Chapter VIII: Introduction (4)

_Movements and Muscles of the Cirri._

Although the cirri have not been described, it will be most convenient here to treat shortly of their muscles. Each cirrus consists of a pedicel, having a long basal and a short upper segment, supporting two multiarticulate rami. The lower segment of the pedicel can be drawn forward by an adductor muscle, attached low down within this segment, and crossing at right angles (at least in the case of the anterior cirri) the corresponding muscle of the opposed cirrus, on the central, ventral surface of the thorax. This segment can also be drawn back by a muscle springing from the dorso-lateral surface of the thorax, and running only a little way within the segment: I am far from sure that the lower segment does not possess other muscles. The short upper segment of the pedicel can be moved backwards and forwards, as I saw in living specimens, independently of the lower segment; this movement being best seen in the anterior cirri, which are much more often moved independently of each other than are the posterior cirri. The rami are capable, I believe, of being moved backwards and forwards _as a whole_, by the movement of the few lower segments, which are generally more or less confluent. They can, also, be curled up and uncurled by the combined movement of each separate segment. The uncurling seems to separate the two rami a little laterally. Each ramus, at least in the two or three anterior pairs, can be moved to a certain extent, independently of the other ramus of the same cirrus; and the few terminal segments, either of both rami or of one ramus, are often a little moved and curled (and this is especially the case with the long anterior ramus of the first pair), without the lower segments or the pedicel being moved.

The flexor and extensor muscles, which, as I believe, move the upper segment of the pedicel (_a_ and _b_, Pl. 29, fig. 1), are attached at their upper ends to its basal margin, and are thus enabled to draw it a little way down within the lower segment, and so move it. The short flexor muscle (_c_), which is attached at its lower end within the upper segment of the pedicel, and the longer extensor (_d_), also, attached within this same lower segment, serve, I believe, to move the lower, partially confluent segments of each ramus as a whole. In the case of these muscles, and of those last mentioned, I am surprised that the extensors (_b_) and (_d_) are not attached nearer to the exterior and dorsal surface. Other muscles (_e_, _f_) attached at their lower ends within the upper segment of the pedicel, run up each of the two rami to their tips, with some of the fasciæ terminating within each segment: of these muscles, the outer one (_f_, _f_) appears to be the extensor, and the inner one (_e_, _e_) the flexor. But besides these, there are other short flexor muscles (_g_, _g_) which run on the anterior face,[31] from segment to segment, serving to pull the front edge of one segment within the edge of the next lower segment. These muscles differ much in plainness in the several genera: they are very distinct in Coronula. In some specimens of this genus, a few of the articulations between the basal segments of the rami having been obliterated, the short muscles (_g_, _g_) running from articulation to articulation were absent, and their presence and nature in the upper segments thus rendered the plainer. The muscular system in the several pairs of cirri seems to be the same, with the exception of the first pair, in which the muscle answering, as I suppose, to (_a_), namely, the flexor of the upper segment of the pedicel, is much spread out at its lower end, and is there attached to the exterior surface of the lower segment.

[31] For a considerable time I thought that there were muscles
going to the spines, especially to those which arise from the upper
dorsal edge of each segment; but I have since ascertained that
these are the cases within which new spines, with their lower ends
doubled like the fingers of a glove hastily pulled off, are in
process of formation.

The backward and forward movements of the segments, both in the rami and in the pedicels of the cirri, are apparently effected, as already noticed, by the outer or inner (as the movement may be) basal edge of one segment being drawn a little way down within the next succeeding lower segment. If, at the same time, both the inner and outer margins of all the segments were drawn one within the other, the whole limb would necessarily be shortened; and I distinctly saw a shortening action, with very slight movement in any other direction, in the first and second pairs of cirri; and I think it almost certain that this movement might be performed by the other cirri. If I correctly understand a statement of Milne Edwards,[32] this is an important fact, as he asserts that only the higher Crustaceans possess the power of shortening their limbs.

[32] 'Annales des Sciences Naturelles,' tom. xviii, 1852, p. 121.

When a Cirripede is alive, the action of the cirri is really beautiful: from the position of the thoracic segments, the posterior cirri (three pairs in the Balaninæ and four pairs in the Chthamalinæ) form a sort of semicircle facing the mouth: the anterior cirri stand further apart, and are opposed in pairs to each other, with the first pair pointing beyond the mouth. Together the cirri form a hollow cone, not circular but elongated, with the mouth situated at the lower anterior end. The posterior cirri are protruded, by the movement of the whole thorax, curled up, close along the carinal end of the orifice; as they are protruded, they diverge, both by the movement of their pedicels, and, as I believe, by the separation of the thoracic segments. As the two rami of each separate cirrus are uncurled, they also diverge a little; as do the double rows of spines on the segments in each ramus, by their elasticity. By the movement of the thorax, the cirri are then swept towards the rostrum; and, lastly, they are brought perpendicularly down towards the mouth with a rapid movement, which would be beautifully adapted to catch any object floating or swimming in the water; hence I have called the action of the cirri, captorial. When the shell of a Balanus is broken open, the second and third pairs of cirri are repeatedly clasped over the mouth with a convulsive movement, in a manner indicating, I think, that their chief function is to seize and carry to the mouth any object entangled by the sweeping movement of the three posterior pairs. The first pair is also well adapted to aid in this seizing action; but I suspect that the long anterior ramus likewise acts as an organ of touch, warning the animal of danger. The mouth being itself moveable as a whole,--the outer maxillæ being capable of a backward and forward sweeping action, and being furnished with orifices apparently olfactory,--the inner maxillæ having more diversified movements,--the toothed mandibles overhanging the œsophagus,--and the œsophagus itself possessing a powerful swallowing movement, are all admirably adapted to secure any prey, when once entangled by the cirri.

