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.