition, and by the structure of their oral system. In but few of the genera is there any marked divergence from this formula, and when any exists it is made manifest by an increase in development of some one of its elements at the expense of another. For instance, in the genus Spirochona, the external edge of the encircling border or peristome is suppressed, while the inner portion is abnormally developed into a transparent and highly elevated spiral membrane. The bell-animalcules usually possess stalks, and are either solitary or form branching colonies. _Conichilus vorticella_ (Plate III., No. 80) is a well-known member of the colony stock, all the zooids of which are united on a slender branching pedicle, which consists of a central contractile cord enclosed within a tubular hyaline sheath. There are many other shrub-like colonies all variously modified in form and character. The _Epistylis opercularia_, or nodding-bell animalcule, is an interesting member of a numerous host of solitary short-stalked forms (Fig. 337). When the animal is disturbed, the heads drop down towards the stalk. This animalcule has been found to form a colony; and another, Carchesium, whose tiny branched tree-like colonies resemble little white globular masses of moulds, are seen at once to drop down towards the base of the colony with a jerky movement if the cell be touched. By a process of encysting, all the Vorticellæ and many of the more highly-organised ciliata have the means of what may be termed self-preservation. Should the water dry up in which they have been living, the little animal encases itself in mud at the bottom of the pool. Should this be baked by the sun not the least injury arises, for at this stage it crumbles into dust, and is carried by the wind to long distances, but the first shower of rain calls it back to active life, and soon after it is seen to issue forth as a free swimming bud.

_Thuricola valvata_ (Plate III., No. 72) possesses a hinge-like process which closes up like a door when the animal contracts itself into its case. This very effectually protects it from assault. Both portions of the valve are capable of extension. Another group of ciliate infusoria also possess a limited number of cilia, but these, although restricted to the under surface of their bodies, have an unrestricted range of motion. The group are all free swimmers, belonging to the genus Oxytricha. They possess two separate alimentary orifices, neither of which are situated at the extremities or encased by a dense integument. Their locomotive organs consist either of setæ, vibratile cilia, or non-vibratile styles or uncini, variously situated, and all serving to make these infusorial animals very active (Plate III., Nos. 73 and 77). A typical species is the mussel-animalcule (Stylonychia, Fig. 338), common in all infusions and pools of water. Its body is oval and flattened, and about 1/100th of an inch in length. At one end a funnel-shaped depression or mouth, with a ciliated margin, leads to the inner part of the body, in which are two oval bodies, a nucleus and a contractile vacuole, which is seen to contract rhythmically. The creature can also stalk along by means of its cilia or setæ, and set up currents to the mouth. Plate III., Nos. 70, 71, 72, 73, and 74, are types of these interesting bodies.

Dr. Balbini believes a true sexual generation occurs among these organisms, but, with the exception of the Paramecium, this has not been seen to take place; even Gruber’s more recent investigations appear to be inconclusive on this point. Conjugation, however, it is said takes place among some attached forms, as in the Stentors. These have been seen to put forth a bud from the body base, and soon after become free swimming bodies. The trumpet-animalcule (Stentor), a conspicuous member of the ciliata, is comparatively large, being about the 1/25th of an inch in length when extended to the full size. It is usually found attached to the under sides of duckweed, and is continually changing its form from that of a small knob when contracted, to the trumpet shape seen in Fig. 339, No. 6, when fully extended, and from which it derives its name. The long cilia projected from the upper part form a spiral within the margin of the open mouth leading to the digestive sac. A contractile vacuole lies to the right of the oral opening. New individuals are produced by the process of budding, and in the form of ciliated embryos from the nucleus. Stentors are commonly met with in fresh water, and are usually of a brilliant green colour. These little bodies will bear cutting up: if only a fragment of the nucleus be included in the section, the injury is soon repaired.