_Mouth._

The mouth, in the sub-family Chthamalinæ, cannot be distinguished from that of the Lepadidæ, which has been pretty fully described in my former volume. In the Balaninæ, however, the labrum differs considerably in not being swollen; that is, in its outer and inner fold of membrane being close together, and in having a central notch: the palpi are also larger, and the lower teeth on the mandibles, are laterally (Pl. 26, fig. 5) double, as will be more particularly stated under these two sub-families. I have given a drawing (Pl. 26, fig. 1) of the mouth, seen from above, of _Balanus perforatus_, with the right-hand palpus (_d′_) and outer maxilla (_a′_) cut off, in order that the labrum (_e_), mandibles (_c_), and inner maxilla (_b_) might be better shown; the cut-off bases (_x_, _x_) of the first cirrus on each side are also shown. In fig. 2 we have the deep supra-œsophageal cavity in _Bal. improvisus_ torn open and laid flat, with the inner surfaces of the labrum (_c_) and outer maxillæ (_a_) exhibited, the palpi, mandibles, and inner maxillæ having been removed. Figs. 3 and 4 will presently be referred to; they are parts of the mouth, with the muscles, &c. removed, of Coronula. The mouth differs extremely little in the different genera and species of the Balanidæ, much less than amongst the Lepadidæ. In the Balaninæ, the crest of the labrum is sometimes hairy, instead of having, as is usual, from two to six teeth on each side of the central notch: in _Balanus improvisus_ (Pl. 26, fig. 2) and _eburneus_, and in _Chelonobia_, the crest on each side of the central notch (_e′_) is furnished with a row of finely graduated teeth. A sub-triangular portion of the inner fold of membrane of the labrum, which overhangs the œsophagus, is always thickened and yellowish; it is also often punctured in patterns (Pl. 26, fig. 2, _f_), which, I believe, give attachment to little muscles that serve to open the upper end of the œsophagus. Opposite to this thickened, sub-triangular portion of membrane, the thin membrane forming the supra-œsophageal cavity (or the cavity surrounded by the gnathites) is strengthened by a pair of curved ribs (_h_, fig. 2) of thickened yellowish membrane, running down from the inner bases (_a′′_) of the bilobed outer maxillæ to the opening of the œsophagus (_g_): a broad branch from each of these ribs supports the sides of the orifice of the œsophagus; and this branch almost joins on to a slightly thickened rim or bar (_f′_), which branches off from the upper part of the sub-triangular (_f_) inner fold of the labrum. This structure, in _Bal. improvisus_, is represented in Pl. 26, fig. 2, as well as it could be, considering that the deep supra-œsophageal cavity has to be torn open; and then laid flat.

The _Palpi_ (Pl. 26, fig. 6) differ little, except in size, in the different genera, being squarish, more or less elongated, or even approaching to club-shaped: in most of the Balaninæ they are larger even than the mandibles, of which they normally form a part. Their upper margins, especially towards their free extremities, are always thickly clothed with spines; and there is generally a single row, either short (_r_) or long, of spines of greater length, which arise from a little above, and stand almost in a parallel line to, the basal margin. On the internal surface, there is sometimes a row (_t_) of very short little spines, which overhang the crest of the labrum. The _Mandibles_ (Pl. 26, fig. 5) have from three to five teeth; the lower point or angle is generally pectinated. In Coronula and its close allies, there are some small teeth intermediate between the four or five main teeth; and in these genera, though members of the sub-family Balaninæ, the lower teeth exhibit only rudiments of being laterally double.[33] The _Maxillæ_ sometimes have a notch under the upper large pair of spines, and in _Octomeris brunnea_ there is a double notch: in many species of Balanus, the inferior corner stands up like a step (Pl. 26, fig. 7, _a_): in many other genera and species, the whole edge is straight. In all, or almost all cases, the row of spines on the middle portion is double. The _Outer Maxillæ_ are always bilobed on their inner faces, and are clothed with bristles. On all the gnathites, the bristles are often doubly serrated.

[33] M. Martin St. Ange describes, in his 'Mémoire sur
l'Organisation des Cirripèdes,' pp. 15 and 32, "_une petite
langue_" in the mouth of Lepas; but I may venture to assert that
such does not exist; it is merely the point of union between the
outer maxillæ. M. St. Ange, in his comparison of the mouth of
Lepas with that of Phyllosoma, compares the mandible of the latter
with the palpus of Lepas; the first maxilla of Phyllosoma with the
mandible of Lepas; and so on with the other gnathites.

_Muscles and functions of the Gnathites, and their confluence._--The outer maxillæ appear at first like a deeply-lobed lower lip, for they reach over almost to the labrum (Pl. 26, fig. 1), and thus partially cover the other organs; they are separately capable of a strong and rapid, to and fro movement, by which no doubt they sweep any prey, entangled by the cirri, towards the other gnathites. Each outer maxilla is furnished with a pair of muscles, apparently a flexor and extensor; there is also a little muscle between the two maxillæ, I presume for the purpose of bringing them together. The outer and inner maxillæ generally stand close together, and in several genera a little way apart, from the mandibles; but there is no trace of any labrum or true lower lip, bounding the mandibles and orifice of the œsophagus. The outer and inner maxillæ and mandibles are not opposed in pairs to each other, but against the thickened inner fold of the labrum; almost in the same manner as the posterior pairs of cirri are not opposed one to the other, but to the mouth.

