186 THE MECHANICAt FUNCTIONS. 



employ them as oars for the purpose of enabling the animal 

 to swim through the water. ' 



Mollusca of this description are found in great abundance 

 in the colder regions of the ocean surrounding both the 

 Y2Q north and south poles; and other species 



are also met with, though in smaller num- 

 bers in the tropical seas. The Clio borea- 

 lis, of which Fig. 120 is a representation, 

 is the most perfect specimen of this form 

 of construction. It swarms in the Arctic 

 seaSj and constitutes the princij^al food of 

 the whale. The position of its gills, which 

 perform 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 Pterojjoda, given by Cuvier to this order of Mol- 

 lusca. 



§ 6. Cej)halopodat 



Following the progress of organic development, we now 

 arrive at a highly interesting family of Mollusca, denomi- 

 nated the Cephalopoda, and distinguished above all the pre- 

 ceding orders by being endowed with a much more elabo- 

 rate organization, and a far wider range of faculties. The 

 Cephalopoda have been so named from the position of cer- 

 tain organs of progressive motion, which are situated on the 

 head, and like the tentacula of the Polypus, surround the 

 opening of the mouth. (See Fig. 121.) These feet, or arms, 

 or tentacula, if we choose so to call them, are long, slender, 

 and flexible processes exceedingly irritable, and contractile, 

 in every part, and provided with numerous muscles, which 

 are capable of moving and twisting them in all directions with 

 extraordinary quickness and precision. They are thus ca- 

 pable of being employed as instruments, not only of progres- 

 sive motion, but also of preliension. For this latter pur- 

 pose they are in many species peculiarly well adapted, be- 

 cause being perfectly flexible as well as highly muscular, 



