CLASS II. PTEROPODA, Cuvier. 



Pelagic animals in which the mid-region of the foot in its primi- 

 tive condition, is relatively largely developed, and drawn out into 

 a pair of wing-like muscular lobes, which are used as paddles. The 

 head is often rudimentary, but may be drawn out into one or more 



iS J J 



pairs of tentacles, simulating cephalic tentacles, and provided with 

 suckers. The hind region is often aborted, but may carry an oper- 

 culum. 



The visceral hump is not twisted, except in the Limacinidce. Jaws 

 and a radula are present. Very few forms show cephalic eyes ; oto- 

 cysts are universally present. The gonads are both male and female 

 in the same individual. The genital aperture is single ; copulatory 

 organs, often of considerable size, are present. 



A mantle-skirt and shell is present in one division of the Pteropoda 

 (Thecosomata), and in these an extensive subpallial chamber is deve- 

 loped, the walls of which, in the absence of ctenidia, have a branchial 

 function. 



In a second division (Gymnosomata) the mantle-skirt is aborted, 

 and there is no shell in the adult animal. 



The Pteropods inhabit the high seas, floating constantly in the 

 water by means of the lateral fins. They are extremely vivacious 

 in their movements, and are frequently together in prodigious num- 

 bers. 



Fossil they first appear in the Palaeozoic. 



ORDER 1. THECOSOMATA, Blainville. 



Pteropoda provided with a mantle-skirt, and with a delicate hya- 

 line shell developed on the surface of the visceral hump and mantle- 

 skirt ; visceral hump, and consequently the shell, spirally twisted in 

 one family, the Limacinidce ; shell often with contracted mouth and 

 dilated body, its walls sometimes drawn out into spine-like processes, 

 which are covered by reflexions of the free margin of the mantle. 



Earn. CYMBULIID^l, Cantraine. 



Hyalceidte, in. part, of authors. Alata, Wagner, 1885. 



Shell straight, bilaterally symmetrical, so-called cartilaginous, quite 

 enveloped in the mantle. The animal^cannot completely retire within 