I have described pretty accurately the muscles of the mandibles in my former volume, and there given a drawing (Pl. 10, fig. 1) of them. There are four muscles: first, the depressor muscle, which is the largest, and is attached, at its upper end, to ligamentous apodemes under the free toothed portion of the jaw; and at its lower end, spreading considerably out, is attached to a concavity close above the basal margin of the labrum; to understand the action of these muscles, it should be borne in mind that the mandible almost faces the labrum. In some genera, as in Coronula,[34] the swelling near the basal margin of the labrum (Pl. 26, fig. 3, _k_), caused by the internal concavity for the above muscle, is conspicuous. The depressor muscle is opposed by a small elevator, attached to the mandible close by the depressor; thence it runs upwards, and is united at its upper end to the base of the palpus, at the point where the latter adheres to the labrum: I have ventured to call this muscle the elevator, from being apparently so well fitted for this purpose; but I feel some little doubt, from having observed an apparent slight movement in the palpi of living Balani; and this is the only muscle entering those organs. The free part of the mandible is articulated on a square, thickened piece of membrane, forming part of the side of the mouth (Pl. 26, figs. 3, 4, _c^1_; and Pl. 10, fig. 1, _a_, _b_, in my volume on the Lepadidæ); to this square piece of membrane, two short muscles are attached, one above the other, and which ought, in the Plate in my former volume, to have been represented crossing the depressor muscle at nearly right angles; at their further ends they are attached to about the middle of the labrum, where, at least in Coronula (Pl. 26, fig. 3, _i_), a slight concavity can be detected. The action of these two muscles must be to draw the whole mandible against the labrum; and the depressor muscle might, at the same time, draw the toothed edge downwards, and thus force any prey into the œsophagus.

[34] This is figured by Burmeister in his 'Beiträge zur
Naturgeschichte der Rankenfüsser,' Tab. 2, fig. 6.

The inner maxillæ are likewise furnished with four muscles, very nearly as figured in my former volume (Pl. 10, fig. 10); namely, two muscles, one inside and the other outside the curious apodeme, which in the Balanidæ (Pl. 26, fig. 7, _b′_) is as invariably present as in the Lepadidæ: these two muscles are attached at their lower ends to the outer membrane of the mouth, close to its basal articulation: the outer one of these two muscles would, I presume, act as an elevator, and the inner one as a depressor; the free part of the organ working on the top of the apodeme, like an axe, on a hinge, on the top of a pole. But there is also a larger depressor muscle, in an analogous position with that (_i. e._ the first-mentioned muscle) of the mandibles; and a fourth muscle, crossing the latter depressor at nearly right angles, and attached (as far as I could make out) on the side of the orifice of the œsophagus, close under the mandibles: the action of this latter muscle would be to draw the whole organ towards the labrum.

I must not conclude my description of the mouth, without drawing particular attention to its peculiar compounded nature. It is prominent, and is capable, as a whole, of movement; it is separated from the body by a fold or articulation, which can be traced all round. It is, as we have seen, composed of a broad labrum and three pairs of gnathites, which latter have only their terminal segments free; and these surround a conical hollow, at the bottom of which lies the opening of the œsophagus. The prominence of the whole mouth appears to result from the lateral fusion of the two basal segments of the three pairs of gnathites. I have examined the mouth of ordinary Crustaceans, and can see no trace of a structure like this. That there has been some union of the parts is indisputable; for the palpi, which in ordinary Crustaceans are quite free, are here firmly united to the upper and outer corners of the labrum; and indeed, at first appear to be more intimately connected with the labrum than with the mandibles. The palpus on its upper and exterior surface, is in direct continuity with the square thickened piece of membrane, on which the mandible is articulated, and likewise with that side of the upper or free portion of the mandible which faces the labrum. This face of the mandible, beneath the toothed edge, is hollowed out or arched (Pl. 26, fig. 5, _p_), owing to the above-mentioned continuity of its membrane with that of the palpus. On the lower surface, the palpus is firmly united to the lateral corners of the labrum; or indeed the corners of the labrum may be almost said to be formed by the soft, swollen bases of the palpi: the point of union, when viewed from the outside, is seen to form a knob on the shoulder of the labrum, beneath the level of its crest, and at this knob (Pl. 26, fig. 3, close to _d′_) several thickened bands in the surrounding membrane unite. The free portion of the palpus stands out transversely behind (_i. e._ anteriorly to, in a homological sense) the labrum. I suspect that the palpus possibly may consist of two segments, of which the terminal one is free, and the lower one confluent with the labrum.

