Part 20

_Compartments._--The external appearance of the shell has already been described. In the most typical species, the parietes consist of an outer and inner lamina, separated by strong longitudinal septa; these septa are denticulated on both sides at their bases, but only close to the inner lamina; in fact the inner lamina is apparently formed by the union, thickening, and production, of some of the denticuli. As it is not the innermost of the denticuli on the basal edges of the longitudinal septa, which thus become united into a solid layer, the longitudinal septa form slightly projecting, longitudinal ribs on the inner lamina. These internal ribs are longitudinally striated; in old specimens they often become obliterated, especially in the upper part of the shell. The parietal tubes or pores (occupied by threads of corium) are generally square and large; but in _B. Ajax_ they are very small, and in _B. glandula_ often extremely minute. In the upper part of the shell, and sometimes low down, they are generally crossed by thin, transverse, calcareous septa: in some species, as in _B. perforatus_, and in some varieties of _B. amphitrite_, the upper ends of the tubes are filled up solidly with shell. In some varieties of _B. crenatus_ and of _amphitrite_, the longitudinal septa, near the outer lamina, divide, thus giving rise to a very imperfect row of outer short tubes. In _B. vinaceus_ (Pl. 2, fig. 7 _d_) the inner lamina is cancellated instead of being solid, which is caused by the basal denticuli of the longitudinal septa being simply united together by their ends and crossed by transverse septa, instead of being consolidated into a mass. In several species, as in _B. Hameri_, the walls consist only of the outer lamina with longitudinal ribs, no inner lamina having been formed; the ribs here evidently answer to the longitudinal septa in the foregoing species. In _B. flosculus_ and _imperator_ the walls are solid, their basal margins being formed of irregular, elongated points, and little ridges (Pl. 8, fig. 4 _c_), which apparently prefigure the more regular longitudinal ribs or septa. In _B. balanoides_ the walls are generally either nearly smooth and solid, or irregularly cancellated; in _B. cariosus_ (Pl. 7, fig. 3 _b_) two or three rows of short irregular tubes are formed by unequally branching septa, almost as in the genus _Tetraclita_.

The _Radii_, in all the species, are constructed essentially on the same plan as the parietes; thus, in the typical forms, there is an outer and inner lamina, with septa, which, near the inner lamina, are furnished with denticuli on both sides; hence the radii are permeated by pores or tubes, like the parietes; but this holds good only in the first section of the genus, for, in the other species, the tubes are filled up quite solidly. The denticuli on the septa often occur only on one side, or disappear altogether; and, lastly, the septa themselves often appear merely like little teeth, or disappear altogether as in _B. Hameri_, or occur only near the bases of the radii, as in _B. amaryllis_. A slight furrow in the compartment, against which each radius abuts, is generally marked by the septa and their denticuli. In regard to the _alæ_, their lateral or sutural edges are either thin and smooth, or, more commonly, finely crenated or ribbed. The little transverse crenations are homologous with the septa in the radii and parietes. The edges of the alæ are usually received in a furrow. The diametric growth of the shell is effected by the growth of the radii and alæ, and chiefly by that of the former. The sutural and lateral edges of both radii and alæ are added to, either quite up to their summits, or only low down, and during the continued growth of the shell, lower and lower down; in accordance with this difference in growth, the summits of the radii and alæ become either very oblique, or they extend parallel to the basis, that is, from tip to tip of the adjoining compartments. When the radii and alæ are added to, as is most usual, above the level of the opercular membrane, and therefore above the sack, ribbons of corium run up the sutures from the sack, higher or lower, according to the height to which, in the different species, the edges of the radii and alæ continue to be added to. The obliquity of the summits of the radii and alæ varies, in some cases, in the same species. It often happens that when the summits of the radii are very oblique, the summits of the alæ are but little so; and the converse; both, however, are often either equally oblique, or both have square summits. The _sheath_ extends either one third or more than half down the shell; its basal margin often (Pl. 25, fig. 1, K′) freely depends or overhangs the inner lamina of the walls.