_Rotifera_, or _Wheel-animalcules_ (Fig. 339).--In this group we have a higher type of animal, with a more complex organisation than those previously noticed. The great majority inhabit fresh water, and are readily developed in hay infusions, in bog-moss, in house-top gutters, everywhere if looked for after a shower of rain. The rotating organs from which these fascinating animalcula derive their name consist of two disc-like bodies whose margins are fringed with rows of cilia, which create currents toward the oral aperture, and which have given rise to the optical delusion of rotating wheels. The disposition of the cilia is so arranged as to bring food to the rotifer and conduct it to the mastax or digesting apparatus--a muscular bulb moved by a series of muscles--the gastric glands and stomach. The great transparency of the whole structure permits of the animal economy being easily studied. The body is covered with a horny envelope of two layers, and is divided into segmental divisions, which slide into each other telescopic fashion. Consequently, as the water dries up, the animal is for a long time rendered indestructible and capable of resisting varying temperatures and the action of caustic reagents.

Rotifers are oviparous, and their eggs are conspicuous and of three kinds. The common soft-shelled eggs produce females, the smaller and more spherical produce males. The ephippial, or summer eggs, are often beset with spines or bosses; these have only a membranous covering, and are hatched soon after they are laid, or before leaving the ova sac. The male rotifer is but a third of the length of the female, often without cilia, and appears to have no alimentary tract; indeed, the only internal organ is a large sperm sac. Rotifers have been divided by Dr. Hudson and the late Mr. Gosse in their charming work on these very interesting “Wheel-animalcules” into four orders, according to their powers of locomotion, as follows:--(1) Rhizota, the rooted; (2) Bdelloida, the leech-like, that swim and creep like a leech; (3) Ploïma, the sea-worthy, that only swim with their ciliary wreath; (4) Scirtopoda, the skippers, that swim with their cilia and skip with arthropodous limbs. These, again, are subdivided into families. With such hardy creatures as Philodina, Adineta, Brachionus, &c., creatures to whom extremes of cold, heat, and drought are the ordinary conditions of life, nothing can be easier to keep going throughout the year. Mr. C. F. Rousselet, who has so thoroughly succeeded in mounting Rotifers with their cilia fully extended, recently exhibited at one of the evening meetings of the Royal Microscopical Society, London, no less than four hundred specimens in a natural and perfect condition, the nervous system being seen more clearly from its successful staining throughout the body than in the living rotifer.

There is also a family of Rotatoria with a single rotatory organ, disposed around the margin of the case. This comprises at present a very small group. The Œcistes is a member of the family (Plate III., No. 69). A single ciliary wreath leads to the alimentary canal, and a pharyngeal bulb or mastax comprises the apparatus of nutrition. The visual organs are red, as in other rotifers, and the ovarium contains several ova, shown in No. 69. The envelope is a gelatinous transparent sheath, into which the animalcule can withdraw itself, its attachment to the bottom being by the end of the foot-like tail. The most interesting among this genus are the Floscularians. These creatures may undoubtedly be described as among the most beautiful and interesting of infusorial animals.

The Stephanoceros, “crowned animalcule,” as it is termed, is about 1/36th of an inch in length, and enclosed in a transparent cylindrical flexible case, beyond which it protrudes five long arms in a graceful manner. These, touching at their points, give a form from which it derives its name. These arms are furnished with several rows of short cilia, which seize the food brought within their grasp until it can be swallowed. In addition to the rotatory organs, they have short flexible processes, or cornu, attached to the outside of one or more of their lobes. The water vascular system consists of two canals arising from a small pyriform contractile vesicle, situated below the stomach. The ova, after leaving the ova sac, remain quiescent until their cilia are developed. Floscularians, like Melicertans, have a certain affinity in form with Vorticellians and Stentors, and also with Campanulariæ, among polypes. Their cilia are less regular when in action than in other Rotatoria. When they retreat into their transparent cells they appear to fold themselves up. Their internal structure can be seen through the external case, and ova are observed enclosed in an ova sac; when thrown off they remain quiescent until the formation of their cilia. The whole family furnish interesting objects for microscopic investigation.