Before proceeding any further, I should observe that figs. 3 and 4, in Pl. 26, represent the membranes of the mouth of _Coronula diadema_, perfectly cleaned. In fig. 3, all the front part of the mouth has been removed, the mandible on one side, the labrum with the two palpi, and the œsophagus being alone left, and these are viewed from the inner side; the front part, however, of the supra-œsophageal cavity has been cut away. In fig. 4, the labrum, with the œsophagus, has been removed, whilst the two outer maxillæ, the right-hand inner maxilla and mandible (with the exterior and basal portions, _d_, _d′′_, of one palpus) are seen from the outside; but in order that these parts should all be shown, the whole of the right-hand side of the mouth has been spread out, for the teeth of the mandible should have stood in a vertical line between the two outer maxillæ. In the mandibles, the free upper part is separated, by a distinct articulation, from the square piece of thickened membrane (fig. 3, _c^1_) on which it is supported; and this latter is separated by a second articulation from a portion of thickened membrane (_c^2_), the basal edge of which forms the third and lowest articulation, separating the mouth from the body. This basal, thickened portion of membrane curls round and inwards, towards the outer maxillæ or front of the mouth, and its terminal points sometimes even penetrate a little way within the muscles, like apodemes: it is not distinctly separated by any line or suture from the membrane, which forms the whole broad labrum; so that I at first concluded that the labrum dipped under the mandibles, and thus afforded a support on which they were articulated; but this appears so opposed to all analogy, that it is more probable that the above basal thickened portion of membrane is truly the basal segment of the mandibles, completely confluent with the labrum; and it is, I think, not very improbable that even a large portion of what in appearance belongs to the labrum, namely, those concavities to which the muscles of the mandibles are attached, may, also, be part of the basal segment of the mandibles. Whether or no there really are two segments beneath the upper free portion of the mandibles, which have become laterally confluent with other parts, I must think that the square thickened piece of membrane (_c^1_) represents at least one segment. I may here observe, that Prof. Milne Edwards seems to consider the mandible of the higher Crustaceans as answering homologically to the haunch of the leg; but, according to M. Brullé,[35] there ought to be two basal segments (sous-maxillaire and maxillaire) bearing the proper mandible, and giving rise, on the outer side, to the palpus,--a structure which perfectly corresponds with my view of the mandible and palpus in Cirripedes.

[35] 'Annales des Scienc. Nat.,' 3d series, Zoolog., tom. ii, p.
271.

_Maxillæ_: the point whence the long apodeme (_b′_, Pl. 26, fig. 4 and fig. 7) arises, according to Audouin's views, must mark an articulation, and this would separate the upper free segment from the lower segments, which I believe to be laterally confluent with the organs on each side. The thickened membrane, of which the upper free part is formed, extends a little distance beyond the insertion of the apodeme; and this small portion beneath the point of insertion may possibly answer to the square, thickened piece of membrane, or second segment, supporting the mandibles. Beneath it, a rather wide expanse of thin, flexible membrane reaches down to the basal fold surrounding the mouth, and may thus form the third segment.

_Outer Maxillæ_: the upper free segment has a spinose lobe (_a′′_, Pl. 26, figs. 2 and 4), on its inner face, which may indicate a lower and second, almost free segment. Passing over this, we have, on the outside of the mouth, beneath the free, upper segments, an expanse of membrane, which, on the side, close to the inner maxillæ, is perforated (Pl. 26, fig. 4, _n_) by orifices which I believe are olfactory. In some species, as in _Bal. eburneus_ and _improvisus_, there is a longitudinal medial suture in this expanse of membrane, which I suppose indicates the lateral confluence of the middle segments of the two outer maxillæ. A short, transverse articulation or fold separates this middle segment (fig. 4, _a^1_) of each maxilla from the third or basal segment; and this latter (_a^2_) is separated from the body by a very distinct fold, which (at least amongst the Lepadidæ) sends inwards a short, medial, tongue-formed apodeme. Here, then, we apparently have, as in the mandibles, two segments under the upper free segment of each outer maxilla, laterally confluent with the adjoining organs. But I must state that, in old specimens, and only in old specimens of _Coronula diadema_, I have found under the outer maxilla an additional transverse ridge and fold, which plainly shows how easily a mere thickening of the membrane might be mistaken for an articulation. I can, however, hardly persuade myself that the articulated membrane, under the free part of the mandibles, which has now been figured and described, has no homological signification; and the fusion of the palpus and labrum seems too plain to be mistaken. Hence I must conclude that the mouth, in the Cirripedia, does truly exhibit a compounded structure of a very peculiar nature.

_Cirri._

There are always six pairs; each biramous and multiarticulated, supported on a pedicel formed of two segments. A shield-like swelling at the exterior bases of these pedicels often appears like another segment; but such, I believe, is not its nature. The five posterior pairs answer to the five pairs of ambulatory legs in the higher Crustaceans; and as in the case of the latter, the three, or the four hindermost pairs almost invariably resemble each other. The first pair, which is homologous with the outer maxillipeds of ordinary Crustaceans, is separated by an interval from the second pair;--though this is not the case with the legs of the pupa, from which the cirri are metamorphosed. These anterior cirri are attached to the lateral edges of the mouth, namely to the thickened rim of membrane, forming the supposed basal segment of the mandibles. They are capable of more diversified movements than the other cirri: the anterior ramus is always elongated, with the terminal segments more or less tapering, and is directed beyond (or anteriorly to) the mouth: the shorter ramus closely resembles in structure the rami of the second pair. In the Chthamalinæ the second pair, and in the Balaninæ the second and third pairs (as will be more particularly described under these sub-families) differ in structure from the posterior pairs, from which they are separated by a slight interval. The number of segments on the posterior cirri is often great, amounting in Chelonobia even to fifty. Each segment normally is furnished on its inner face, which is usually somewhat protuberant, with from two to rarely eight or ten pairs of long spines or bristles, placed in a double row; the two spines in the lower pairs stand nearer to each other, and are shorter than the spines in the upper pairs. Between each pair of spines there is either a single, very thin bristle, or often a tuft of such. The pairs are directed somewhat upwards, and they diverge when the cirri are uncurled; their function is obviously to entangle the prey. On the dorsal or exterior surface of each segment, close to its upper margin, there is a tuft of spines, often composed of thicker and thinner spines; these, I believe, serve to prevent any creature intruding within the sack. On both sides of the upper margin of each segment, there is generally a row of short, blunt, excessively minute spines, which only deserve notice, inasmuch as it is by their increase in number and size, and by the spreading out of the dorsal tufts, and, lastly, by the increase of the little tuft intermediate between the pairs of spines situated in front, that the segments on the two or three anterior pairs of cirri become covered, like brushes, with bristles. The bristles or spines on the second and third cirri are often, especially in Tetraclita, doubly and coarsely pectinated. The bristles on the pedicels follow the same arrangement as on the rami; namely, being in regular pairs on the posterior cirri, and crowded thickly, like a brush, on the anterior cirri. The segments in the shorter ramus of the first cirrus, and in both rami of the second, and often of the third cirrus, are broader than the segments of the posterior cirri; they are, also, especially in the genus Balanus, frequently produced in their upper, ante-lateral corners, into remarkable prolongations (see Pl. 29, fig. 4, of the third cirrus of _Bal. perforatus_), clothed on their inner surfaces, and at their extremities, by numerous bristles. The number of the segments in each cirrus is in some degree variable, and increases with age; this is likewise the case, to a certain extent, with the number of the spines borne on each segment.