_Basis._--In typical species the basis is calcareous, and consists of an upper and lower lamina, separated by radiating septa, forming pores. In the same manner as the septa of the parietes sometimes, though rarely, become irregularly divided near the outer lamina, forming outer pores, so it is, but in a much more marked degree, with the basis. The basis in such cases becomes extremely thick, and consists of an upper, thin lamina, with the regular radiating septa and pores, and of an underlying, thick, cancellated mass, which seems wholly to result from the dividing and sub-dividing of the septa. The basal radiating pores, like the parietal pores, are closed at intervals by calcareous transverse septa. The basal points of the parietal septa enter the orifices of the basal pores, and the threads of corium pass into the latter, between the denticuli of the parietal septa. In some species, as in _B. crenatus_ and _Hameri_, the basis is perfectly solid, the upper lamina being absent, just as in some species, the internal lamina of the parietes is absent. In _B. flosculus_ the basis is calcareous, but consists of so excessively thin a film as hardly to be distinguished: it presents, moreover, as also is the case with _B. imperator_, a beaded structure. Again, in some few species, as in _B. balanoides_, the basis is simply membranous. When the basis is thin, it is always flat, and is closely moulded to the irregular surface of attachment; and in this case, when specimens are crowded together, their elongation is effected exclusively by the growth of the walls; but, when the basis is thick, it sometimes becomes, in crowded groups, deeply, but irregularly, cup-formed, or cylindrical, as in _B. psittacus_ and _perforatus_. In _B. allium_, however, which inhabits massive corals, the basis is as regularly concave or cup-formed as in the genus Pyrgoma: in _B. calceolus_ and its allies, and in some varieties of the fossil _B. inclusus_, the basis is boat-formed, with its lower surface deeply grooved longitudinally from clasping the stem of the Gorgonia or other zoophyte, to which it was attached. In certain varieties of _B. lævis_ it is very remarkable that the deeply cup-formed basis becomes, owing apparently to the whole shell having grown too deep for the animal, half-filled up with irregular, calcareous, transverse plates (Pl. 4, fig. 2 _a_), resting one upon the other by irregular points or pillars. The cementing apparatus has been sufficiently described under the Family.

_Mouth._--The _labrum_ is always notched; sometimes it has no teeth, but generally there are three on each side; in _B. balanoides_ there are five or six on each side; and in _B. improvisus_ and _eburneus_ there is a whole row of teeth (Pl. 26, fig. 2, _e′_), graduated in size, on each side of the notch. The _palpi_ are large, with their apices nearly touching, and furnished with long spines. The _mandibles_ have, as it appears, normally, five teeth, but the two lower teeth are always small and often rudimentary, and almost confluent with the inferior, sometimes spinose angle. The _maxillæ_ have either a simple edge, or a notch under the pair of large upper spines, or the lower part forms (Pl. 26, fig. 7) a step-formed projection: there are generally two lower spines, placed singly or not in pairs, larger than the others, with the exception of the uppermost pair. The _outer maxillæ_ are, on their inner faces, obscurely divided into two lobes.

_Cirri._--The rami of the first pair are unequal, the shorter one sometimes not being more than half the length of the other ramus: the segments of the shorter ramus are broad, and are, together with the lower segments of the longer ramus, thickly clothed with spines; in some species, as in _B. perforatus_, the anterior surfaces of the segments, more especially of the shorter ramus, and of both rami of the second pair are produced (Pl. 29, fig. 4), so as sometimes to form very remarkable projections. The segments of the second and third pairs are always thickly clothed with spines, as also are their pedicels. The third pair is rather longer than the second; but in _B. vestitus_ and _imperator_ it is much longer, and is otherwise somewhat different. The dorsal and basal margin of the pedicel of the third pair, in some of the species, as in _B. tintinnabulum_, is produced backwards on the thorax, and forms a membranous plate fringed with fine spines. The three posterior and longer pairs of cirri have from three to rarely eight or ten pairs of long spines on each segment, with generally one or two minute spines in the middle between each pair: their pedicels have a regular double row of spines.

The _penis_ is long and hairy: in most of the species there is, at its dorsal basis, a small, sharp, flattened, imperforate projection; first observed by Poli: but this is sometimes absent, as in _B. crenatus_, though present in the closely allied _B. balanoides_; and its presence is variable in _B. tintinnabulum_. All the species have large plicated _branchiæ_. The base of the _sack_ in several species is furnished with inwardly projecting filamentary appendages. In _B. perforatus_, _crenatus_, and _improvisus_, and I believe in other species, the upper part of the _stomach_ is furnished with a circle of branching cæca.