_Melicerta ringens_ (“beaded Melicerta”).--Of all the Melicerta, or “horny floscularia,” this is the most beautiful. Its crystalline body is enclosed in a pellucid covering, wider at the top than the bottom, of a dark yellow or reddish-brown colour, which gradually becomes encrusted by zones of a variety of shapes, cemented together with a peculiar secretion that hardens in water. It derives its name from these pellets, which have the appearance of rows of beads. Mr. Gosse furnished an excellent account of the architectural instincts of _Melicerta ringens_: “An animalcule so minute as to be with difficulty appreciable by the naked eye, inhabiting a tube composed of pellets, which it forms and lays one by one. It is a mason who not only builds up his mansion brick by brick, but makes his bricks as he goes on, from substances which he collects around him, shaping them in a mould which he carries on his body.

“The pellets composing the case are very regularly placed in position; in a fine specimen, about the 1/30th of an inch in length, when fully expanded, as many as fifteen longitudinal rows of pellets were counted, which gave about thirty-two rows in all. As it exposes itself more and more, suddenly two large rounded discs are expanded, around which, at the same instant, a wreath of cilia is seen performing surprising motions.

“On mixing carmine with the water, the course of the ciliary current is readily traced, and forms a fine spectacle. The particles are hurled round the margin of the disc, until they pass off in front through the great sinus, between the larger petals. If the pigment be abundant, the cloudy torrent for the most part rushes off, and prevents our seeing what takes place; but if the atoms be few, we see them swiftly glide along the facial surface, following the irregularities of outline with beautiful precision, dash round the projecting chin like a fleet of boats doubling a bold headland, and lodge themselves, one after another, in the little cup-like receptacle beneath. Mr. Gosse, believing that the pellets of the case might be prepared in the cup-like receptacle, watched the animal, and presently had the satisfaction of seeing it bend its head forward, as anticipated, and after a second or two raise it again; the little cup having in the meantime lost its contents. It immediately began to fill again; and when it was full, and the contents were consolidated by rotation, aided probably by the admixture of a salivary secretion, it was again bent down to the margin of the case, and emptied of its pellet. This process he saw repeated many times in succession, until a goodly array of dark-red pellets were laid upon the yellowish-brown ones, but very irregularly. After a certain number were deposited in one part, the animal would suddenly turn itself round in its case, and deposit some in another part. It took from two-and-a-half to three-and-a-half minutes to make and deposit a pellet.”

Melicerta may be found in clear pools, mill-ponds, and other places through which a current of water gently flows. If a portion of water-weed be brought home and placed in a small glass zoophyte-trough, and carefully examined with a magnifying power of about fifty diameters, a few delicate-looking projections of a reddish-brown colour will probably be seen adhering to the plant; these are the tubular cases of Melicerta, which, after a short period of rest, will be seen to be animals of 1/12th of an inch or more in length.

Porifera. Spongiadæ.

_Sponges._--The term Porifera, or “canal-bearing zoophytes,” was applied by the late Dr. Grant to designate the remarkable class of organisms known as sponges, met with in every sea, and numbering about two thousand species, varying in size from a pin’s head to masses several feet in height; and weighing from a few grains to over a hundred pounds. Sponges assume an endless variety of shapes, as cups, vases, spheres, tubes, baskets, branched-like trees, but often as shapeless masses. When living they are all colours and all consistences, soft and gelatinous, fleshy, leathery or stony. A fuller knowledge of sponges was gained in 1825, when Dr. Robert Grant examined a fragment of living sponge under the microscope. On bringing it to the side of the glass cell in which he had preserved it, he beheld this living fountain pouring forth a torrent of liquid matter in rapid succession, and he was at once convinced that a current flowed out of the larger orifices. He introduced a small portion of fine chalk, and saw particles driven into the interior, and pass out again by different ways. To determine the cause of the currents, it was necessary to make a closer examination of the anatomy of the sponge. For this purpose he cut or peeled off thin sections, and saw that the whole substance was divided into flagellated chambers, enclosing spherical and other bodies, and perforated by pores. Each chamber proved to be about 1/500th of an inch in diameter, groups of them opening by a wider orifice into a common space, or canaliculus, and joining others to form canals terminating in larger oscular canals. The walls throughout are lined with flat cells, but in the flagellated chambers the living cells are more or less cylindrical, and each is provided at the free end with a whip-like appendage, or flagellum. Furthermore the upper margin was seen to be expanded into a thin hyaline collar, so that the whip appeared to have its origin in the centre of a basin or funnel. The currents of water traversing the body of the sponge are kept up by the movements of the flagella of the collar-cells. These beat the water in the flagellated chambers into the rootlets of the canals leading to the oscules. To replace this, water flows into the flagellated chambers from the rootlets of the canals passing down from the groups of pores in the skin. The currents entering the sponge bring in oxygenated sea-water and minute food particles, such as diatoms and infusorial organisms; the currents from the oscules contain an excess of carbonic acid of waste products, resulting from vital activity and indigestible remains. The cells lining the canals effect the exchange of gases, and take up food particles.

