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. Spirilla 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 

 Tetmbranchiata, there are four branchiae, 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; 



