Part 11

The optic nerve (_i_), running from the plexus to the eye, is of considerable size; it runs nearly parallel to the main antennular nerve, diverging from it a little. It retains nearly the same diameter throughout; and gives off only one single, small, inner branch. It can be traced beyond the basal edges of the scuta, to just under the upper edge of the transparent opercular membrane, which unites the scuta to the sheath of the rostrum. The nerve itself, at a little distance from its further end, was, in a full-sized specimen, 5/1000ths of an inch in diameter; widening a little, it expands slightly, and abruptly terminates in a circular disc, about 8/1000ths of an inch in diameter, (see Pl. 27, fig. 5). The nerve just beneath this slight expansion, is coated all round by pellets of dark purple pigment-cells, but not actually united into a continuous layer. These pigment-cells are the more conspicuous from the surrounding parts being colourless. I could not make out distinctly any cornea; and I suppose the external transparent membrane, to which the above slight circular expansion is attached, acts as such. This description very closely agrees with that given of these organs in _Bal. rugosus_ of Gould, (_B. crenatus_?) by Dr. Leidy,[41] who first discovered the eye in the adult cirripede, but he did not observe the ophthalmic ganglion. These eyes differ from those in some of the genera of the Lepadidæ, only in the greater length of the optic nerve, and by standing laterally further apart from each other.

[41] 'Proceedings of the Acad. Nat. Sciences of Philadelphia,' vol. iv, 1848, p. 1. I may add that I have, also, observed the supra-œsophageal and ophthalmic ganglions in _Bal. perforatus_.

I may here mention that I tried a few simple experiments on the senses of _Balanus balanoides_, _B. crenatus_, and _Chthamalus stellatus_. I found these three species very sensitive to shadows, that is, to an object like my hand passing even quickly, and at the distance of about a foot, between them and the source of the light.[42] They were indifferent to a gradual change from bright to obscure light; but instantly perceived and drew in their cirri, when my hand was passed between the basin in which they were kept and the window, even when this was tried rather late on a dusky evening; and likewise when my hand was passed between them and a single candle. I took, of course, the precaution of passing my hand in other directions, but this never produced any effect. These species are moderately sensible to any vibration in the vessel in which they were kept, but they were indifferent to noises made in the air, or in the water. I found it impossible to touch, under water, an individual shell ever so lightly with a needle, without all the immediately surrounding individuals, when several adhered together, perceiving it, and retracting their cirri: it made no difference whether the one touched had already withdrawn its cirri and was motionless: from this fact, and from seeing that a similar but slighter effect was produced by touching the rock on which the specimens adhered, I infer that the perception by the others of the one being touched, is communicated by vibration. When an individual was touched under water, not by a needle, but by a pointed camel-hair brush, it generally withdrew its cirri, but the neighbouring specimens took no notice: when touched by a single hair of the brush, no notice was taken, unless the skin of the orifice leading into the sack was so touched. In these trials, it is of course necessary carefully to avoid intercepting the light. I could not make out that cirripedes perceived odours diffused in the water.

[42] I find that this fact was long ago observed by Von Siebold, 'Anatomie Comparée,' tom. i, p. 434.