Professor Grant’s careful and instructive researches were begun on the smaller kind of British sponges hanging down from rocks (_Spongia coalita_), and on which he gazed for “twenty-five minutes, until obliged to withdraw his eyes from fatigue.” This sponge fixes itself by a root; and the currents enter through the stem and body, and leave principally by oscules placed on the branches.

At present too little is known as to the physiology of digestion in sponges to permit of a definite statement on the subject. In specimens fed upon carmine the collar-cells have been found loaded with granules; in others, again, the flat cells lining the subdermal cavities have been found gorged with colour granules. From Bowerbank’s monograph on the British Spongiadæ (1864 and 1874) nothing of importance can be gained on the subject; in fact, it relates almost entirely to the structure and organisation of sponges in their dried or preserved condition, and therefore is only of value for purposes of specific identification. One of the simplest of living sponges, the microscopic structure of which it is possible to trace, _Ascetta primordialis_, is found on seaweeds in the Mediterranean. In its simple unbranched condition it forms a minute white sac about one twenty-fifth of an inch in height, opening above by a wide round oscule and narrowing below to a stalk (Fig. 342). The walls are very thin and perforated by pores, through which the water passes into the interior. The walls of the sac are composed of two layers, an inner lining of collar-cells, and an outer layer consisting of a gelatinous matrix containing amœboid bodies and transparent three-rayed spicules. These serve to support the walls and as a frame-work for the pores, as in all the sponges. By eliminating the spicular skeleton, and by supposing the tube to be more globular, the “olynthus form” will be obtained, which has been regarded as the hypothetical ancestor of all sponges. A canal system arises when the walls grow thick or form folds, or give off pouches or tubes. From these channels arise incipient in-current canals, between the inside or lumen of the folds and that forming the out-current canal system.

There is a common ciliated Sycon found on seaweed round the British coast; it has the appearance of a white sac about an inch in height, with a crown of glassy spicules around the orifice. The vertical cavity of the sac is surrounded by a wall of closely-packed horizontal tubes, opening at their inner ends into the central cavity, but externally ending blindly. The central cavity of the sac is surrounded or lined with flat-cells, and the radial tubes with collar-cells, and the walls of the tubes are perforated. Here the spaces between and outside the densely-packed tubes are the in-current canals. In an equally common British sponge, Grantia, which forms small flat white bags, a rudimentary cortex covers the outer ends of the tubes. In Grantiopois, the cortex becomes quite thick; as the radial tubes in this species become more branched and the mesoderm thicker, so the passages or in-current canals become more complicated. Common silicious, sponges develop in a different manner from the calcareous ones, namely, from a hollow conical sac open at the top and with a flat base; the spherical flagellated chambers at a very early stage forming a mammillated layer in the walls. Plakina, one of the simplest silicious sponges, encrusts stones with a fleshy crust, consisting of a sac with a flat base attached to the stone in sucker-like fashion, and with the rest of the walls forming simple folds. The spaces between and outside the folds form the in-current, and those in the lumen of the folds the out-current, channels. Each of the flagellated chambers in the walls of the folds communicates with the in-current spaces through several pores, and opens into the out-current spaces by one large pore, the currents of water passing out by the central oscule. Here we have a general idea of the formation of all the commoner forms of sponges. In the more delicate species, as that of Venus’ flowerbasket, the cells are formed by a trellis work of large spicules of silica. Groups of cells congregate in the ground substance and secrete a network of cylindrical fibres and spicules, which, although they remain to a certain extent separate, are always beautifully adapted for purposes of support. In addition to the support these afford, the skeleton spicules afford a means of defence against the attacks of small animals.[68]

A fairly good idea will be gained of the internal structure of sponges from the section made of a _Geodia Barretti_, Fig. 343.