As compared with ordinary Crustaceans, I presume the two rami answer to the "_tige_" and "_palpe_" of Milne Edwards; and the pedicel (as I have called it) to the two basal segments of the leg.[36] The "_fouet_" or flabellum does not appear to be developed in any Cirripede; for though the filamentary appendages in certain genera of Lepadidæ, might at first be thought to be of this nature, yet their usual position _beneath_ the basal articulation of the first pair of cirri, and the occasional presence of more than one, proves, I think, that such is not the case.

[36] According to this author's new nomenclature, the pedicel would
consist of the coxopodite and basipodite; the tige would be the
ischiopodite and following segments; and the palpe would be the
exopodite; the epipodite or flabellum being absent. ('Annales des
Sciences Naturelles,' tom. xviii, 1852.)

Though the structure of the cirri is very uniform, yet we meet with some peculiarities. In Chelonobia, the segments of the posterior cirri bear only two pairs of main spines; whereas in some varieties of _Balanus balanoides_, they carry as many as ten pairs in a longitudinal row; but in this latter species, the number of these spines varies, in a singular manner, from six to ten pairs. In Tubicinella, the pairs of spines on the segments of the posterior cirri are arranged so closely one under the other, that they appear almost like a single transverse row. Considering the whole family, the third pair of cirri differs most in structure in the different genera. Thus, in _Chthamalus antennatus_, the anterior (or outer) ramus (Pl. 29, fig. 3) is thicker and much longer than the posterior (or inner) ramus; the number of the segments in one instance being, in the two rami, 53 and 18; in the longer ramus, the spines are arranged abnormally, tending to form a little circle round each segment; and the whole ramus may be said to be antenniformed, and I believe acts as an organ of touch: the relative number of the segments, I may add, in the two rami and the arrangement of their spines varies greatly in this species. In two other species of the same genus Chthamalus, we have _occasionally_ the anterior ramus in some degree antenniformed, so that this whole structure is variable. In the allied _Chamæsipho columna_, it is the posterior or inner ramus which is antenniformed, but this peculiar development is more plainly marked in the case of the second pair of cirri than in that of the third pair. In _Tetraclita porosa_ it is, also, the posterior ramus of the third pair which is antenniformed; in this third pair, and indeed in the other cirri, the relative numbers of the segments vary extremely. A similar structure in the third pair, but in a lesser and variable degree, may be observed in some of the other species of Tetraclita. In _Balanus vestitus_, also, we have, in the third pair, an analogous structure. It is scarcely possible to believe that the circumstance of the second pair of legs, which answer to the third pair of cirri, being antenniformed in certain decapod Crustaceans, is an accidental coincidence; it must be owing to some special affinity in the two groups.

In Chelonobia, the third pair of cirri is of unusual length compared with the second pair, but does not otherwise differ from the type of its sub-family: in Coronula and its allies, on the other hand, the third pair is very short and broad, as may be seen (Pl. 29, fig. 5) in Xenobalanus: in this latter genus, the front surfaces of the segments of the pedicels (fig. 6) of the posterior cirri, are extremely protuberant, almost as in _Scalpellum vulgare_.

The last peculiarity in the cirri at all worth mentioning, is in the sub-genus Acasta, in which, differently from in all other known Cirripedes, the anterior ramus of the fourth pair does not absolutely resemble the rami of the fifth and sixth pairs; in most of the species, the spines on this anterior ramus are more crowded together, are larger, and are mingled with some short thick points; and the spines in the dorsal tufts are also longer than in the two posterior pairs of cirri; but in _A. sulcata_ (Pl. 29, fig. 2), and in a lesser degree in _A. cyathus_ and _A. purpurata_, the front margins of the lower segments of this anterior ramus, and of the upper segment of the pedicel, are developed into strong, downwardly curved teeth: it is very remarkable that so beautiful a structure should be extremely variable, as it certainly is in _Acasta sulcata_.

_Caudal Appendages._

With extremely few exceptions, these are present in all the Lepadidæ and Verrucidæ; whereas amongst the Balanidæ they occur only in the two species of Pachylasma, and in one species of Catophragmus; these being the genera most closely allied to the Lepadidæ, and where, consequently, their presence might have been anticipated. These appendages are seated close together over the anus; they are multiarticulate, each segment being sub-cylindrical, with a few small bristles round its upper edge.

