322 NATURAL HISTORY. 
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 lmed. 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, with 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. ae 
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 their organization. It has powerful jaws armed 