_Reproduction._--As regards the modes of reproduction, both male and female cells are found in the mesoderm. The male cells generally give rise by division of the nucleus to masses of spermatozoa, each of which possesses a conical head and a long vibratile filament. The ova appear as large round cells, and when conglomerated in masses, resemble those of Micro-gromia, which, after fertilisation, undergo segmentation or division, first into two cells, and again dividing and sub-dividing, until a cluster or mass of cells results (as seen in Fig. 343). The outer layer of the egg-shaped embryo becomes more cylindrical in shape, and is now provided with cilia, and soon appears as an independent minute oval body. If a bread-crumb sponge be cut open in the autumn, the embryos will be seen as bright yellow spots within the body-substance. By keeping specimens in a vessel of water, the embryos will be seen to escape from the oscules, and swim freely about with the broad end forwards. After twenty-four hours of independent existence, the embryo remains stationary, and fixes itself by its broad end, which becomes flattened out. By a remarkable transformation, the larger granular cells of the interior burst out and grow over the outer flagellate layer of cells, and the latter become the collar-cells of the adult sponge. A minute sponge with one oscule results from the development of the fertilised ovum. An extensive crust with numerous oscules may be regarded either as a colony in which each oscule represents an individual, or simply as one individual in which the growth of the body necessitates the formation of new channels for the conveyance of food materials. The embryos of some of the fresh-water sponges (Spongillidæ) living in ponds, canals, lakes and rivers all over the world, as soon as they become fertilised undergo segmentation, and form oval ciliated bodies, in appearance somewhat resembling the gastrula of Monoxenia, one of the simplest kinds of corals. Fresh-water sponges are green in colour, due to the granular bodies which crowd the cells near the surface of the sponge; that this colour is not due to the formation of chlorophyll is seen on keeping them in a shady place, when they become pale grey or yellowish-brown, and if kept quite in the dark they entirely lose all colour.

A few sponges possess no skeleton whatever, excepting the gelatinous ground substance; in some specimens the skeleton is mainly or entirely composed of foreign particles of sand or the remains of Foraminifera. Others are composed of calcium carbonate, and form the class Calcarea, the spicules of which are white, and opaque in mass; but on placing portions in hydrochloric acid, the skeleton is dissolved away with effervescence, and the spicules are left behind transparent and glassy. A great variety is seen in the different species, as will be gathered from the few typical forms shown in Plate XVI., and which even in their fossilised state remain unaltered, the silica which enters so largely into their composition being indestructible, the calcareous matter alone becoming separated in exposure to the action of air, or by boiling in hydrochloric acid. The only perceptible difference noticed is an increase in transparency, and this, on mounting them in Canada balsam, adds to their beauty when examined by polarised light.

Hyalonema, the “glass-rope” sponge of Japan, consists of a bundle of from 200 to 300 threads of transparent silica, glistening with a satiny lustre like the most brilliant spun glass; each thread is about eighteen inches long, in the middle the thickness of a knitting-needle, and gradually tapering towards either end to a fine point; the whole bundle coiled like a strand of rope into a lengthened spiral, the threads of the middle and lower portions remaining compactly coiled by a permanent twist of the individual threads; the upper portions of the coil frayed out, so that the glassy threads stand separate from each other. The spicules on the outside of the coil stretch its entire length, each taking about two and a half turns of the spiral. One of these long needles is about one-third of a line in diameter in the centre, gradually tapering towards either end. The spirally-twisted portion of the needle occupies rather more than the middle half of its entire length. In the lower portion of the coil, which is embedded in the sponge, the spicule becomes straight, and tapers down to an extreme tenuity, ultimately becoming so fine that it is scarcely possible to trace it to its termination.

