Chapter IX: Introduction (5)

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._

The larvæ in this stage are known, amongst the Balanidæ, in Balanus, Pyrgoma, Coronula, Platylepas, and Chthamalus; and these genera include all the principal forms. Amongst the Verrucidæ they are known in its one genus, Verruca. Amongst the Lepadidæ, in Scalpellum, Ibla, Alcippe, Lepas, Conchoderma, &c.; and in all these genera the larvæ present no important difference--hardly any difference which could be viewed as generic, were these larvæ independent animals,--as may be inferred, chiefly, from Mr. C. S. Bate's descriptions.[51] The abstract given in my former volume was not accompanied by any illustrations, and I have consequently here given (Pl. 29, fig. 8), a view of the larva, in the first stage before moulting, of _Scalpellum vulgare_: the natatory legs are not drawn with accuracy, only the relative position of the several organs having been carefully attended to. I have also had copied from Mr. Bate's memoir, a figure of the larva (Pl. 29, fig. 9) of _Balanus balanoides_, in its first stage, _before_ moulting, with its ventral surface exhibited; and another figure (with a few trifling alterations made after examining specimens most kindly sent me by Mr. Bate) of the larva of _Chthamalus stellatus_ (fig. 10), in its first stage, but _after moulting once_. It should be observed that Mr. Bate has given a drawing of the larva of this latter cirripede, in the first stage, _before moulting_; and it does not differ essentially from that just referred to (fig. 9), of _B. balanoides_, but is rather more fully developed. These drawings suffice to show the character of the larvæ in the first stage, both before and after the first moult, and even after the second moult, throughout the Order of Thoracica. The larvæ sometimes undergo their first moult within the sack of their parent, as I have been informed by Mr. Bate, and as I have observed in Coronula.

[51] 'Annals and Magazine of Natural History,' vol. viii (2d
series), 1851, Plates 6, 7, and 8.

I will now make a few remarks on these larvæ in the first stage, before and after the first moult, supplemental to those in my former volume. Their shape is oval, and the whole dorsal surface is evidently covered by a carapace. It is remarkable that the body exhibits no distinct articulations; those given by Goodsir[52] being certainly erroneous. Commencing at the anterior extremity, the eye varies considerably in the state of its development; in _Platylepas decorata_ it is nearly circular, and in most of the specimens very distinct; whereas in the allied _Coronula balænaris_, before the first moult, it is very imperfect, but afterwards square and of considerable size. In _Balanus galeatus_, in the immature larvæ dissected out of the egg, the cellular matter which was in process of conversion into the eye, formed a transverse band, obscurely divided into two portions, and this seems to indicate that the single eye is in fact formed by the confluence of two eyes. In _Scalpellum vulgare_, this heart-shaped eye lies between a V-shaped muscle, the nature of which I cannot understand, and which has not been represented in (Pl. 29, fig. 8, _a_). I need only further add, that in _Chthamalus stellatus_, after the first moult, the eye exhibits, in specimens sent me by Mr. Bate, some appearance of tending to become double.

[52] 'Edinburgh New Philosophical Journal,' July, 1843, Pl. 3, 4.

Arising posteriorly to the eye, we see, in _Scalpellum vulgare_, a pair of minute curved horns (_b′_), directed backwards; and within these horns I distinctly saw an articulated organ. These horns are difficult to be distinguished, and probably could not be made out previously to the first moult, in any larva of less size than that of _Scalpellum vulgare_. But after the first moult, Mr. Bate has seen, in two species of Balanus, in Verruca and in Chthamalus (fig. 10, _b_), a pair of articulated organs, in this same position, evidently now forming antennæ, and directed anteriorly, and free from any envelope. It is somewhat important, as we shall presently see, to bear in mind that these antennæ first appear within an envelope or horn; and that I detected that they included an articulated organ, before I had heard of Mr. Bate's observations. These antennæ, from their small size, from being seated internally with respect to the horns containing the other pair of antennæ, and from the position which the latter assume in the later stages of the larva, I believe to be the first or anterior pair. Their position in appearance posteriorly to the large lateral horns, containing the second pair of antennæ, is probably due to the anterior cephalic segments having been driven inwards, the truncated outline of the front of the head, and likewise, probably, the position of the mouth between the bases of the natatory legs being thus caused.

