PEDUNCLE. 81 



colour, and becomes much harder and more persistent ; 

 and as the whole valve is formed of component laminae 

 thus edged (the once continuous laminae of non-calcified 

 chitine connecting the valves, having disintegrated and 

 disappeared) the surfaces of the valves are generally left 

 covered by a persistent membrane, constituted of these 

 edgings : this membrane has been called the epidermis. 

 In some genera, as in Lepas, this so-called epidermis is 

 seldom preserved, excepting on the last zone of growth : 

 in Scalpellum and Pollicipes it usually covers the whole 

 valves. It appears to me that the laminae of chitine, 

 and of calcified chitine composing the valves, are both 

 formed not by secretion, but by the metamorphosis of an 

 outer layer of corium into these substances. 



Within the capitulum is the sack, which, together with 

 the upper internal part of the peduncle, encloses the 

 animal's body. The sack is lined by a most delicate mem- 

 brane of chitine, under which there is a double layer of 

 corium ; this double layer is united together by short, 

 strong, transverse bundles of fibres, branched at both 

 ends :* in some genera, the ovarian tubes extend between 

 these two layers. We have seen, under the head of the 

 Metamorphoses, that the delicate tunic lining the sack is 

 simply a duplicature of the thick membrane and valves 

 forming the capitulum, the whole being the posterior 

 portion of the carapace of the larva slightly modified. 



Peduncle. — Its length varies greatly in different species, 

 and even in the same species, according to the situation 

 occupied by the individual; its lower end is some- 

 times pointed, but generally only a little narrower than 

 the upper end. In outline, the peduncle is usually 

 flattened, but sometimes quite cylindrical. It is com- 

 posed of very strong, generally thick, transparent mem- 

 brane, rarely coloured reddish, and often penetrated by 

 numerous tubuli. The underlying corium is sometimes 



* I am much indebted to Mr. Inman of Liverpool for having kindly sent 

 me excellent specimens illustrating this structure. 



