322 NATUEAL HISTOET. 



ment or reparation of the shell. It is by the action of 

 the arms as oars, and by the forcing out of water from 

 the gill-chambers, that the animal can swim. Both the 

 Argonaut and the Cuttle-fish use their arms as feet to 

 walk along on the bottom of the sea. 



554. The Pearly Nautilus is so called from the " nacre," 

 or mother-of-pearl with which its shell is lined. It is 

 found on most shores between the tropics. It is pecul- 

 iar in having many separate chambers in its shell, in only 

 one of which, the largest and the outermost, the animal 

 lives. It has a connection, however, w'ith the other cham- 

 bers by a membranous tube called the siphuncle. It is 

 supposed that the animal, when it wishes to sink in the 

 water, can force some water into this siphuncle, thus in- 

 creasing its specific gravity ; and that the reverse takes 

 place when it wishes to rise. Some doubt this, and con- 

 sider the design of the siphuncle and the chambered 

 structure as yet a mystery. 



555. The Pteropoda, or wing-footed Mollusks, consti- 

 tute a small and aberrant group. The animals included 

 in it may be considered as having the same place in the 

 Molluscous kingdom that the Birds have in the Verte- 

 brate and the Insects in the Articulate. They fly in the 

 water, having for the purpose a pair of fin-like organs, or 

 wings, which are an expansion of the mantle on each side 

 of the neck. Though the number of species in this group 

 is small, the number of individuals in some of the species 

 is often enormous in some localities. Some have a shell 

 and some have not. 



556. The Clio Borealis,Fig. 254, one of the best known 

 of this class of Mollusks, is very abundant in the arctic 

 seas, and is one of the principal articles of food of the 

 Whalebone Whales (§ 192). These little animals are 

 sometimes so numerous that the Whale can not open its 

 mouth without ingulfing thousands of them. The Clio 

 has eyes, which, though exceedingly small, are very per- 

 fect in theiv organization. It has powerful jaws armed 



