280 COMPARATIVE ZOOLOGY. 
straggler of a mighty race dwells in the deep parts of the 
Indian Ocean, crawling on the bottom; and, while the 
shell is well known, only a few specimens of the animal 
have ever been obtained. 
2. Dibranchs .—These are the most active of Mollusks, 
and the tyrants of the lower tribes. Among them are 
the largest of invertebrate animals. They are naked, hav¬ 
ing no external shell covering the body, but usually a 
horny or calcareous part within. They have a distinct 
head, prominent eyes, horny 
mandibles, eight or ten arms 
furnished with suckers, two 
gills, a complete tubular fun¬ 
nel, and an ink-bag contain¬ 
ing a peculiar fluid ( sepia ), of 
intense blackness, with which 
the water is darkened to fa¬ 
cilitate escape. They have 
the power of changing color, 
like the Chameleon. They 
crawl with their arms on 
the bottom of the sea, head 
downward, and also swim 
backward or forward, usual¬ 
ly with the back downward, 
by means of fins, or squirt 
themselves backward by forc¬ 
ing water forward through 
their breathing funnels. 
The Paper Nautilus (Ar- 
gonctuta) and the Poulpe ( Octopus ) have eight arms. The 
female Argonaut secretes a thin, unchambered shell for 
carrying its eggs. The Squid ( Loligo ) and Cuttle-fish 
{Sepia) have ten arms, the additional pair being much 
longer than the others. Their eyes are movable, while 
Fig. 248.—Cuttle-fish (Sepia officinalis ); 
one fifth natural size. Atlantic coasts. 