_On the variation of the species_; _their arrangement and affinities_; _value of the characters used_; _changes during growth_.--Owing to the great variation in external characters, to which almost all the species are subject, and likewise to the genus being a very natural one, that is, to the species following each other in close and natural order, it is not easy to exaggerate the difficulty of identifying the species, except by a deliberate examination of the internal and external structure of each individual specimen. Every one who has collected sessile cirripedes must have perceived to what an extent their shape depends on their position and grouping. The surface of attachment has a great effect on that of the shell; for as the walls are added to at their bases, every portion has at one time been in close contact with the supporting surface; thus I have seen a strongly-ribbed species (_B. porcatus_) and a nearly smooth species (_B. crenatus_) closely resembling each other, and both having a peculiar appearance, owing to their having been attached to a pecten. Dr. Gray has pointed out to me specimens of _B. patellaris_, curiously pitted like the wood to which they had adhered; and numberless other instances might be added. Quite independently of the effect produced by the surface of attachment, the degree to which the longitudinal folds and ribs are developed on the parietes, is variable in most of the species, as in _B. tintinnabulum_, _vestitus_, and even in _B. porcatus_; the presence or entire absence of these ribs often surprisingly alters the whole aspect of the shell. The persistence of the so-called epidermis is in some degree variable; and in _B. lævis_ we have groups of specimens absolutely naked, and others uniformly clothed with a brown membrane. Again, some species in certain localities are all subject to the disintegration of the entire outer lamina of the walls; and in such cases (as with _B. perforatus_) there is not the smallest resemblance between the corroded and perfect specimens. The size of the orifice, and consequently of the operculum, compared with the shell itself, varies accordingly as the shell is more or less conical or cylindrical; in the latter case, the summits of the radii are generally more oblique and the aperture consequently more deeply toothed than in the more conical varieties. Size is a serviceable character in some cases, but very many specimens are required to ascertain the average or maximum size of each species, for there is no method of distinguishing a half-grown from a full-grown specimen; and I believe, as long as the individual lives, so long does it go on moulting and growing. _Colour_ is of very considerable service; though the precise tint varies greatly in almost every species; and what is a far more serious evil, the majority of the species have their white or nearly white varieties, the latter sometimes as numerous as the coloured ones: in _B. perforatus_, _lævis_, _flosculus_, _amphitrite_, and in several other species, the common white varieties are eminently deceptive.

Besides the slight variation in the obliquity of the summits of the radii and alæ, dependent on the more or less cylindrical form of the shell, in some species, as in _B. tintinnabulum_, _amphitrite_, _improvisus_, _trigonus_, and _porcatus_, their obliquity also varies occasionally from unknown causes, and thus greatly affects the general appearance of the shell. In some few species, as in _B. perforatus_, the radii are often either not at all developed, or are of very variable width; in others, when the shell has become cylindrical, or when very old, the radii cease to grow, and from the disintegration of the whole upper part of the shell, with the continued growth of the lower part, the radii at last come to exist as mere fissures: I have seen instances of this in _B. psittacus_, _nigrescens_, and _porcatus_. Nevertheless, the obliquity of the upper margin, and the breadth of the radii are useful characters; and still more useful is the fact whether the upper margins are smooth and arched, or straight and jagged. The fact of the terga being more or less beaked is useful: as is, likewise, the presence of striæ, or furrows, or rows of pits, radiating from the apices of the scuta; but to ascertain the presence of these marks, it is almost invariably necessary to take out the scuta, clean, and examine them with a lens; these ridges and furrows, moreover, in some species, as is strikingly the case with _B. tintinnabulum_, and in less degree with _B. trigonus_, _lævis_, and _concavus_, appear and disappear, and vary without any apparent cause.

Now if we reflect that form, size, state and nature of the surface, presence of epidermis, relative size of the orifice, presence of longitudinal ribs, tint, and often the existence of any colour, are all highly variable in most of the species; and that the obliquity of the summits of the radii, and the presence of longitudinal striæ on the scuta, are variable in several species; we shall perceive how difficult it must ever be to distinguish the species from external characters. As some evidence of this, I may mention that, after having described nearly 40 species, and when my eye was naturally able to appreciate small differences, I began carefully to examine varieties of _B. tintinnabulum_, _amphitrite_, _improvisus_, _porcatus_, _vestitus_, &c., without even a _suspicion_ that they belonged to these species, at that time thoroughly well known to me; yet in the cases here referred to, there could be no doubt, when a perfect series was examined, that the specimens were only varieties. From this cause the labour of naming a collection is great. Let no one attempt to identify the species of this genus, without being prepared to disarticulate, clean, and carefully examine with a microscope the basis and parietes, and both the under and upper surfaces of the opercular valves; for I feel convinced, that he will otherwise throw away much labour. Moreover, in many cases, it is almost necessary, on account of the variability of the characters, to possess several specimens. From these facts, I have not hesitated to form my sections on characters which require close examination, though I would gladly have seized on external characters, could I have found such even moderately constant.

