GENERAL ZOOLOGY 



Fig. 13.20. An octopus. (Photograph by John F. Storr.) 



deeply pigmented fluid stains the water being pumped from the mantle cavity 

 and spreads to hide the movements of the squid. In feeding, the animal de- 

 pends on its great speed and agility to capture fish, which are grasped in the 

 arms and brought to the mouth. Here, the beak-like jaws cut ofT coarse 

 pieces of the prey, which are passed into the stomach and caecum for diges- 

 tion. It is interesting to note that the squid bears in the floor of the buccal 

 cavity a vestigial radula resembling the feeding organ of chitons and of 

 gastropods. In fact, radulas occur in all the modern molluscan classes 

 except the pelecypods. 



The greater efficiency and higher specialization of the circulatory and 

 sensory-neuro-muscular systems of the squid are presumably related to its 

 more active way of life, as compared with the more sedentary representatives 

 of other molluscan classes. 



Other dibranchiates are the giant squids, which inhabit the ocean far from 

 land but are occasionally washed ashore; the cuttlefish. Sepia, known for 

 its pigment and for its shells, which are the light, calcareous "cuttlebones" 

 sold as bill sharpeners for cage birds; the "devil fish," Octopus, a less active 

 animal than the squid, with eight arms and no vestige of a shell (Fig. 13.20); 

 and the paper nautilus, Argonaula. The paper nautilus builds a very thin and 

 fragile spiral shell, resembling in form that of the nautilus, but this shell is 

 used chiefly as an egg case and does not enclose the body of the adult. 



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