In this same larva of _Scalpellum vulgare_, within the great lateral horns just alluded to (fig. 8, _c_), filiform organs, supporting rows of spines, could be distinguished; and these appeared to me to be antennæ. These horns or cases resemble in structure the smaller pair just described; they arise from the ventral surface, and can hardly, therefore, be considered as prolongations of the carapace. After the first moult (fig. 10, _c_) they are seen to have increased much in length: in some cases they are of considerable length before the first moult, as in Lepas: in the Balanidæ they seem to be generally shorter than in the Lepadidæ; but in _Balanus galeatus_ I found them one third of the entire length of the animal. Whilst within the egg, these horns are adpressed laterally to the body, and so point posteriorly; afterwards they project rectangularly from the sides, or, as in _Scalpellum vulgare_, are directed somewhat anteriorly. As in the larvæ of all ordinary Crustaceans, as yet known, the antennæ are amongst the earliest developed organs; and as the first pair of natatory legs (Pl. 29, figs. 8-10, _e_) in these Cirripedial larvæ, might so very naturally be thought to be antennæ (as has been remarked to me by Mr. Dana), both from their structure and from their position a little anteriorly to the mouth, I am well aware that to prove my view correct, namely, that these horns are the second pair of antennæ in process of formation, it is not sufficient merely to have seen organs resembling antennæ within them; nor is it sufficient to advance the strictly analogical fact of the first-mentioned pair of antennæ, which in Scalpellum indisputably appear in their earliest condition within an envelope or horn. Further evidence is required, and this is presented in Cryptophialus, in which the lateral horns of the egg-like larva, in its first stage (Pl. 24, fig. 16), can be actually followed step by step until, in the second stage (fig. 17), just before passing into the pupa, the horns are seen to have become larger and more nearly approximated to each other on the ventral surface; and whilst in this condition, I several times dissected out the prehensile antennæ of the future pupa with _every character perfectly recognisable_. Hence I cannot doubt that in the larvæ of Cirripedes the law of development is, that in their very earliest condition, the small first pair of antennæ are enclosed in cases; and that the large second pair remains thus enveloped until the pupal stage. This conclusion, we shall immediately see, is in harmony with the late development of the succeeding appendages or organs of the mouth, which certainly do not appear in the first larval stage, and are not known to appear even till after the final metamorphosis.[53]

[53] According to M. Joly, ('Annales des Sciences Naturelles,' 2d
series, tom. xix, p. 59) in the larva of the macrourous Caridina,
the natatory legs appear before the gnathites or parts of the
mouth; so that in ordinary Crustaceans there is no invariable order
of development from the anterior towards the posterior end of the
body, as has sometimes been supposed.

The mouth is more or less probosciformed (Pl. 29, figs. 8-10, _d_), differing considerably in this respect in different species of the Lepadidæ; and this, probably, is due to the larva being born in a more or less mature condition. Its exact position likewise varies, for it arises either between the first or second pairs of natatory legs. It is known, from Mr. Bate's observations, to have the power of movement. It is directed posteriorly, the œsophagus extending anteriorly; both these directions being the same as in the mature cirripede. Certainly during these early stages there are no jaws or gnathites; but the margin, answering to the labrum, is furnished with some short, thick, sharp spines, and with hairs. In _Scalpellum vulgare_ the orifice of the œsophagus seems to lie rather beneath the upper prominent spinose edge, which, as just remarked, probably answers to the labrum; but this is one of the species in which the probosciformed mouth, at least before the first moult, is not much developed.

