(1172). In the outer maxillae, at their bases, where they are united together, but above the basal fold separating the mouth from the body, there are, in all the Lepadidge, a pair of orifices, sometimes seated on a slight prominence, or else on the summit of flattened tubes projecting upwards and towards each other. Each of these orifices leads into a deep sac lined by pulpy corium, and closed at the bottom, over which a nerve of considerable size is distributed. That this closed sac is an organ of sense, of some kind or other, there can be little doubt; and, judging from its position, Mr. Darwin is induced to consider that the two constitute an olfactory apparatus.

* Proceedings of the Academy of Natural Sciences of Philadelphia, Jan. 1848, 1 Monograph on the Subclass Cirripedia, by Charles Darwin, F.R.S., 1841.

(1173). At a little distance beneath the basal articulation of the first cirrus, on each side, there may be seen a slight swelling, and on the under side of this a transverse slit-like orifice (fig. 234, e), one-twentieth of an inch in length, in Conchoderma, but often only half that size: this is regarded by Mr. Darwin as the organ of hearing. The external orifice leads into a deep and rather wide meatus, which, enlarging upwards, is lined by a thick pulpy corium, and is closed at the upper end; from its summit is suspended a flattened sac, variously shaped in different genera. In all cases the sac 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 sac is closed by a tender diaphragm, through which Mr. Darwin thinks he saw a moderate-sized nerve enter; and as the first pair of cirri seem, to a certain extent, to perform the office of antennae, therefore the position of an acoustic organ at their bases is analogous to what exists in Crustacea; but there are not here any otoliths, or the siliceous particles and hairs, as described by Dr. Farre in that class (§ 1050.) Nevertheless the sac is so highly elastic, and its suspension in a meatus freely open to the water seems so well adapted for an acoustic organ, that Mr. Darwin considers such to be its function.

(1174). With respect to the organization of the reproductive system in these creatures, the most discordant opinions are expressed by different writers, no two authors agreeing either concerning the names or offices which ought to be assigned to different parts of the generative apparatus. It must therefore be our endeavour, in considering this part of their economy, to separate as far as practicable all conjecture and hypothetical reasoning from the simple facts which anatomy has placed at our disposal, and leave disputed questions to be solved by careful experiment and research. According to the dissection of John Hunter, the internal generative apparatus is double, occupying both sides of the alimentary canal. Covering the stomach (fig. 235, d), there is found a vascular substance, which the above-named illustrious anatomist regarded as probably constituting the tubular parts of the testicle, from which a tortuous canal with very thick walls (vas deferens) runs upwards along the side of the intestine to the root of the fleshy prolongation (k), at which point it is joined by the corresponding tube from the opposite side of the body. The common canal thus formed is extremely slender, and passes in a flexuous manner through the whole length of the tubular organ (k), named by Hunter, apparently for the sake of brevity, the penis, to terminate by a minute orifice at its extremity. Yet, notwithstanding the name applied to the termination of the sexual canals, Hunter was well convinced that the Cirripeds were hermaphrodites, - as he expressly says *, "It is most probable that all Barnacles are of both sexes and self-impregnators; for I could never find two kinds of parts, so as to be able to say, or even suppose, the one was a female, the other male".

Structure of Conchoderma: a, external layer of integument; a a, internal layer.

Fig. 234. Structure of Conchoderma: a, external layer of integument; a a, internal layer; b b, ova, forming a layer around the body; c, ovipositor; d, mouth; e, presumed organ of hearing; f, aperture communicating with the interior of the pedicle; g, h, pedicle. (After Darwin).

(1175). Cuvier found the vascular mass, considered by Hunter as being the tubular portion of the testis, to be composed of granules, which he deemed to be ova; and conceived the delicate white vessel seen to ramify through the ovarian mass, as represented in the figure, to be the oviduct, whereby the eggs were taken up and conveyed into the thick and glandular canal (h), from the walls of which he imagined that a fecundating liquor might be secreted for the impregnation of the ova in transitu. He therefore regarded the proboscidiform tube (h) as an ovipositor, whereby the ova derived from both sides of the body are expelled. Before scattering them abroad, as Cuvier noticed, the animal retains them for a considerable length of time concealed between the body and the mantle, where they form two or three irregularly-shaped layers. When the eggs are found in this situation, he observed that the ovaria were empty and the testicles much less tumid - circumstances which indicate the season of oviposition to be at an end.

Anatomy of Pentelasmis vitrea: a, external envelope of the body.

Fig. 235. Anatomy of Pentelasmis vitrea: a, external envelope of the body; b, the mouth; c, the oesophagus; d, the stomach; e,f, tract of the intestine; g, bristle inserted into the anal orifice; h, the oviduct. (After Hunter).