_Alimentary Canal._

I have not much on this head to add to what I have said under the Lepadidæ. As in that family, the strong internal membrane of the œsophagus terminates in a remarkable, bell-shaped expansion (Pl. 26, fig. 3, _g′_), which, as observed by M. St. Ange, serves to keep the upper broad end of the stomach expanded. The œsophagus is well furnished with constrictor and radiating muscles for closing and opening it; and it is thus capable of a strong swallowing movement. The stomach runs down to the lower end of the prosoma, and then doubling back on itself extends to the anus. As the prosoma is much elongated in Tubicinella and Xenobalanus, so is the stomach of unusual length in these genera. In several species of Balanus, the upper edge of the stomach is surrounded by from six to eight cæca; these cæca I ascertained, in _Balanus perforatus_, are branched, and penetrate a considerable way into the body; and some of them at least expand a little at their extremities. Each cæcum, from the manner in which it retained fluid, must, I think, be furnished, at the point where it enters the stomach, with a sphincter muscle. In Tetraclita, Chthamalus, Tubicinella, Coronula, and Xenobalanus, there are no cæca; but in Xenobalanus and _Coronula balænaris_, there are longitudinal, approximate folds in the upper, broad end of the stomach, which would serve to expose the food to a greater extent of digesting surface.[37]

[37] The presence and absence of these cæca in genera so closely
allied as Balanus and Tetraclita, shows, I think, that these
cavities are not of high importance; and I must doubt whether Von
Siebold's view ('Anatomie Comparée,' tom. i, p. 445), that these
cæca form a passage to a true or isolated liver, such as exists in
the higher Crustacea, can be admitted. Cæca are said by Von Siebold
to occur in some of the Entomostraca, as Daphnia, Argulus, &c.

As in the case of the Lepadidæ, a transparent, structureless, epithelial tube, composed of chitine, containing more or less digested food, is found, in specimens preserved in spirits, occupying the whole length of the stomach, and where there are cæca, sending branched prolongations into them. It does not extend into the œsophagus or into the rectum. This epithelial tube or model of the stomach, filled with excrement, is expelled by the rectum, whole, that is in a single piece, as I observed in some living specimens of _Balanus balanoides_: in some specimens, however, of _Chthamalus stellatus_, the excrement was ejected, perhaps from the animal being confined, in fragments, and the sack thus became befouled. Beneath the epithelial layer, the stomach is lined by a delicate, pulpy and cellular mucus layer, which easily peels off in flakes: this is surrounded by a muscular layer with the fibres closely approximated and transverse; and this by a layer of stronger, longitudinal muscles, but more distant from each other. Lastly, outside this double muscular layer, there is a rather thick, somewhat laminated, pulpy layer, abounding with cells, often nucleated, and frequently containing much oily matter. This structure agrees closely with Dr. C. H. Jones's[38] account of the external covering of the stomach in Daphnia, and which he believes to be hepatic: as in Daphnia, there does not seem to be any ducts. I may here observe, that within the upper part of the prosoma, but not immediately connected with the stomach, I have often observed much white pulpy substance, permeated by lacunal passages, and exhibiting no structure except some excessively minute cells.

[38] 'Philosophical Transactions,' 1849, p. 116. Karsten ('Nov.
Actorum Acad. Nat. Cur.,' 1845, tab. xx) has excellently figured
the testes, as the hepatic glands; and has indicated the ovaria
as salivary glands; it is singular that this anatomist overlooked
the ducts which lead from his supposed hepatic glands, into the
vesiculæ seminales, within which he observed spermatozoa.

The rectum, lined by membrane continuous with that investing the thorax (and seen through it, in Pl. 26, fig. 8, _c_), extends inwards to about opposite the bases of the third or fourth pairs of cirri. It is longitudinally plaited; the ends of the folds forming a sort of valve where joined on to the stomach. It is coated by circular, transverse muscular fibres: judging from the movements, the anus itself is surrounded by a strong sphincter muscle. The anus opens on the dorsal surface of the thorax (fig. 8, _b_); but as in the genera, in which caudal appendages occur, it opens under them, the orifice, I believe, is homologically terminal, and owes its dorsal aspect to the aborted state of the whole abdomen, and to the great development of the probosciformed penis; for the anus may be said to be situated on the dorsal base of this organ.

Altogether we see that the alimentary canal is of a very simple structure. The food, judging from the contents of the stomach, seems generally to be composed of infusoria and minute animals: but in the case of Tetraclita, I have been surprised at the size and number of the included amphipod, isopod, and entomostracan Crustaceans, in one case, together with an annelid. I have, also, sometimes seen some confervoid matter within the stomach.

_Circulatory System._

On this subject I can add nothing, except to express my conviction that there is no heart, or true vessels; the circulation being strictly lacunal. A passage has often been quoted from Poli, in which he states that he saw a pulsating organ, close above the anus; but I have seen this movement, which appeared to me to be a convulsive twitching of the sphincter muscle of this orifice. The largest lacunal channel extends down the middle of the rostral compartment of the shell: and this answers to the rostral channel down the peduncle in the Lepadidæ. Large nerves and the main pair of unbranched ovarian tubes (Pl. 25, fig. 1, leading into _g_) extend along this channel. At the basis (at least in Coronula) this channel joins on to a large circular one, running all round the sack, and sending off branches into the mass of ovarian tubes and cæca.