We come, now, to the three pairs of natatory legs: the first (Pl. 29, figs. 8-10, _e_) has throughout the order only one ramus, whereas the two succeeding pairs (_f_, _g_) are biramous. I must here remark that the straight and strong, and the curved plumose spines, with which these limbs, after the first moult, become furnished, now appear to me as more probably prehensile, rather than masticatory as I imagined in my former volume. That these spines are important organs to the larvæ I do not doubt. With regard to the homologies of these three pairs of limbs, my first impression was that they were the mandibles and the two pairs of maxillæ in their earliest condition; but I consider this view as quite untenable, for several reasons; viz., the wide interval between their bases and the mouth itself,--the somewhat variable position of the mouth with respect to the legs,--and the position which the latter occupy in the _second_ larval stage.[54] A far more tenable view is that these three pairs of legs are the three pairs of maxillipeds, in their earliest condition, in accordance with the view of M. Joly[55] on the nature of the three very similar pairs of natatory legs in the larva of Caridina, a macrourous Crustacean. But, in Cirripedes, the three pairs of natatory legs, in the larva in the first stage, are apparently the very same as the first three pairs, in the larva in the second stage, and in the pupa. And in the pupa the first three pairs, which certainly correspond with the first three pairs of cirri in the mature animal, seem to me, for reasons presently to be assigned, to be the second, third, and fourth thoracic limbs. Hence I am led to the conclusion that the first pair of legs in the larva in the first stage, are homologically the second thoracic (answering to the third pair of maxillipeds in the higher Crustaceans), and that the two succeeding pairs are the third and fourth thoracic limbs; to be succeeded, in the _pupal_ stage, by the fifth, sixth, and seventh thoracic appendages.

[54] Mr. Dana, moreover, has remarked, ('Crustacea: United States
Exploring Expedition,' p. 1386), "that he knows of no instance of
a mandible becoming so completely a leg, as to lose wholly the
mandibular function even of its basal portion."

[55] 'Annales des Sciences Naturelles,' 2d series, tom. xix, 1843,
p. 34. M. Joly's observations were made on the Caridina. I owe to
the great kindness of Mr. C. Spence Bate, an examination of some
larvæ of the allied genus _Hippolyte varians_, and I found, on
dissection, the view of M. Joly, that the three pairs of natatory
legs are the maxillipeds, so far strongly confirmed, that they
followed closely, with equal intervals, the mandibles and two pairs
of maxillæ. The first pair of natatory legs in Caradina, in its
earliest condition within the egg, is uniramous, like the first
pair in the larvæ of Cirripedes. There is one fact which seems
rather strongly opposed to the view of these three pairs of legs
in the larvæ of the macrourous Crustaceans being the maxillipeds,
which is that Capt. Du Cane ('Annals of Nat. Hist.,' 1838, vol.
ii, pl. 6, and 7) observed only three pairs of limbs in process
of formation posteriorly to the first three pairs, whereas there
should be found, in accordance with M. Joly's view, five pairs, _i.
e._ all five pairs of ambulatory legs. This one fact countenances
the view, which I hold on the nature of the legs in the larvæ of
Cirripedes during their early stages, namely, that they are the
second, third, and fourth thoracic limbs, to be succeeded by only
three additional pairs.

Lastly, behind the natatory legs, on the ventral surface, (Pl. 29, figs. 8, 9, _i_), the body is much produced, and terminates in a horny fork, which, after the first moult (fig. 10, _i_), becomes much elongated. Anteriorly to this fork, on the ventral surface, there is another fork (_l_), and again above this I could distinguish, in _Chthamalus stellatus_, after the first moult, another fork (_m_), or at least a pair of short thick spines. From the structure of the forked abdomen in the known larvæ of the Podophthalmia, I presume that this portion of the body is the abdomen of the young Cirripede, but it is not at all plainly articulated. After the first moult, the posterior end of the carapace (_h_), which is always pointed, becomes much elongated and serrated on both sides;[56] reminding one of the structure of the carapace of the so-called Zoea, or larva of certain Podophthalmia. Situated under this posterior prolongation of the carapace, there is a swelling (_n_, with long hairs on both sides), which apparently lies on the dorsal surface of the spinose and forked abdomen; here, when the larva is compressed, the cellular and oily contents of the body burst forth; and I suspect that this swelling is the anus, for it is known from the researches of Rathke,[57] that the anus in the higher Crustaceans opens during the earliest periods dorsally.

[56] I suspect that the account given by Goodsir ('Edinburgh New
Phil. Journal,' 1848) of the posterior points of the carapace and
abdomen in the larva of a Balanus, is not quite accurate.

[57] 'Annales des Scienc. Nat.,' tom. xx, p. 451.

