204. INVERTEBRATE ANIMALS. 
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 discolour the water for 
many miles. Minute as they are, they constitute in high lati- 
tudes one of the staple articles of diet of the whale, and they 
themselves in turn are probably carnivorous, feeding upon small 
crustaceans and other diminutive creatures. Though all the 
living forms are small, geology leads us to believe that formerly 
there existed comparatively gigantic forms, which appear to be 
truly referable to this class. 
