MOLLUSCA: PTEROPODA. 



417 



operculum (Limadnida). There is usually a symmetrical, 

 glassy, sometimes chitinous, shell (fig. 222), either consisting 

 of a dorsal and ventral plate united, or forming a spiral (fig. 



Fig. 223. Hyalea tridentata, show- 

 Fig. 222. Pteropoda. a, Cleodora pyrnmidata ; ing the shell and the lateral fins 

 b Cuvieria columnella. (After Woodward.) attached to the sides of the head 



215, B), but in some cases the body is naked, the mantle 

 being absent or rudimentary. The head is rudimentary, and 

 bears the mouth, which is occasionally tentaculate, and which 

 is furnished with an odontophore. There is a muscular 

 stomach and a well-developed liver; and the flexure of the 

 intestine is neural, so that the anus is situated on the lateral 

 or ventral surface of the body. 



The heart consists of an auricle and ventricle. The res- 

 piratory organ is very rudimentary, and consists of a ciliated 

 surface, which is either entirely unprotected, or may be con- 

 tained in a branchial chamber. 



The ganglia of the nervous system "are concentrated into 

 a mass below the oesophagus " (Woodward), united by a com- 

 missure above the gullet ; and the eyes are rudimentary. 



The sexes are united in all the Pteropods, and the young 

 pass through a metamorphosis, having at first a bilobed ciliated 

 veil attached to the sides of the head. 



The Pteropoda are divided into two orders, termed Thecoso- 

 mata and Gymnosomata ; the former characterised by possess- 

 ing an external shell and an indistinct head ; the latter by 

 being devoid of a shell, and by having a distinct head, with 

 fins attached to the neck. 



The Pteropoda, as already said, are found swimming near 

 the surface in the open ocean, and they are found in all seas 

 from the tropics to within the arctic circle, sometimes in such 

 numbers as to discolour the water for many miles. They are 

 nocturnal in their habits, and, minute as they are, they con- 

 stitute in high latitudes one of the staple articles of diet of 

 the whale. They themselves are, in turn, carnivorous, feeding 



2 D 



