Double eyelength26/6000
width13/6000
Diameter of single lens 6/6000
Ophthalmic ganglionlength16/6000
breadth11/6000
Supra-œsophageal ganglion, transverse or longest axis of both together126/6000
Supra-œsophageal ganglion, longitudinal axis of45/6000
Infra-œsophageal ganglion, transverse axis of120/6000
Infra-œsophageal ganglion, longitudinal axis of114/6000

[16] Proceedings of the Academy of Natural Sciences, Philadelphia. No. i, vol. iv, Jan. 1848.

In [Conchoderma aurita] the ophthalmic ganglia are much smaller, and nearer to the supra-œsophageal ganglion, than in L. fascicularis. In [Alepas cornuta] the ophthalmic chords run towards each other from the two distant and separate supra-œsophageal ganglia; and the ophthalmic ganglia, (instead of being quite separate, as in L. fascicularis,) are united by their front ends, and the two eyes instead of standing some way in front, with nerves running to them, are embedded on the double ophthalmic ganglion; the pigment-capsules here, also, have the shape of mere saucers, and are joined back to back, with the two lenses projecting far out of them. In neither sex of Ibla could I perceive that the eye was double. In [Pollicipes spinosus] the ophthalmic ganglion stands in front of the single supra-œsophageal ganglion, and shows no signs of being formed of a lateral pair; the eyes themselves, however, differently from, in all the foregoing cases, are, though approximate, quite distinct. In Pollicipes mitella I did not see the eyes; but the ophthalmic ganglion consists, as I believe, of a single globular one, placed exactly between the two globular, supra-œsophageal ganglia, all three being of nearly equal size. Professor Leidy does not mention the ophthalmic ganglia; hence I infer that in Balanus, which is a more highly organised Cirripede, they are fused into the supra-œsophageal ganglion.

In all the genera, the double eye is seated deep within the body; it is attached by fibrous tissue to the radiating muscles of the lowest part of the œsophagus, and lies actually on the upper part of the stomach; consequently, a ray of light, to reach the eye, has to pass through the exterior membrane and underlying corium connecting the two scuta, and to penetrate deeply into the body. In living sessile Cirripedes, vision seems confined to the perception of the shadow of an object passing between them and the light; they instantly perceived a hand passed quickly at the distance of several feet between a candle and the basin in which they were placed.

As the infra-œsophageal ganglion sends nerves to the trophi and to the first pair of cirri, it must correspond to the segments, from the fourth to the ninth inclusive, of the archetype crustacean. The state of the supra-œsophageal and ophthalmic ganglia appears to me very interesting: I do not believe that in any mature ordinary crustacean, the first or ophthalmic ganglion can be shown to be distinct from the two succeeding ganglia, or to be itself composed of a pair laterally distinct. The ganglia, corresponding with the second and third segments of the body, which should normally support two pair of antennæ, are in the Lepadidæ united together; but laterally they are generally distinct in outline, and are actually separate in Alepas: the supra-œsophageal ganglion shows also its double nature, by giving rise to a pair of large double nerves, evidently corresponding with the two pair of antennular nerves in ordinary crustaceans. The embryonic condition of the whole supra-œsophageal portion of the nervous system in the Lepadidæ, corresponds with the rudimentary state of the only organ of sense supplied by it, namely, the eye, which in size and general appearance has retrograded to the state in which it was in, during the first stage of development of the larva;—I have used the term embryonic, because, in the embryos of ordinary crustacea, all the ganglia are at first longitudinally distinct, and laterally quite separate. The conclusion at which we before arrived from studying the metamorphoses, namely, that the whole peduncle and capitulum consisted of the first three segments of the head, is beautifully supported by the structure of the nervous system, in which these parts are seen to be supplied with nerves exclusively from the supra-œsophageal ganglion: now in ordinary crustacea the supra-œsophageal ganglion sends nerves to the eyes and the two pair of antennæ corresponding, as is known by embryological dissections, to the first three segments of the body. Moreover, it is asserted that the carapace which covers the thorax in crustacea, is not formed by the development of the first segment; and this, likewise, may be inferred to be the case with the peduncle and capitulum in the Lepadidæ, as the nerves of the ophthalmic ganglia go exclusively to the eyes. Finally, I may remark that in Pollicipes, looking to the whole nervous system, the state of concentration nearly equals that in certain macrourous decapod crustaceans, for instance the Astacus marinus, of which a figure is given by Milne Edwards.