_Larva, Second Stage._

I have given, from Burmeister,[58] a lateral view (Pl. 30, fig. 1) of the one single specimen, ever observed of a larva in this stage, belonging, as is supposed, to the genus Lepas. The carapace has now greatly altered its character. The two fleshy projections, as so called by Burmeister, by which the larva adhered to the sea-weed, were supposed by this author to include the great prehensile antennæ of the pupa; from my observations, already alluded to, on the two projections (Pl. 24, fig. 17) in the closely analogous egg-like larva, in the second stage, of Cryptophialus, by which it also adheres, I have not the least doubt that this is the case. The small, internal, and anterior pairs of antennæ, are, as it would appear, now aborted. The eye, according to Burmeister, has commenced becoming double; but the two approximate eyes are not as yet compound. The mouth is probosciformed (_m_), and does not differ much from its condition in the first stage; no gnathites were observed by Burmeister, and they could not be expected to be present, for they are not found even in the pupa. The mouth, which in the larva in the first stage differs in different genera, in being more or less advanced forward, here stands some way anteriorly to the natatory legs, as in the pupal condition. The first pair of legs is uniramous, and the two other pairs biramous; this fact, together with the number of the legs in this second stage being still three, and their structure being not very different, leaves little doubt on my mind that we here have the same three pairs as during the first stage. The abdomen has become much shortened, but still space is left for the development, in the pupa, of the three posterior pairs of legs. I may here remark that in the pupa the anterior natatory legs have become, like the others, biramous; but yet, as it were for the purpose of showing their metamorphosis from the uniramous legs of the earlier stages, they have their bristles arranged rather differently from those on the succeeding five pairs of legs.

[58] 'Beiträge zur Naturgeschichte der Rankenfüsser,' tab. 1, figs.
3, 4.

_Larva in the Last or Pupal Stage._

I have given a lateral view of the pupa of _Lepas australis_ (Pl. 30, fig. 2), illustrative of the description in my former volume: the specimen is drawn as if transparent, and it was to a certain extent thus rendered by boiling in caustic potash. A sketch of the position of the young Cirripede within the pupa, was made by the camera. At first the drawing will perhaps hardly be comprehended: the darker shaded portion to the left of the letter (_b_) shows the extent of the sack, with the included thorax and natatory legs of the pupa: to the right of the same letter, if we do not consider the young included Cirripede, the only organs distinguishable in the mass of cellular and oily matter, are the alimentary canal, the cement-glands (_t_), _i. e._ the incipient ovaria, and the cement-ducts (_t′_) which enter the antennæ. A view is also given (fig. 4) of the ventral surface of the pupa; and a transverse section (fig. 7) of the carapace, taken close to the eye-apodemes. On comparison with the larva in the second stage, the changes in external appearance and structure are not very great; the prehensile antennæ are freed from their cases; the two eyes stand further apart; the three posterior pairs of legs have been developed, and a small abdomen has become distinctly separated from the thorax. Before proceeding to make a few additional remarks and corrections to my former description of the pupa, it will be advisable, on account of the importance of the subject, to discuss the homologies of the limbs.

From the presence of eyes and of two pairs of antennæ in the larva, during its earlier stages, the front of the head consists, in accordance with all analogy, of three segments; the mouth, likewise, from being formed of three gnathites (which can be detected by dissection in the pupal state), consists, also in accordance with all analogy, of three segments, making altogether six segments--on the nature of which I apprehend no objection will be raised. In two out of the three orders into which Cirripedes may be divided, the mouth is succeeded, in the adult animal, by eleven most distinct segments; of which the first (_i. e._ the seventh cephalic) differs from the succeeding seven thoracic segments; and these seven again differ from the three abdominal and terminal segments. Hence it must be admitted that, as far as the cephalo-thorax of the archetype Cirripede is concerned, it consists, like that of the archetype Crustacean, of fourteen segments, of which eight succeed the first-named six that form the mouth and front of the head; and that, with the three abdominal segments, there are altogether seventeen segments. In the order Thoracica, however, which includes all common Cirripedes, both in the pupa and in the mature animal, only six thoracic segments with their appendages, succeed the mouth, two having been lost; and the question arises which are these two, whether the seventh and eighth, or the thirteenth and fourteenth (_i. e._ the two terminal thoracic) segments; for there is no reason to suspect any other segments of having disappeared. In my former volume, I inferred, without sufficiently entering into my reasons, that it was the seventh and eighth, _i. e._ the last cephalic and first thoracic segments, which had disappeared; but I now find that Mr. Dana[59] believes that, in ordinary Crustaceans, the abortion of the segments with their appendages almost always takes place at the posterior end of the cephalo-thorax. Nevertheless, after due deliberation and fresh examination of the pupa, I must retain my former opinion, that it is the last cephalic and first thoracic segments which have either coalesced with the others, or wholly disappeared. In the pupa, the mouth, although functionless, has its place most plainly marked by being slightly prominent, and by the presence of a sort of labrum and of a shrivelled œsophagus, round which latter the gnathites and the new œsophagus of the future young cirripede are in process of formation. Now between the mouth of the pupa and the first pair of natatory legs, there is a space of membrane, equalling, when stretched out, the three succeeding thoracic segments in length and breadth: this interspace, I conceive, must have some homological signification; here then we have at least an appearance of the abortion of appendages; whereas, at the posterior end of the cephalo-thorax, no such appearance is presented. Moreover this interspace of membrane is divided nearly in the middle by a most conspicuous fold, which, on the view here adopted, would mark the separation of the seventh (cephalic) from the eighth (thoracic) segment; and the interspace and fold are thus simply explained. Lastly, I have shown, in the Introduction (p. 18), that the first and five succeeding pairs of cirri of the mature Cirripede present certain small, but significant, resemblances in structure and in the origin of their nerves, with the outer pair of maxillipeds and with the five pairs of ambulatory legs in the Podophthalmia; which resemblances are all futile, if the cirri belong to the 7th, 8th, 9th, 10th, 11th, and 12th segments of the cephalo-thorax, or those immediately succeeding the mouth; but are full of meaning, if the six pairs of cirri belong, as I believe, to the 9th, 10th, 11th, 12th, 13th, and 14th segments, or the six posterior segments of the cephalo-thorax.

