A Monograph on the Sub-class Cirripedia · Charles Darwin
TABLE OF CONTENTS.
_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]
_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._
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.
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]
_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.
_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.
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.
Ideas discussed in this chapter
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