_Nervous System._

It has been shown in my former volume, that in Lepas and in some other genera of the Lepadidæ, there are six main ganglions; one supra-œsophageal, and five infra-œsophageal, or thoracic. In Pollicipes, however, there are only four thoracic ganglions, the last ganglion supplying the three posterior pairs of cirri with nerves, whereas in the other genera, the last ganglion supplies only the fifth and sixth pairs of cirri. In this genus, moreover, the lateral fusion of the ganglions has been so complete, that there is no evidence of their having been formed by the union of two. Amongst sessile cirripedes, we discover evidence of much higher concentration even than in Pollicipes. My chief examination has been confined to _Coronula diadema_, and to _Balanus tintinnabulum_: and in these genera we find (and the fact appears to me highly remarkable) as high a degree of concentration in the infra-œsophageal ganglion as in any decapod Crustacean, for instance, as in Maia, judging from the figure given by Milne Edwards; for all the nerves, with the exception of those connected with the supra-œsophageal ganglions, radiate from a single great ganglion.[39] The nervous system is, moreover, otherwise complicated.

[39] It must, however, be observed that, according to Mr. Dana,
there is in certain suctorial Entomostracans, as in Caligus,
only one infra-œsophageal ganglion. Mr. Dana speaks of this as
resulting from reduction. In Cirripedes, from the gradation which
may be observed from Lepas through Pollicipes into Balanus, the
ganglions are certainly not reduced but concentrated. In Van de
Hoeven's figure of the nervous system in Limulus, there is seen to
be no chain of thoracic ganglions; all the nerves rising from the
circa-œsophageal collar; but this, on the other hand, seems hardly
developed into a ganglion.

To begin with _Coronula diadema_ the great infra-œsophageal ganglion (Pl. 27, fig. 1, A) is seated nearly opposite to the anterior margin of the second pair of cirri, which are homologous with the first pair of legs in the decapod Crustaceans. This ganglion shows no trace of any longitudinal medial suture; its shape is hardly discoverable, for it is formed by the union of eleven principal pairs of nerves, besides several arising from its under surface; in outline, however, it may be said to be divided into a posterior and anterior half; the latter being somewhat heart-shaped, and the posterior half elongated. The nerves going to the five posterior pairs of cirri rise from the posterior margin of the ganglion, and run for some distance in a sheet, parallel and close together; the pair, however, going to the second pair of cirri soon branches off from the others. Each of these nerves enters at the inner and posterior margin of the cirrus to which it belongs, and, at least in the case of the first pair, divides into two branches as it enters. The nerves (Pl. 27, fig. 1, _r^5_, _r^6_) going to the fifth and sixth pairs of cirri are more closely united together than are the others, and appear, till they branch off, like a single large nerve. That which belongs to the sixth cirrus gives off, opposite to the fifth cirrus, a branch (_s_) nearly as large as itself, which enters the probosciformed penis. I may remark, that homologically this is the only abdominal nerve in any cirripede of the Order. From the under side of the nerves which run to the five posterior pairs of cirri, small branches are given off, extending dorsally into the thorax.

The anterior end of the great infra-œsophageal ganglion is formed by the union of a set of nerves, extending parallel in a bundle in a directly opposite direction to those running to the five posterior pairs of cirri. These nerves consist of an outer larger pair (_r^1_) entering the first pair of cirri; and within these, and rather dorsally to their roots, we have the circa-œsophageal chord (_c_, _c_), or collar nerve; between the roots of the latter, and on the ventral surface (or near side of the figure), there are three closely united, small pairs, running to the gnathites, and, as I believe, to the olfactory sacks. From the under (or dorsal) surface of the anterior end of the ganglion, two nerves, larger even than the circa-œsophageal chord, and which I shall call the splanchnic pair (_d_, _d_) arise; and the singular course of these nerves will presently be described; between this great pair, there is a single (_b_) medial nerve, which runs down and branches into that large diverging muscle, which is attached to the upper ventral surface of the stomach. Posteriorly to these three nerves, we have two pairs of much smaller nerves (not figured), running dorsally into the body, so that we have seven nerves rising from the under surface of the infra-œsophageal ganglion. I need only further add, that on each side of this ganglion, between the nerves going to the first and second pairs of cirri, there is a moderately sized nerve (_k_), which appeared to run into the muscles of the thorax: a nerve in a similar position is figured by Milne Edwards in Maia.

The circa-œsophageal chord (_c_, _c_) nearly equals in length the whole distance from the centre of the main ganglion to the posterior end of the thorax. This collar bows out on each side, where passing the œsophagus (_œ_), which is seated at its anterior end. From the collar a branch is given off on each side, which I traced as far as between the mandibles and maxillæ; from analogy with other Crustaceans, it perhaps runs to the mandibles. The collar has not a transverse commissure, such as described by Milne Edwards in the Podophthalmia, and as figured by Van de Hoeven in Limulus.

The supra-œsophageal ganglions (B) present a singular contrast with the infra-œsophageal ganglion in their little development, size, or degree of confluence. They lie directly under the basal edge of the labrum. They are laterally quite distinct, and consist merely of a slight enlargement of the circa-œsophageal chord. From the anterior edge of each ganglion, a broad nerve (_f_) extends for some distance in a straight line, and, on close examination, can be seen to be formed of two nerves closely united, of which the inner and smaller one, after a space, appears to cross over the larger nerve: both become at this point tortuous, and, giving off branches (_m_, _m_), form a plexus. The two nerves (_f_) then bend inwards, and almost touching each other, run down, together with the two ovarian simple ducts, along the rostral compartment of the shell. No doubt, if the smaller branches from these nerves could be traced, they would be seen to form a network over the whole sack; and would therefore enclose, as in a cage, the rest of the nervous system. These nerves correspond, I believe, to the two pair of antennular nerves of ordinary Crustaceans, and hence I will call them by this name. Just in front, at the outside corners of the two supra-œsophageal ganglions (B), a branch (_e′_) arises, which I traced to the ends of the adductor scutorum muscle, and to those several muscles which serve to retract the interspace of membrane between the mouth and the adductor.