Olfactory Organs.—In the outer maxillæ, at their bases where united together, but above the basal fold separating the mouth from the body, there are, in all the genera, a pair of orifices ([Pl. X], [fig. 16]); these are sometimes seated on a slight prominence, as in Lithotrya, or on the summit of flattened tubes ([Pl. X], [fig. 17]), projecting upwards and towards each other, as in Ibla, Scalpellum, and Pollicipes. In Ibla these tubular projections rise from almost between the outer and inner maxillæ. It is impossible to behold these organs, and doubt that they are of high functional importance to the animal. The orifice leads into a deep sack lined by pulpy corium, and closed at the bottom. The outer integument is inflected inwards, (hence periodically moulted,) and becoming of excessive tenuity, runs to near the bottom of the sack, where it ends in an open tube: so excessively thin is this inflected membrane, that, until examining Anelasma, I was not quite certain that I was right in believing that the outer integument did not extend over the whole bottom. I several times saw a nerve of considerable size entering and blending into a pulpy layer at the bottom of the sack of corium; but I failed in tracing to which of the three pair of nerves, springing from the front end of the infra-œsophageal ganglion, it joined. I can hardly avoid concluding, that this closed sack, with its naked bottom, is an organ of sense; and, considering that the outer maxillæ serve to carry the prey entangled by the cirri towards the maxillæ and mandibles, the position seems so admirably adapted for an olfactory organ, whereby the animal could at once perceive the nature of any floating object thus caught, that I have ventured provisionally to designate the two orifices and sacks as olfactory.

Acoustic (?) Organs.—A little way beneath the basal articulation of the first cirrus ([Pl. IX], [fig. 4 d], and [Pl. IV], [fig. 2 e]), on each side, there may be seen a slight swelling, and on the under side of this, a transverse slit-like orifice, 1/20th of an inch in length in Conchoderma, but often only half that size. In Ibla this orifice is seated lower down ([Pl. IV], [fig. 8 a´, e]), between the bases of the first and second cirri, which are here far apart: in [Alepas cornuta] it is placed rather nearer to the adductor scutorum muscle, namely, beneath the mandibles. The orifice leads into a rather deep and wide meatus; the external integument is turned in for a short distance, widening a little, and then ends abruptly. The meatus, enlarging upwards, is lined by thick pulpy corium, and is closed at the upper end; from its summit is suspended a flattened sack of singular and different shapes in the different genera. This, the so-called acoustic sack of [Conchoderma virgata], is figured [Pl. IX], [fig. 6.] The deep and wide notch faces towards the posterior end of the animal; the inferior lobe, thus almost cut off, is flattened in a different plane from the upper part; the lobe is lodged in a little pouch of corresponding form, leading from the open meatus in which the upper part is included. In [Conchoderma aurita], the top of the acoustic sack is narrower and more constricted, the whole more rounded, and the lobe more turned down. In [Lepas fascicularis] the notch is not so deep or wide, and the lobe larger. In [Ibla Cumingii] the sack is of the shape of a vase, with one corner folded over. In Scalpellum vulgare it is small, oval, with the lower end much pushed in, and furnished with a little crest. Lastly, in [Pollicipes mitella] it is simply oval. In all cases the sack is empty, or contains only a little pulpy matter: it consists of brownish, thick, and remarkably elastic tissue, formed, apparently, of transverse little pillars, becoming fibrous on the outside, and with their inner ends appearing like hyaline points. The mouth of the acoustic sack (removed in the drawing) is closed by a tender diaphragm, through which I saw what I believe was a moderately-sized nerve enter; I have not yet succeeded in tracing this nerve. The first pair of cirri seem, to a certain extent, to serve as antennæ, and therefore the position of an acoustic organ at their bases, is analogous to what takes place in crustacea; but there are not here any otolites, or the siliceous particles and hairs, as described by Dr. Farre, in that class. Nevertheless, the sack is so highly elastic, and its suspension in a meatus freely open to the water, seems so well adapted for an acoustic organ, that I have provisionally thus called it. In the larva, as I have shown, a pouch, certainly serving for some sense, I believe for hearing, is seated in quite a different position at the anterior end of the carapace. I may mention that I found sessile Cirripedes very sensitive of vibrations in objects adjoining them, though not, apparently, of noises in the air or water. In a group of specimens, I could not touch one even most delicately with a needle, without all the adjoining ones instantly withdrawing their cirri; it made no difference if the one touched had its operculum already closed and motionless.