_Acoustic Organs._

These are situated in the same position as in the Lepadidæ, namely, in a slight swelling on the sides of the thorax (Pl. 25, fig. 1, _d′_) just beneath the basal articulation of the first pair of cirri. The orifice in Tubicinella and Xenobalanus is slightly produced, or is tubular; the free part in the former genus projecting 5/100ths of an inch. The structure of all the parts is essentially the same as in the Lepadidæ, but I think all are proportionally larger. The external membrane of the body is turned inwards at the orifice, as a short flattened tube, which widens considerably (being, in a middle-sized specimen of Coronula, 4/100ths of an inch in width) before it abruptly terminates. The meatus, as I have called the sack-like cavity which encloses the true acoustic sack or vesicle, is formed of pulpy membrane, and is apparently continuous with the corium of the whole body, but by dissection it can be separated entire. The acoustic vesicle is of various shapes, as we shall immediately see; but in all essential respects it is identical with the same part in the Lepadidæ; it is formed of the same peculiar, soft, elastic, brownish, transparent tissue, which seems to be composed of fine, transverse pillars, becoming towards the outside fibrous, and at their inner ends appearing when viewed vertically from above, like hyaline points. In _Coronula diadema_, I observed on the outside of the acoustic vesicle, some excessively minute bristles, only 1/3000ths of an inch in length, seated on little eminences. I examined carefully the contents of the vesicle in this species, in specimens well preserved in spirits, and there was nothing within but a very little, thin, pulpy fluid, and a few yellowish nucleated cells, here and there aggregated into small groups. In Coronula, the flattened acoustic vesicle is elongated, with a somewhat sinuous, but not very irregular margin (Pl. 27, fig. 4), and is without any ridges on the surface; its neck or orifice projects at right angles to the elongated portion, which stands obliquely to the tubular orifice of the meatus. In a moderately-sized specimen of _Coronula diadema_, the elongated portion of the acoustic vesicle was, 6/100ths of an inch in length. In Tubicinella, the acoustic vesicle is heart-shaped, with the neck attached to its broader end; and the surface is covered by zig-zag ridges. In _Balanus tintinnabulum_ (fig. 3), the acoustic vesicle is almost square at the lower end, with the neck placed at one of the upper corners; on the external surface, there is an oblique prominent ridge or fold, which sends off downwards another ridge; its length, in a large individual, was 5/100ths of an inch.

In all these cases, the acoustic vesicle is mainly attached by its neck, to the upper end of the sack-like meatus; but there is likewise a layer of soft, pulpy, cellular matter, slightly connecting that side of the vesicle which is opposite to the neck, with the walls of the meatus or outer sack. The mouth or orifice of the vesicle is closed by a delicate lid or diaphragm, which can easily be separated; and this diaphragm is formed by the expansion of a large nerve, which here abruptly terminates. In a very large specimen of _Coronula diadema_ I clearly made out the existence of this nerve, and traced its course for some distance from the point where the summit of the meatus and the neck of the vesicle are joined together; the nerve first runs posteriorly, and then turns inwards and doubles back or anteriorly; and I clearly followed it to the antero-lateral sides of the uppermost end of the stomach, where it seemed to enter a ganglion, so that I unfortunately cut it off, but found only a slight plexus, with the cut off nerve apparently running onwards with nearly the same diameter. The diameter is great, fully equalling, in its widest part, that of the circa-œsophageal chord; but it is very much flattened, and so has not nearly so much bulk as that nerve. Before it reached the stomach, it gave off one branch, which ran towards the mouth. The only nerves which, from their size, could, I think, be continuous with this from the acoustic sack, are the main branches proceeding from that plexus (_d′_) formed by the interbranching of the splanchnic and supra-splanchnic nerves.[43]

[43] I have always feared that anatomists would reject my view of these organs being acoustic, owing to the absence of otolithes; but I observe that so high an authority as Von Siebold ('Anatomie Comparée,' tom. i, p. 433) does not believe that otolithes occur in the acoustic organs even of the highest Crustacea. He considers an "ampoule volumineuse, a parois mince, remplie d'un liquide transparent," and a "membrane tympanique," though having a fissure in the centre, as sufficient. I may here remark, that the nerve proceeding from the acoustic vesicle in Cirripedes, and apparently running to the splanchnic nerve, may easily be placed in connexion with the antennular nerves, by the second plexus (_m_) in figs. 1 and 2, pl. 27. I should infer from Von Siebold's remarks on his ampoule volumineuse in the higher Crustacea, that my acoustic vesicle answered to the labyrinth in higher animals.

_Olfactory Sacks._

I can add nothing to the account given of these organs under the Lepadidæ: I saw them in all the genera which I examined for this object. In _Coronula diadema_ the orifices are large; they are seated in the usual position (Pl. 26, fig. 4, _n_), in the confluent segments, beneath the free part of the outer maxillæ, and somewhat exteriorly, or as near as possible to the inner maxillæ. In no sessile cirripede are the orifices produced or tubular, as is the case with several genera amongst the Lepadidæ. I failed, as heretofore, in tracing with certainty the nerve, which appears to enter the base of the sack, to its ganglion.