The pair of great splanchnic nerves above alluded to, which arise from the anterior and dorsal surface of the infra-œsophageal ganglion, are in Pl. 27, fig. 1, _d_ _d_, (and in fig. 2), laid flat; but in nature they first bow outwards, and then, penetrating deeper into the body, approach each other, and running nearly parallel, pass round the lower end of the œsophagus: their course consequently is nearly similar to that of the circa-œsophageal chord, with this difference, that the outwardly bowed portion is situated near the infra-œsophageal, instead of near the supra-œsophageal ganglion. The splanchnic nerve, a little beyond the supra-œsophageal ganglion, joins a plexus (_d′_); and into this plexus another large nerve (_e_) which I will call the supra-splanchnic nerve, sends branches; this nerve takes an almost semicircular bend over the ovarian glands. The supra-splanchnic nerves (_e_, _e_), though appearing to spring from the supra-œsophageal ganglions, do really arise, as may be seen by tracing the constituent fibres, from the circa-œsophageal chord. The plexus (_d′_) lies close to the coats of the upper end of the stomach: several branches, proceeding from it, run further on, but I was able to trace only a few of them: one went (at least in the case of _Balanus perforatus_), to the adductor scutorum muscle: another branch spread out on the flanks of the prosoma: I strongly suspect that one branch goes to the acoustic sack: it appeared, also, as if some of the small branches entered the second plexus (_m_), where the inner antennular nerve and ophthalmic nerve cross over the outer antennular nerve.

I have called the nerves (_dd_, _ee_) splanchnic and supra-splanchnic, from their course and apparent function in supplying the viscera. In the descriptions of the nervous system of other Crustaceans I can find nothing analogous to my great splanchnic nerve (_dd_); the so-called supra-splanchnic nerves (_ee_), which arise from the circa-œsophageal chord, seem to be the analogues of the ordinary splanchnic nerves, though these latter are always described as uniting into a single medial branch. The plexus (_d′_) is the cervical ganglion of M. Martin St. Ange,[40] who has likewise indicated the course of my splanchnic and supra-splanchnic nerves; but the plexus, when viewed as a transparent object, hardly appears to me to be ganglionic in its nature. In my former volume on the Lepadidæ, I quite misunderstood the course of these splanchnic nerves.

[40] 'Mémoire sur l'Organisation des Cirripèdes,' p. 19.

From the commissure between the two supra-œsophageal ganglions, a straight chord (Pl. 27, fig. 1, _g_) arises, which terminates in a small ganglion (C), barely exhibiting traces of being formed of two laterally confluent ganglions. This is the ophthalmic ganglion. The chord connecting it with the two supra-œsophageal ganglions is accompanied by a small nerve (_h_) which runs on to the muscles round the adductor scutorum muscle; the chord is encased by much fibrous tissue, and its dissection is thereby rendered difficult. From the ophthalmic ganglion, on each side, a nerve (_i_) goes forth and crosses the antennular nerve; these, if I could have traced them, would have been found to run, as may be safely inferred from what is known in _Balanus tintinnabulum_, to a pair (D, D) of eyes.

In _Balanus tintinnabulum_, the structure of the great infra-œsophageal ganglion (Pl. 27, fig. 2, A) is essentially the same as described under Coronula. The great pair of splanchnic nerves springing from its under side, are here actually twice as large as the circa-œsophageal chord. The plexus (_d′_) formed by the splanchnic nerve (_d_), on each side, with the supra-splanchnic nerve (_e_), which arises close posteriorly to the supra-œsophageal ganglion, is here much less complicated, but is perfectly distinct; and there was no appearance of the cervical ganglion of M. Martin St. Ange. The chord (_g_) running from between the two supra-œsophageal ganglions to the ophthalmic ganglion, is nearly as large as the double antennular nerve (_f_) on each side of it. The ophthalmic chord (which is accompanied in its whole course by a small branch running to the adductor scutorum muscle) terminates in a small ophthalmic ganglion (C), which seems to be formed by the almost complete fusion of two ganglions. This ganglion is hardly larger than the chord which it terminates: it appeared to me to give rise to more than one pair of nerves, and a single nerve (to my surprise) joined the branch just mentioned, which goes to the adductor scutorum muscle.

From each supra-œsophageal ganglion, two closely united antennular nerves (_f_) extend, of which the inner one crosses over the main or exterior nerve, nearly opposite to the ophthalmic ganglion, and here forms (_m_) a plexus. The structure of this plexus I was not able, any more than in Coronula, to make out thoroughly, but I traced quite distinctly a long nerve (_i_) running from it into what must be considered as the eye. As in the case of Coronula, I traced a nerve on each side from the ophthalmic ganglion into the plexus, where I lost it; and as here in Balanus, I saw on each side of the ophthalmic ganglion a cut off nerve, of about the size of that which runs from the plexus on each side into the eye, I think we may safely conclude that the latter or optic nerve does really arise from the ganglion here called ophthalmic. I may add that the analogy of the nervous system in the Lepadidæ most strongly confirms the view of this latter being the ophthalmic ganglion.

_Eyes and Vision._