PTEROPODA. 13 



The Nautilus occasionally swims, like other members of its class, 

 at the surface of the sea, but mostly crawls about leisurely on its 

 feet at the bottom in search of food, which consists chiefly of small 

 crabs or Mollusca, which it crushes with its strong calcareous 

 parrot-like mandibles. 



The chambered shell is pearly within, and covered with an external 

 calcareous layer. The chambers are connected by a slender tube 

 or siphon, the function of which is not at present thoroughly 

 understood. The septa, or partitions across the shell, indicate 

 periods of growth. When the Nautilus outgrows the capacity of 

 the outer chamber, in which it resides, it constructs a new one of 

 larger size, separating the additional chamber from the preceding 

 one by a transverse partition. 



PTEROPODA*. 



The Pteropods are sometimes called Sea-butterflies, and, like the [Casel k.] 

 preceding group (Cephalopods), are organized for swimming freely 



Fig. -j. 



a *- 



Shell-bearing- Pteropod (Cavolina tridentatd). 



a. Shell and animal, b. Side view of shell, c. Dorsal view of shell. 



in the ocean. They have a pair of fins developed from the sides of 

 the mouth or neck, which perform a flapping movement during 

 progression. Some Pteropods (Thecosomata) arc provided with 



* From the Greek : pte?-o>t, wing, and pous, foot. 



c2 



