2-'j8 the mechanical functions. 



ploy them as oars for the purpose of enabling the 

 animal to swim througli the water. 



MoUusca of this description are found in great 

 abimdance in the colder regions of the ocean 

 surrounding both the north and 

 south poles ; and other species 

 are also met with, though in 

 smaller numbers, in the tro- 

 pical seas. The Clio borealis, 

 of which Fig. 120 is a repre- 

 sentation, is the most perfect 

 specimen of this form of con- 

 struction. It swarms in the 

 Arctic seas, and constitutes the principal food of 

 the whale. The position of its gills, which per- 

 form the office of oars or feet, at the same time 

 that they resemble in their shape and action the 

 wings of an insect, are characters which have 

 suggested the title of Pteropoda, given by 

 Cuvier to this order of MolUisca. 



^ 6. Cephalopoda. 



Following the progress of organic develope- 

 ment, we now arrive at a highly interesting 

 family of MoUusca, denominated the Cephalopoda, 

 and distinguished above all the preceding orders 

 by being endowed with a much more elaborate 

 organization, and a far wider range of faculties. 



