106 ANIMAL BIOLOGY 



chambered nautilus and other molluscs, but by the ex- 

 panded posterior arms; it occurs only in the female where 

 it serves as an egg case. 



Most of the other living cephalopods have a relatively 

 small shell which is overgrown by the mantle. This may 

 be calcareous as in the cuttle fish which furnishes the 

 " cuttle fish bone" that we often give to canaries, or it 

 may be chitinous as in the "pen" which lies along the dorsal 

 side of the body of the squid. It is commonly said that the 

 squid carries its own pen and ink; the animal is furnished 

 with an ink-sac containing a black fluid which may be dis- 

 charged in times of danger, producing a black cloud in the 

 water which facilitates the escape of the animal. There are 

 many species of squid living in various parts of the world. 

 One of the forms most easily obtainable for study is the 

 small Loligo pealii found along our Atlantic coast. Of 

 the ten sucker-bearing arms extending in front of the head 

 two are longer than the others. In the center of the circle 

 of arms is the mouth with its two strong, horny jaws 

 resembling the beak of a parrot. As in other cephalopods 

 the head bears two large, well-developed eyes. The 

 squid swims backward by suddenly expelling water from its 

 mantle chamber through the siphon which is a short tube 

 below the head. This siphon can be turned in various 

 directions so that the reaction of the expelled water may 

 cause the animal to turn in different ways. The mantle 

 cavity contains the gills of which there are two pairs. 

 Squids are very active animals, living on small creatures 

 which they capture by means of their arms. They are 

 remarkable for the rapid changes of color which may 

 pass over the skin, especially when 'they are disturbed. 

 There are a few species called the giant squid which attain a 

 very large size, with a body nine to ten feet long and with 

 the longest pair of arms reaching a length of forty feet. 