Reproductive System,—Male Organs.—All the Cirripedia which I have hitherto examined, with the exception of certain species of Ibla and Scalpellum, are hermaphrodite or bisexual.[17] I shall so fully describe the sexual relations of the several species of these two genera, under their respective headings, and at the end of the genus of Scalpellum, that I will not here give even an abstract of the grounds on which my firm belief is based, that the masculine power of certain hermaphrodite species of Ibla and Scalpellum, is rendered more efficient by certain parasitic males, which, from their not pairing, as in all hitherto known cases, with females, but with hermaphrodites, I have designated Complemental Males.

[17] I am compelled to differ greatly from the account given by Prof. Steenstrup of the reproductive system in the Cirripedia, in his ‘Untersuchungen über das Vorkommen des Hermaphroditismus, ch. v, 1846;—a translation of which I have seen, owing to the great kindness of Mr. Busk. Mr. Goodsir has described (‘Edin. New Phil. Journal,’ July 1843,) what he considers the male of Balanus; but I have seen this same parasitic creature charged with ova, including larvæ! From the resemblance of the larvæ to the little crustacean described by Mr. Goodsir, in the same paper, as a distinct parasite, I believe the latter to be the male of his so-called male Balanus, and that all belong to the same species, allied to Bopyrus. This genus, as is well known, is parasitic on other crustacea; and it is a rather interesting fact thus to find, that this new parasite which is allied to Bopyrus, in structure, is likewise allied to it in habits, living attached to Cirripedia, a sub-class of the crustacea.

The male organs have been well described by M. Martin St. Ange, whose observations have since been confirmed by R. Wagner.[18] The testes are small, often leaden-coloured, either pear or finger-shaped, or branched like club-moss,—these several forms sometimes occurring in the same individual; they coat the stomach, enter the pedicels, and even the basal segments of the rami of the cirri, and in some genera occupy certain swellings on the thorax and prosoma, and in others the filamentary appendages: the testes seen in the apex in one of these appendages in Conchoderma, is represented in [Pl. IX], [fig. 5]. The two vesiculæ seminales are very large; they lie along the abdominal surface of the thorax, and generally (but not in some species of Scalpellum) enter the prosoma, where their broad ends are often reflexed; here the branched vessels leading from the testes enter. The membrane of the vesiculæ seminales is formed of circular fibres; and is, I presume, contractile, for I have seen the spermatozoa expelled with force from the cut end of a living specimen. The two canals leading from the vesiculæ generally unite in a single duct at the base of the penis; but in [Conchoderma aurita], half-way up it. The probosciformed penis, except in certain species of Scalpellum, is very long; it is capable of the most varied movements; it is generally hairy, especially at the end; it is supported on a straight unarticulated basis, which in [Ibla quadrivalvis] alone ([Pl. IV], [fig. 9 a)], is of considerable length; in this species, the upper part is seen to be as plainly articulated as one of the cirri; in Alepas, the articulations are somewhat less plain, and in the other genera, the organ can be said only to be finely ringed, but these rings no doubt are in fact obscure articulations. In the females of [Ibla Cumingii] and [Scalpellum ornatum], there is, of course, no penis.