[59] 'Crustacea: United States Exploring Expedition,' p. 22.

Before commencing on details, I may premise that I have examined the pupa of _Lepas australis_, _pectinata_, _fascicularis_, and _anatifera_, of _Conchoderma virgata_, partially of _Dichelaspis Warwickii_, of _Ibla quadrivalvis_, and of _Alcippe lampas_; and in the Balanidæ, of _Balanus balanoides_ and _Hameri_. In the pupæ of all these genera there is a most close general agreement in structure, excepting in minute details: I was surprised to find exactly the same slight differences in the spines on the first pair of natatory legs, as compared with the succeeding pairs, in _Balanus Hameri_, as in Lepas. The abdomen and caudal appendages of the pupa in the abnormal Alcippe, as we shall presently see, offer the only marked exception to this uniformity of character throughout the Thoracica. The outline of the carapace or shell is usually not so blunt at the anterior end, as in the pupa of _Lepas australis_ (Pl. 30, fig. 2); more commonly the shape is that of the pupa of Alcippe (Pl. 23, fig. 16). In _Lepas pectinata_ the two posterior points of the carapace are produced into two short spines. The surface of the carapace in _L. australis_ is lined, as represented in fig. 4: the colour of this species when alive was blue:[60] in _L. fascicularis_ the surface is punctured: in _L. pectinata_ it is marked with curious points of various shapes, often star-shaped, in parts reticulated, and confluent along the dorsal margin, and in parts lined: in _B. balanoides_ it is very obscurely punctured, and in _B. Hameri_ the punctures pass into lines. The whole of what is externally visible consists of the carapace, for this is produced not only backwards, so as to enclose the thorax and abdomen with their appendages, but also forwards, so as to overhang the whole front of the animal; and the prehensile antennæ, in Lepas, Ibla, Balanus, and probably in all the genera, can be retracted within its lower edge. The protection afforded by the carapace to the antennæ is aided by two crests (Pl. 30, fig. 7, _c_) parallel to this lower edge. The whole sternal surface is very narrow (fig. 4), and is likewise protected by the carapace; that is, when the two sides are drawn together by the adductor muscle. The shell, however, when thus drawn together, gapes a little at the two ends, at least in the case of _Lepas australis_. The adductor muscle, if introduced in fig. 4, would have crossed close anteriorly to the basal margin of the mouth; and in fig. 2, its end on the near side would have been attached under the dark cæca, which enter the upper end of the stomach. The adductor is shaped almost like an hour-glass, and so differs from this muscle in the mature Lepas, in which it is of the same thickness throughout. I may here add that the pupa of _Lepas australis_ could swim very rapidly, and often on one side in a circle; it could walk by the aid of its antennæ, but often fell over; being thus locomotive, and, as we shall immediately see, well provided with senses, it cannot be considered as very lowly organised.

