196 MOLLUSKS: CEPHALOPODS. 



of an arrow. Immense numbers are used for bait in 

 the cod-fishery. 



Cuttle-Fishes resemble Squids, but have two of the 

 arms or tentacles much lengthened and expanded at 

 their tips ; and they have a broad, internal shell, called 

 cuttle-bone. This is the " cuttle-bone " which is given 

 to canary-birds. On the coasts of the Eastern Mediter- 

 ranean cuttle-fishes are so abundant that the cuttle- 

 bones are thrown up by the waves into ridges miles in 

 length. Like other cephalopods, cuttle-fishes have the 

 power of clouding the water by ejecting an inky fluid 

 into it when they wish to escape. This ink, when dried 

 and prepared, is the " sepia " used in painting. 



SPIRULAS. 



The Spirulas resemble those just described, but have 

 a coiled shell inside, Figure 362, and the shell is di- 

 vided by partitions into chambers. 



NAUTILI AND AMMONITES. 



The Nautilus is the only living Cephalopod which has 

 an external chambered shell. Figure 363 shows the 

 Nautilus as it appears when cut open ; the animal is 

 in the outer chamber, which communicates with all 

 the others by means of a tube called the siphuncle. 

 The animal has occupied each chamber in turn, mov- 

 ing forward, and making a partition behind as often 

 as it outgrew its old home. 



Ammonites, Figure 361, are chambered-shelled Ceph- 

 alopods that lived in the seas ages ago ; hundreds of 

 kinds of these, from an inch to a yard in diameter, 

 are found imbedded in the rocks of this and other 

 countries. 