Within the mesoderm, and in oscule, was noticed a deep brownish-orange coloured shrunken membrane; this was traced to a parasitic polyp. Since this was first observed on an early specimen of the Japanese glass-sponge, the same parasite has always been found growing on and in all these curious sponges. The surface of the stalk above the portion embedded in the mud is seen to be covered with a warty crust of parasitic polyps. All the specimens of Hyalonema in the European museums in 1860 had their stalks overgrown with Palythoa, while many had their bodies also covered with another parasite, and which, fortunately for the sponge, did not form a sandy crust. The polyps, having no skeleton, dry up entirely, and leave behind no trace except the stain first referred to. Unlike a parasite, however, the polyps do not feed upon the juices and soft parts of the sponge, nor indeed do they share its food, but simply settle upon the sponge and feed upon any food that may chance to come within their reach.

The dredgings of the _Challenger_ brought to the surface many entirely new forms of glass-sponges and from great depths. One of the most beautiful, known as Carpenter’s glass-sponge (Pheronema), is composed of concentric laminæ of silica deposited around a fine central axial canal. These form a gauze-like network throughout, but with no regularity of structure.

_Clionæ._--Not the least wonderful circumstance connected with the history of sponges is the power possessed by certain species of boring into substances, the hardness of which might be considered as a sufficient protection against such apparently contemptible foes. Shells (both living and dead), coral, and even solid rocks are attacked by these humble destroyers, gradually broken up, and, no doubt, finally reduced to such a state as to render substances which would otherwise remain dead and useless in the economy of nature available for the supply of the necessities of other living creatures.

These boring sponges constitute the genus Cliona of Dr. Grant. They are branched in form, or consist of lobes united by delicate stems, and after having buried themselves in shells or other calcareous objects, preserve their communication with the water by means of perforations in the outer wall of the shell. The mechanism by which a creature of so low a type of organisation contrives to produce effects so remarkable is still doubtful, from the great difficulties which lie in the way of coming to any satisfactory conclusions upon the habits of an animal that works so completely in the dark as the _Cliona celata_. Mr. Hancock, in his valuable memoir upon the boring sponges, attributes their excavating power to the presence of the multitude of minute silicious crystalline particles adhering to the surface of the sponge; these he supposes are set in motion by ciliary action. In whatever way this action may be produced, however, there can be no doubt that these sponges are constantly and silently effecting the disintegration of submarine calcareous bodies--the shelly coverings, it may be, of animals far higher in organisation, and in many instances they prove themselves formidable enemies even to living molluscs, by boring completely through the shell. In this case the animal whose domicile it so unceremoniously invades has no alternative but to raise a wall of new shelly matter between himself and his unwelcome guest, and in this manner generally succeeds in barring him out.

From a close examination of the structural and developmental characters of the Spongideæ, it must be conceded that they belong rather to the flagellata Protozoa than to any other order. This was the view held by the late Professor Clark, and Mr. Saville Kent quite concurs in it.[69] Summing up the entire evidence adduced, scarcely a shadow of doubt is admissible concerning the intimate relationship that subsists between the Choano-flagellata and other flagellate Protozoa and that of sponges. The primary and essential element of the apparently complex sponge stock is the assemblage of collared flagellate zooids that inhabit its interstitial cavities under various plans of distribution. Individually these collared zooids correspond structurally and functionally in every detail with the collared units of such genera as Codosiga, Salpingœca, and Proto-spongia. The collar in either case presents the same structure and functions, exhibits the same circulatory currents or cyclosis, and acts in the same way for the capture of food. The body contains an identical centrally located spheroidal nucleus or endoplast, and a corresponding series of rhythmically pulsating contractile vesicles. The developmental reproductive phenomena are also strictly parallel. Both originate as simple Amœba or simple flagellate Monads, exhibiting no trace in their earliest stage of the subsequently acquired characteristic collar. Both again after a time withdraw their collar and flagellum, and assume the amœboid state; then, coalescing, enter upon a quiescent or encysted condition, and break up into a number of sporular bodies, and thus provide for the further existence and distribution of the species. The whole process again is much akin to that which obtains in the protophytic type, _Volvox globator_, which liberates from its interior free swimming gemmules that take the form of spherical aggregation of biflagellate daughter-cells. In their isolated state, on the other hand, the swarm gemmules of the sponge stock are directly comparable with the free swimming subspheroidal colony stock of the flagellate infusoria Synura, Syncrypta, and Uroglena, or with the attached subspheroidal clusters of Codosiga and Anthophysa.