_Male Organs of Generation._

All the Cirripedes of the family we are now describing, are bisexual or hermaphrodite; and no instance has been observed of the presence of males or complemental males. I have very little to add to the observations made by M. Martin St. Ange and R. Wagner,[44] and to those given in my former volume. The testes seem always to be confined to within the thorax, including the prosoma. With their ducts, they resemble club-moss or stag's horns, with the extremities a little enlarged: a figure[45] of a small portion from _Balanus perforatus_ is given in Pl. 25, fig. 2. It is quite surprising how like in structure and appearance the branching ovarian tubes often are to the testes with their ducts; but the latter are of smaller diameter. Two main ducts generally unite just before entering the broad, often reflexed, end of the vesicula seminalis: in _Coronula balænaris_, however, I observed four ducts entering this receptacle. The two vesiculæ seminales, lying within the thorax and prosoma, are usually very long and tortuous: they are formed of a thin inner tunic, which is strengthened by thicker reticulated lines, and of an outer layer of transverse fibres, which are either elastic, or probably muscular, as they serve to expel the contents with force when the end is cut off. The inner tunic is prolonged up the probosciformed penis, at the base of which the two vesiculæ unite.[46] The contents of the vesiculæ are commonly pulpy and cellular; and from the cells the spermatozoa are developed; soon after their development, they are, as it appears, expelled.

[44] The 'Report' on M. Martin St. Ange's memoir was laid before the Academy of Sciences, July 14, 1834, so that I suppose it was read previously to this date. R. Wagner's paper was published in 'Müller's Archiv,' 1834, p. 467. Burmeister's 'Beiträge zur Naturgeschichte der Rankenfüsser,' was published this same year, 1834; so that these three authors published almost contemporaneously.

[45] A far better figure is given by Karsten ('Nov. Act. Acad. Cæs. Nat. Cur.,' 1845, Pl. 20, figs. 2, 3, 4), but under the erroneous supposition that these organs were hepatic.

[46] In _Conchoderma aurita_, the ducts, as shown by Burmeister ('Beiträge,' &c. tab. ii, fig. 17), unite half way up the probosciformed penis.

I have seen the spermatozoa in _Balanus crenatus_, _perforatus_, and _balanoides_, and in _Chthamalus stellatus_. The cells, from which the spermatozoa are developed, and which are often found in vast numbers within the vesiculæ, are on an average about 1/5000th of an inch in diameter. The spermatozoa differ remarkably within the vesicula of the same individual, according to their state of development. I have observed in _B. perforatus_ and in the Chthamalus, that the shortest, and therefore, I presume, the youngest (Pl. 29, fig. 7, _a_), had a globular head with no projection in front: as they increased in size, this head became less in diameter, and a short tapering filament, (_a_, _b_,) like the tail, projected out of it. This anterior filament does not lie in exactly the same line with the posterior filament, which is straight as an arrow. In _Bal. crenatus_, the anterior filament was 1/2000th of an inch in length, and the posterior filament 4/2000th, giving a total length of 5/2000th: in the longest and best developed specimens of _Chthamalus stellatus_, the nodular enlargement was much elongated and spindle-shaped, and not above half the diameter it had in the earliest stage; the posterior filament (measured from the front of the enlargement, this consequently being included) was 5/2000th in length, and the front part only 1/4000th, giving a total length of 11/4000ths of an inch. These observations agree pretty well with Kölliker's;[47] but this author states, that perfectly developed spermatozoa are absolutely without any nodular enlargement: if this be the case, I have never chanced to see the spermatozoa in their perfect condition. Mr. Bate, also, figures some (Pl. 29, fig. 7, _c_) in this state, without any enlargement.

