Alimentary Canal.—The membrane lining the œsophagus is unusually thin: it is furnished with the ordinary constrictor muscles, and others radiating from them like spokes of a wheel. The stomach is lined by unusually prominent biliary folds, which in the duodenum are transverse, sending forth, however, short folds at right angles; and these latter, in the proper stomach, become so much developed that the folds appear longitudinal. The rectum extends inwards, about as far as the base of the fourth pair of cirri, but is very short, owing to the little development of the three posterior segments of the thorax. The anus is seated in its usual place, at the dorsal basis of the penis, and is hidden by loose folds of skin; but there are no distinct caudal appendages. The stomach, in the specimen examined, was quite empty.

Reproductive Organ.—The penis ([fig. 2, c]) is thick, short (about twice as long as the sixth cirrus), constricted at the base, ringed, spineless, with the terminal aperture large; internally it is well furnished with muscles. The two vesiculæ seminales, appeared to be unusually small; and one was much smaller than the other; they do not (I believe) become united into a common tube, till near the apex of the penis. They were empty; and, I presume, from the state of the ova, that their contents had lately been discharged. The whole thorax was filled with a white, fibrous and cellular mass, consisting perhaps of the testes in their undeveloped state. The individual dissected by me, appeared to have been defective in its last act of reproduction, for there were only two or three ova attached to the frænum on one side, and not very many on the other. The ova are much less elongated than is usual; they are of a remarkable size, namely 22/1000ths of an inch in their longer diameter; the membrane by which they are united into a pair of lamellæ is remarkably strong; the frænum ([Pl. IV], [fig. 2 f]) on each side is large, strong, with rounded edges, pale coloured and hence conspicuous; on the side nearest the body, the whole surface is covered with club-shaped glands, having very short footstalks, and being in total length 5/6000ths of an inch; these glands secrete a reticulated layer of gut-formed fibres, attached to the ovigerous lamellæ. In the specimen described by Lovén, the lamellæ ([fig. 1], and [fig. 2, b, b]) appear to have been very large: and in that examined by myself, the peduncle was gorged with immature ova, showing that the female reproductive powers were ample, though at the foregoing period, only a few eggs had been formed.

Habits.—According to Lovén, this species lives imbedded in the skin of Squalus maximus and spinax, in the North Sea: I suspect that it is not closely compressed in its cavity, otherwise, I do not see the use of the two layers of muscles round the whole peduncle; it probably adheres to the sides of the cavity by the tips of the branched, root-like filaments; owing to the flexible nature of the capitulum, this Cirripede can offer little resistance to the water, and, therefore, is little likely to be torn out of its cavity. I have no doubt that it can fold the membrane of the capitulum, like a cloak, round its thorax and cirri; but it certainly can offer far less resistance, than other Cirripedes, to any enemy. This creature must obtain its food, and considering its productiveness much food must be required, in a manner quite different from nearly every other member of its Order. As the whole of the peduncle is imbedded, and as the mouth is probosciformed, with the labrum a little curled over the adductor muscle, I conclude that this Cirripede can reach minute animals crawling by on the surface of the shark’s body.

It must be borne in mind that the mouth, as in all Cirripedes, has the power of independent movement, and that the mandibles and maxillæ are here beautifully adapted to catch and force down any small living creature into the muscular œsophagus; the rudimentary outer maxillæ, moreover, no doubt have the power of scraping, like a lip, anything towards these prehensile organs. It will hereafter be seen, that the male of [Ibla Cumingii], in which the cirri are quite rudimentary, obtains its food in a somewhat analogous manner, though in this case the whole peduncle moves, and not merely a probosciformed mouth: it deserves attention, that in the male Ibla and in Anelasma, in neither of which the cirri are prehensile, the palpi are rudimentary and useless. I am tempted to believe, that the largely developed olfactory sacks, and perhaps, likewise, acoustic (?) sacks, in Anelasma, replace, by giving notice of the proximity of prey, the loss of tactile cirri. It should be remembered that all Cirripedes subsist on animals which happen to swim or float within reach of the cirri; but here it is only those which happen to crawl within reach of the probosciformed mouth. It would, however, be rash to assert that the cirri in Anelasma, considering their muscular though feeble structure, may not be of some slight use, when thrown over the prey, in preventing its escape.

Professor Steenstrup informs me that, from late observations, it appears that this animal always adheres to the shark’s body in pairs. I regret extremely that I have not been able to examine a pair: that the individual examined by me was bisexual, I can hardly doubt, though the male organs certainly were feebly developed; it appears probable, that the individual described by Lovén was likewise bisexual: but after the facts presently to be revealed regarding the sexes in Ibla and Scalpellum, it is quite possible that the male and female organs may be developed in inverse degrees in different and adjoining individuals.

The genus Anelasma is, I think, properly placed between Alepas and Ibla. In several of its characters, such as the absence of calcareous valves, the broad blunt end of the peduncle, the spineless cirri, the small size of the trophi, and more especially the absence of transverse striæ in those muscles, which in mature cirripedes are thus furnished, we see that this genus is in some degree in an embryonic condition.

Genus—Ibla. [Pls. IV], [V.]

Ibla. Leach. Zoolog. Journal. vol. ii, July, 1825.

Anatifa. Cuvier. Mem, pour servir, ... Mollusques, Art. Anatifa, 1817.

Tetralasmis. Cuvier. Regne Animal, 1830.