CEPHALOPODS 467 
the animal swims backward; but it can be turned back over the 
edge of the mantle, giving a forward movement. 
The surface of the mantle is covered with pigment-cells (chro- 
matophores). There are sets of chromatophores containing differ- 
ent colors. The cells are opened or closed at will by muscular 
action of their walls. When open the cells seem to fuse together, 
giving a solid color-surface, or spots of color as desired, in blue, 
red, yellow, or brown; when closed they seem as specks on the 
almost transparent tissues of the animal. Flashes of changing 
color follow one another with great rapidity over the living ani- 
mal. In swimming it assumes the color of its surroundings. 
Another curious means of protection possessed by the dibran- 
chiate cephalopods is an ink-bag, the brown or black secretions 
of which are ejected through the siphon, clouding the water when 
the animal wishes to escape from danger. The ink taken from 
the ink-sac of Sepia is an article of commerce. 
Only Nautilus and the female Argonauta have the characteristic 
external shell of mollusks. In all other forms the shell is internal 
or is invested in the integument of the mantle. Of such is the 
cuttlebone of commerce, which is a calcareous leaf-like body 
obtained from Sepia, the cuttlefish. The common squid Loligo 
has a horny substance situated in the dorsal side of the mantle, 
called the pen. Spirula has a spiral internal shell divided into 
chambers. Vast numbers of these shells are cast upon the 
beaches of the Pacific Islands, and they have also been found on 
the shore of Nantucket. 
Cephalopods are separated into two subclasses. In the first, the 
Tetrabranchiata, there are four branchie, four nephridia, and four 
auricles. They are without an ink-sac, and have the foot divided 
into lobes bearing sheathed tentacles. Nautilus is the only genus. 
SUBCLASS TETRABRANCHIATA 
Genus Nautilus 
Probably the best-known cephalopod is Nautilus—the pearly- 
shelled, chambered nautilus made immortal in the beautiful poem 
of Oliver Wendell Holmes. The shell of Nautilus is a flat spiral ; 
the interior is divided by septa forming a series of chambers; 