[47] 'Annales des Sciences Naturelles,' (2d series), tom. xix, p. 348. Kölliker refers to Wagner's paper on the same subject, in Wiegmann's 'Archiv,' 1835, part ii, pl. iii, fig. 9. He also refers to Von Siebold's observations. Mr. C. Spence Bate has figured, in the 'Annals and Magazine of Natural History' (vol. viii, 2d series, 1851), the spermatozoa of _Balanus balanoides_, _perforatus_, and of _Verruca (Clitia) Strömia_, and of these I have given copies, Pl. 29, fig. 7.

The probosciformed penis lies adpressed on the under side of the thorax, with its apex generally projecting between the first and second pairs of cirri. It presents the same ringed or articulated structure as in the Lepadidæ: it arises from an unarticulated projection or support, which also forms the posterior border to the anus. This support often terminates, as first observed by Poli, in a very sharp point; but this point cannot be of much functional importance, for though present in _Balanus balanoides_, it is absent in the closely allied _B. crenatus_; in Tubicinella there is only a rudiment of this point; I have not observed it in any member of the Chthamalinæ. The strong, transverse and longitudinal muscles with which the penis is furnished, are attached to this support. The apex or orifice of the penis is, I believe, invariably surrounded by some bristles. Its length varies much, according to its state of contraction or relaxation; and this again, I believe, is dependent on the condition of the male secreting organs. In a small specimen of _Elminius modestus_, the penis was actually thrice as long as the whole thorax, including the prosoma: in Pachylasma and in _Octomeris angulosa_, the penis is very short, being equal only to once and a half the length of the pedicel of the sixth cirrus: in _Octomeris brunnea_, the unarticulated support is much elongated, being as long as the pedicel of the sixth cirrus, in which respect this organ resembles that of _Ibla quadrivalvis_, and of no other Cirripede. From the attachment of the penis at the posterior end and on the under side of the anus--from the position of the caudal appendages (where such occur) over the anus--from the position of these same appendages in the pupa--and lastly, from the position of the papilla-like penis in the abnormal _Proteolepas_, I infer that, homologically, the penis is situated at the apex of the abdomen, on its ventral surface; and that, consequently, this organ cannot be considered as the abdomen itself in a modified condition.

_Female Organs of Generation._

I have scarcely anything to add to the statements in my former volume. These organs consist of the true ovaria, or glandular bodies seated on each side, not far from the basal edge of the labrum; of the main or unbranched ovarian ducts; and of the (Pl. 25, fig. 1, _g_) ovarian branching tubes and cæca. I traced distinctly in Balanus, Tetraclita, and Coronula, the two main ovarian ducts, running from within the prosoma to the layer of inosculating, branching, ovarian cæca[48] which overlie the basis. In _Coronula diadema_ one of these main ducts was 1/100th of an inch in diameter. Though I traced these ducts near to the grape-like, glandular masses,[49] which I cannot doubt are the true ovaria, I did not succeed in tracing them into actual connection. As in the Lepadidæ, these ovarian glands lie on the sides, near the basal margin of the labrum, and almost under, but rather to the outside of the antennular nerves. The branching and inosculating ovarian cæca form a layer, which corresponds with the mass filling up the peduncle in the Lepadidæ. In Tetraclita they do not cover the whole basis, but are confined to the circumference; they, however, likewise extend up between the two layers of corium round the walls of the shell, and chiefly in the interspaces between the depressor muscles of the opercular valves. In Chelonobia, they enter between the radiating septa in the thickness of the walls: in _Coronula diadema_, they extend from over the basal membrane into the six large square chambers (Pl. 16, fig. 7, _v_) separating the radii and alæ: in Tubicinella they are confined to the basis: in Xenobalanus, they form a layer over the basis and likewise round the upper part of the peduncle-like body, which answers to the shell of other sessile cirripedes.

[48] These are well described in Lepas, by R. Wagner, in 'Müller's Archiv,' 1834, p. 467. Von Siebold, I observe, refers to Burmeister as the first author who discovered the ovarian cæca within the peduncle; I had thought that M. Martin St. Ange had a prior claim.

[49] These are obscurely figured by Karsten ('Nov. Act. Acad. Cæs. Nat. Cur.,' 1845, Pl. 20, fig. 1_d_) as salivary glands; they were so considered by Cuvier and M. Martin St. Ange: I may observe that salivary glands have not been positively recognised in any Crustacean.