[60] I took this species alive in the Southern Atlantic Ocean; and,
mistaking it for an independent Crustacean, was much perplexed
where to class it. I had overlooked these specimens when publishing
my former volume.

_Acoustic Organs._--Commencing at the anterior end, two small elongated orifices, 10/6000th of an inch in diameter, (_e_, fig. 4, Pl. 30), may be seen; these lead, as described in my former volume, into a sack, with a bag suspended in it, which is provided with a large nerve, and which I believe to be the acoustic vesicle. These orifices occur in the carapace, either in the same position, or a little more posteriorly, in the pupæ of all Cirripedes. In _Balanus balanoides_ they are minute, being only 2/6000th in diameter, but are surrounded with a border: in _Conchoderma virgata_ they are also surrounded by a border: in _Lepas pectinata_, the orifices are 3/6000th of an inch in diameter, and are very singular from being seated on rounded prominences, causing the carapace to have two short, blunt horns in front. In _Lepas australis_, and I believe in the other species, the corium round the acoustic orifices is darkly coloured; and these coloured marks can be distinguished for some little time on the peduncle of the young Cirripede, after the metamorphosis, and after the entire organ, together with the whole pupal carapace and eyes, has been moulted. Knowing the connection in the higher Crustacea, of the acoustic organs and the antennæ, and seeing the very backward position (figs. 2 and 4) of the one great pair of antennæ, I have always imagined that these orifices probably marked the normal position of the anterior pair of antennæ, which, since the earlier larval stages, have disappeared. And I now find[61] that Schödler affirms, that in most, if not in all Daphnidæ, there is a black spot in front of the eye, which is connected with an opening in the basal portion of the anterior antennæ, and he concludes that it is an organ of hearing.

[61] Quoted by Dana, 'Crustacea of United States Exploring
Expedition,' p. 1264.

_Antennæ._--These, from their present position, and from standing, in their earlier stages whilst within their envelopes or horns, exteriorly to the small medial pair (since aborted), I believe to be the second pair; and this is Mr. Dana's opinion. In my former description of these very singular and important organs (Pl. 30, figs. 4 and 8), I have fallen into some considerable mistakes: the two plates or segments (fig. 4, N), of which the posterior margins are inflected as apodemes (_n_), carrying the eyes, are certainly, as may be clearly seen in the pupa of Alcippe, Pl. 23, fig. 16, and as affirmed by Burmeister,[62] the basal segments of the antennæ. The second or main segment (formerly called by me the basal segment) has in some species an upper portion of the membrane of which it is composed, next to the body, excessively thin, and separated from the rest of the membrane composing the segment, by an oblique line (fig. 8, _o_), which I mistook for its articulation with the body.[63] We then come to the disc or third segment; and lastly to the fourth, or ultimate segment. This ultimate segment, generally, has its external corner projecting up, as a step; and this sometimes, as in _Dichelaspis Warwickii_, gives the appearance of its consisting of two segments; but a careful examination of this part in Ibla, in which the step-like structure is carried to an extreme, makes me believe that there is only one segment.[64] The prehensile antennæ, therefore, like the natatory legs, are formed of four consecutive segments, of which the basal segments give rise to the singular apodemes, presently to be noticed (fig. 7), that carry the great compound eyes. This basal segment, in all Cirripedes, is moulted with the eyes, the three other segments invariably remaining cemented to the surface of attachment.

[62] 'Beiträge zur Naturgeschichte der Rankenfüsser,' p. 19.
In tab. 1 of this work there are good drawings of the general
structure of the pupa of a species of Lepas, probably _L.
australis_. I believe this author was the first who made out the
structure of the abdomen of the pupa.

[63] In the table of measurements of the antennæ of the several
genera and species of the Lepadidæ (p. 286) of my former volume,
the articulation, called by me _basal_, I now know to be really
the articulation between the basal and second segment. In the
fourth column, headed "Length from end of the disc to the inner
margin of the basal articulation," the term inner margin really
applies to the oblique curved line separating the thin and scarcely
visible membrane from the thicker membrane of that segment. These
corrections do not in the least affect the object for which the
table was given.

[64] In a sketch, sent me by Mr. Dana, of this organ in the pupa
of a Lepas from the Antarctic Ocean, I observe that he divides my
ultimate segment into two segments.