34 BALANID.E. 



the mouth and rest of the body. The anterior extremity of 

 the shell is situated in the centre of the basis, where indeed, 

 by due care, the antennae of the pupa may be always detected; 

 and the posterior extremity is directed vertically upwards. 



When the shell of a sessile Cirripede or barnacle, for 

 instance, of a Balanus, is first examined, the structure ap- 

 pears extremely complicated ; but this can hardly be con- 

 sidered as really the case. The structure will, I think, be 

 best understood by recalling to mind that of Pollicipes, — 

 the oldest known genus, from which, in one sense, all ordi- 

 nary Cirripedes, both sessile and pedunculated, seem to 

 radiate. I must premise, and the fact in itself deserves 

 early notice, that the homologies of the several parts in the 

 pedunculated and sessile Cirripedes admits of no doubt, — 

 that is, if amongst the pedunculated, the genus Pollicipes, 

 or certain species of Scalpellum, be taken as a standard of 

 comparison.* The peduncle corresponds with the basis, as 

 may be clearly seen, if a Pollicipes with a short peduncle, 

 and a Balanus, with a deep cup-formed or cylindrical basis 

 be compared, for the contained parts are similar, and both 

 grow at their upper edges upwards and outwards. Secondly, 

 the valves round the lower part of the capitulum of a Pol- 

 licipes, though generally much more numerous, and forming 

 more than one whorl or circle, and not so closely packed 

 together, answer to the compartments forming the shell of 

 a sessile Cirripede ; this is shown by their lateral and down- 

 ward growth, by their upper ends generally projecting freely 

 above the cavity in which the animal's body is lodged ; and 

 in the case Pollicipes mifella, by an actual resemblance in 

 outline, some being triangular, some broad at the upper 

 end, and some sub-rhomboiclal, and, lastly, in the manner 

 in which they slightly overlap and indent each other : more- 

 over further resemblances in the relative position and even 

 in the size of the several valves, will hereafter be pointed 

 out between certain sessile genera amongst the Chthamalina) 



# 



Dr. J. E. Gray long ago observed these homologies. If Lepas be taken, 

 the comparison is not quite so simple, owing to the growth of all the valves in 

 that genus being upwards ; but in several species of Scalpellum we may see the 

 intermediate steps between the normal downward growth of the valves in 

 Pollicipes, and the abnormal upward growth in Lepas. 