The least varying, and therefore most important characters, must be taken from the internal structure of the parietes, radii, and basis: not that these characters are absolutely invariable; thus the porosity of the parietes is slightly variable in _B. glandula_, and highly variable in the fossil _B. unguiformis_; it is also highly variable in _B. balanoides_, but under a systematic point of view this is unimportant, as the section including this latter species is well defined by the membranous basis. The porosity of the basis is in some degree variable in _B. nubilus_, _improvisus_, and _patellaris_; and in _B. flosculus_ we see signs of a passage from a calcareous to a membranous basis. Characters derived from the general shape, and from the ridges and depressions on the under side of the scuta and terga, especially of the scuta, are highly serviceable. The terga, indeed, in many species, as in _B. amphitrite_, vary considerably, and are affected by the general shape of the shell. Unfortunately the differences are not very great between the scuta of the different species. The cause of the opercular valves offering more useful characters, as far as outline is concerned, than do the walls of the shell, is no doubt due to their being almost independent of any influence from the nature of the surface of attachment. Even the ridges and depressions on the under side of the scuta, which are in direct connection with the muscles and soft parts of the animal, vary to a certain extent: thus the length and prominence of the adductor ridge is decidedly variable in _B. concavus_ and _tintinnabulum_, and in a less degree in _B. lævis_; the size and form of the little cavity for the lateral depressor muscle varies in many species; so does the exact shape and degree of prominence of the articular ridge. There is one character in the terga, which at first would be thought very useful, namely, whether an open longitudinal furrow, or a closed fissure runs down the valve from the apex to the spur; but it is found that the furrow almost always gradually closes up during growth; and as a consequence of this, the width of the spur compared to that of the whole valve, as well as its distance from the basi-scutal angle, and the form of its basal extremity, all vary in some degree. The length of the spur sometimes varies considerably, as in _B. concavus_ and _amphitrite_. The parts of the mouth are only occasionally serviceable; for the teeth on the labrum, and the state of the lower teeth on the mandibles, and the presence of a step-formed projection at the lower angle of the maxillæ, are all often variable. The relative lengths of the two rami of the first pair of cirri, the degree of protuberance of the segments, and the number of pairs of spines on the segments of the posterior pairs of cirri, are sometimes useful; but the relative lengths of the cirri, and more especially the numbers of pairs of spines on the posterior cirri, are apt to vary. Finally, I must express my deliberate opinion, that every part and organ, internal and external, in Cirripedes, is liable to some amount of variation in some of the species.

I must now point out the principal changes which supervene during growth, and which cannot properly be called variations. In the first place, I think, it is scarcely possible to recognise a species when under the 1/10th of an inch in diameter. In some cases, as in _var. d'Orbignii_ of _B. tintinnabulum_, the shell is invariably coloured when old, but quite white when very young. Generally the tints become very much darker with age. Some species, which usually or invariably have, when mature, longitudinally folded walls, as _B. flosculus_ and _balanoides_, are perfectly smooth in youth. The walls in _B. eburneus_, when young, have white, hyaline, longitudinal lines, and are naked; whereas, with advancing age, these lines disappear, and the subsequently formed shell becomes covered with membrane. The summits of the radii are apt to be oblique in the young of _B. Capensis_, _psittacus_, and _tintinnabulum_, whereas they are generally quite square in old specimens. In _B. eburneus_, _cariosus_, and in a lesser degree in _B. psittacus_, the scuta become longitudinally striated only with age. On the other hand, in very young specimens of _B. tintinnabulum_, the scuta sometimes are deeply impressed by little pits placed in rows. I have already alluded to the longitudinal furrow on the tergum so entirely changing its character, owing to the edges becoming, during growth, folded inwards; this likewise causes a decrease in the proportional breadth of the spur. In old specimens of _B. flosculus, var. sordidus_, the whole tergum is much more elongated than in young specimens. The basal margin of the sheath is hollow beneath in the young of _B. cariosus_ and of some other species, but in the old it is continuous with the inner surface of the walls. The inner lamina of the parietes generally loses, to a certain extent, its longitudinally ribbed character in old age. The basis is solid, instead of being porose, in very young specimens of _B. improvisus_. In all the species, the carino-lateral compartments, in early age, are very narrow in proportion to the width of the lateral compartments; and in all, at this early period, the operculum is large in proportion to the whole shell. The number of spines on the edge of the maxillæ, the number of segments in all the cirri, and the number of spines on each segment, are few in early youth, and go on increasing with each successive exuviation: the pedicels of the cirri are long in proportion to the rami at this same early age.[84]

[84] In some specimens of _Balanus perforatus_ I made the following enumeration of the number of segments in the cirri:--

+--------------+--------------+--------------+------------------+ | |Basal diameter|Basal diameter| Medium sized | | | of shell | of shell | specimen about | | | 1/20th of | 1/5th of |3/4ths of an inch | | | an inch. | an inch. |in basal diameter.| +--------------+--------------+--------------+------------------+ |First cirrus }| ? | 11 | 17 | |shorter ramus}| | | | | | | | | |Second cirrus | 4 or 5 | 9 | 13 | | | | | | |Sixth cirrus | 9 or 10 | 19 |31, in another 36 | +--------------+--------------+--------------+------------------+

In the specimen 1/5th of an inch in basal diameter, each segment of the posterior cirri carried five pairs of spines; whereas, in full-grown specimens, there are six or seven pairs. In the 1/20th of an inch specimen, on the inner maxillæ, there were no spines between the upper large and the lower large pair of spines; whereas, in the 1/5th of an inch specimen, there were five intermediate spines, and in larger specimens nine or ten spines.