54 LEPADID^E. 



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 cor- 

 responding 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 Scal- 

 pellum 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 appear- 

 ing 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 ex- 

 tent, to serve as antennae, 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, appa- 

 rently, of noises in the air or water. In a group of 

 specimens, I could not touch one even most delicately 



