LECTURES ON MOLLUSCA. 95 



numerous ; their tiny, fragile, transparent forms being found in vast 

 shoals, so filling the sea, that even in the Arctic regions the water is 

 often discolored by th^m. They never willingly approach the shore, 

 not having the muscular power of the Cephalopoda to swim away from 

 danger : but their delicate glassy shells line the sea bottom at enormous 

 depths, and in man} r districts will form almost the only fossils by which 

 future geologists will recognize the strata. The living forms of Pter- 

 opods are all very small, the largest scarcely reaching two inches in 

 length. They first appear in the Eocene beds. There are, however, 

 certain puzzling shells, found in the palaeozoic rocks, which may have 

 belonged to gigantic animals of the tribe. 



ORDER I. GYMNOSOMATA. (Naked Pteropods.) 



These creatures have no mantle or shell, and the gills are indistinct. 

 They have however a respectable head, and a tongue-ribbon of nu- 

 merous rows of hooked teeth, as in the Opisthobranchs. Like all the 

 other Pteropods, they are carnivorous, preying on minute Crustaceans, 

 Jelly Fish, or Infusoria. 



Family PNEUMODERMONIM. 



The Pneumodermons have the body shaped something like a Cuttle- 

 fish,, and highly colored. There are two tentacles, copiously fringed 

 with tiny anther-like suckers. The gills are leaf-like projections at 

 the tail. When touched, they fold their wings round their neck, roll 

 themselves into a ball, and fall to the bottom. In Spongiobranchia, 

 the gills form a spongy ring round the tail ; and the tentacles have 

 cup-shaped suckers, forming a close approach to those of the Cuttles. 

 In Tricliocydus , there are no gills ; but three rows of tiny hairs round 

 the head, tail, and middle take their place. 



Family 



Clio was the name given by Linnasus to all the Pteropods then 

 known. It is now restricted to rather slender animals which, small 

 and delicate as they are, form the principal food of the mighty whale. 

 The monstrous creature opens his enormous mouth; takes in a sea of 

 water ; filters out his Clios through the whalebone sieve ; and ejects 

 the water through his nose. The Clios have a number of small ten- 

 tacular processes round the mouth, furnished with minute suckers. 

 In swimming, it touches the ends of its fins on each side. In Cliodita 

 the tentacles are obsolete. In Pelagia the head (to speak respectfully 

 of this indistinct organ) is truncated in front. 



Family CYMODOCEID^E. 



Cymodocea differs from other Pteropods in having a second pair of 

 club-shaped wings, behind the ordinary ones. 



ORDER II. THECOSOMATA. (Clothed Pteropods.) 



In these headless tribes, the body is generally shortened, and inclosed 

 in a p-lassv. hornv. or cartilaginous shell. 



