126 A Guide to the Zoological Collections in the 



the visceral hump is generally reduced in size, while some 

 part of the foot — usually the middle portion, or mesopo- 

 dium — is specially developed to form a fin, upon which an 

 adhesive sucker is commonly found. The shell, when 

 present, is small and thin, and like the rest of the animal 

 is transparent, in harmony with the environment. The 

 animals are pelagic, swimming at the surface of the open 

 seas in the warmer parts of the world, and in habit are 

 predaceous. Professor Ray Lankester divides the Sec- 

 tion into three sub-Orders, namely, (i) the Atlantacea, 

 in which the visceral hump, mantle, and shell are large ; 

 and all parts of the foot are well developed, — the whole 

 form, in short, being not very different from the ordinary 

 Azygobranch type ; (2) the Carinariacea, in which the 

 visceral hump, mantle, and shell are small, and the 

 foot is not demarcated from the rest of the body, except 

 in its middle portion, or mesopodium, which forms a 

 large fin ; and (3) the Pterotracheacea, in which the 

 visceral hump is still further reduced in size, the mantle 

 rudimentary, and the shell absent, the foot being similar 

 to that of the Carinariacea, The group is exemplified 

 in Case 166C, by models and enlarged drawings, and by 

 some shells and spirit specimens. The last, which are 

 placed above Case 166C, illustrate the transparency of the 

 body. 



ill. GASTROPODA [ANISOPLEURAJ OPISTHOBRANCHIA. 

 [ara0t0 167JI-168(E]. 



In the adult forms of this degenerate Order the foot is 

 large, and the visceral hump small ; the mantle is always 

 small and is often entirely wanting, and in correlation 

 with the condition of the mantle the shell is almost always 

 more or less delicate and deficient, or is completely absent. 

 In the larval stage, however, the larval nautiloid shell of 

 the typical Moilusk is present. The typical gill-plume is 

 sometimes present unmodified, is in other cases present 



