MOLLUSCA — CEPHALOPODS 



269 



a means of defense, since under cover of the blackened water the 

 animal may escape. Indications of this ink bag are often 

 present in fossil forms in the shape of an external mold of the 

 sac, and even in carbon particles, the remains of the ink itself. 



The typical internal shell is seen in Belemnites, a fossil form 

 described below. The shell is absent in the devil-fish (Octopus). 



The Dibranchiata are known from the Triassic to the present. 



Fig. 121. — An American squid, Ommastrephes illecebrosa Les. (xi), from the 

 Atlantic Ocean at Provincetown, Mass. A, dorsal view. B, side view of animal 

 asleep ; note how the water for respiration is kept pure by the elevation of the 

 anterior mantle slit above the mud. C, the jaws in position. D, side view of tip 

 of skeleton. E, upper view of entire skeleton. (All drawn from life by J. Henry 

 Blake.) 



Ommastrephes (Fig. 121). Living. 



This marine, free-swimming animal is world-wide in its 

 distribution. In general all forms similar to this genus are 

 commonly called squids. It has a distinct head bearing ten 

 long, very muscular arms and two large, highly developed eyes ; 

 two of the arms are much larger than the other eight. The 

 head is separated from the trunk by a constricted region, the 

 neck. The trunk is elongate, shield-shaped, bordered laterally 

 by a fleshy fin. The entire body is surrounded by a thick 

 muscular mantle which is free from the body except along the 

 median dorsal line. 



