186 



INVERTEBRATE ANIMALS. 



This class is a very small one, and includes a number of minute 

 oceanic Mollusks, which are found swimming near the sur- 

 face in the open ocean, far from land, and often in enormous 

 numbers. The organs of locomotion are two wing-like fins 

 (Fig. 88) attached to the sides of the head, and formed by a 



FIG. 88. Pteropoda. a Cleodora pyramidata ; 6 Cuvieria columnella. 

 (After Woodward.) 



modification of a portion of the foot. The body is usually pro- 

 tected by a symmetrical glassy shell (Fig. 88), consisting of 

 two plates united along their edges, or in other cases forming 

 a spiral. In some, however, there is no shell, and the body is 

 quite naked. The head is rudimentary, and bears the mouth, 

 which is furnished with an odontophore. The heart consists 

 of an auricle and ventricle, and the respiratory organs are 

 extremely rudimentary. The sexes are united in the same 

 individual in all the Pteropoda. 



The Pteropoda occur, as already said, 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 discolor the 

 water for many miles. Minute as they are, they constitute in 

 high latitudes one of the staple articles of diet of the whale, 

 and they themselves in turn are probably carnivorous, feed- 

 ing upon small Crustaceans and other diminutive creatures. 

 Though all the living forms are small, geology leads us to be- 

 lieve that formerly there existed comparatively gigantic forms, 

 which appear to be truly referable to this class. 