As after the most careful and repeated examinations of various Lepadidæ, I was convinced that there were no oviducts, so I have come to a similar conclusion in regard to the Balanidæ; the ova being brought to the surface, by the formation of a new membrane round the sack underneath them, and by the subsequent exuviation of the old membrane. The ova are united together by a most delicate tunic investing each egg; the ovigerous lamellæ being thus formed, as in the Lepadidæ. In the cases of _Chthamalus stellatus_, _Balanus balanoides_, and _Platylepas decorata_, I saw a pair of very distinct but fragile lamellæ. In Xenobalanus, the two ovigerous lamellæ form two sub-cylindrical packets, pointed at their lower ends and often cohering. There are no ovigerous fræna, for the attachment of the lamellæ; the ova being sufficiently well retained, as it would appear, by the well-closed shell. I have elsewhere stated my full belief that it is the ovigerous fræna which have been metamorphosed into the branchiæ of the Balanidæ. Most sessile cirripedes breed when very young; and I have every reason to believe that they breed several times in the year. The ova are ovate, and vary in length from 14/2000th of an inch in Chthamalus, to 19/2000th in some species of Balanus, in which this greater length was owing to a more elongated shape,--up to 25/2000th in some other species of Balanus. The ova are wonderfully numerous, especially in the genus Coronula.

I may here mention the singular case of some elongated specimens of _Balanus balanoides_, from Tenby, in South Wales: some of these presented nothing abnormal; but in no less than seven specimens, the two, three, or four posterior pairs of cirri, either on one or both sides, were in an almost rudimentary condition, being of small size and having a shrunk and wasted appearance. In six out of these seven specimens, the probosciformed penis was quite short and abruptly truncated, as if from abortion. By cutting off the truncated apex, and cleaning the external tissue, I ascertained that it was imperforate, apparently in all the cases, and I am certain of this fact in several of the cases. In three of the specimens, I examined the vesiculæ seminales; in one, I found some spermatozoa, but cohering together in a peculiar manner; in the second, there were no spermatozoa; and in the third, the vesiculæ were shrunk, empty, and quite rudimentary in size. So that these three individuals certainly could not have impregnated their own eggs; nevertheless, within the shell of these very three, there were perfectly developed larvæ: I am led to conclude from this fact, that adjoining specimens in a perfect condition had, by means of their long probosciformed penis, effected the fecundation of their imperfect neighbours. I need only further add, that some out of the above six specimens, with more or less aborted cirri and imperforate male organs, were infested by a peculiar parasite, allied to Bopyrus,[50] and that these specimens did not contain ova.

[50] I have given a short notice on this parasite, in my former volume on the Lepadidæ, in a foot-note to p. 55.

_Metamorphoses and Homologies, throughout the Order of Thoracica._

In my former volume, the metamorphoses were described under three principal stages or heads; but whether these three included all the main changes, I was then hardly able to conjecture. But now I have reason to believe that such is the case, for in the genus Cryptophialus, belonging to the Abdominalia, the whole course of the metamorphosis, from the egg to the pupa, takes place within the sack of the parent; and I found, when having, on the coast of South America, numerous specimens to examine, that the egg-like larvæ (Pl. 24, fig. 15-18) could be naturally grouped into two main stages, but with many transitional intermediate grades (answering to the successive moults in the first stage of ordinary larvæ), before they passed into the third or pupal stage. And the first two stages in these egg-like larvæ of Cryptophialus, clearly seem to correspond with the first two stages in ordinary larvæ; for in both the chief changes are, the shortening of the terminal projection--the increase in size and approximation on the ventral surface of the anterior horns or cases for the antennæ--and the compression of the whole body. In all members of the Thoracica, the metamorphosis seems to run a remarkably uniform course. The larva in the first stage undergoes several moults and slighter changes--how many is not known--before arriving at its second main stage, which has been observed only in one single instance; and judging from Cryptophialus, this second stage passes abruptly by one moult into the pupal stage; and this, certainly, passes abruptly into the Cirripedial or mature stage.

_Larva, First Stage._