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EVOLUTION OF ANIMALS 



Part V 



coupled with the development of an acute sense of sight, a contrast to the lack 

 of vision in the lethargic clams. 



The mantle is a conical envelope from which the head and siphon protrude, 

 the latter structure representing the front and rear of the body; the digestive 

 tube is bent double like a jackknife (Fig. 31.3). The shell is a quill feather- 

 shaped plate of chitin buried under the skin on the dorsal side of the body. 



The jaws resemble a parrot's beak and with them a squid can kill a fish by 

 a single bite through the spinal cord or head. The ink sac is a relatively large 

 pear-shaped organ. It consists of a gland which secretes the ink, a sac for 

 storage, and a duct leading to the anal chamber from which the ink is ex- 

 pelled. 



The sexes are separate, each with one gonad opening toward the siphon. 

 When mating a sperm packet is transferred by the specialized right arm of the 

 male to the mantle cavity of the female where fertilization eventually occurs. 

 The eggs are laid in long capsules of jelly from which the young ones emerge, 

 minute but in perfect squid form (Fig. 31.18). 



Fig. 31.18. Common squid {Loligo pealii). Squids stand on their heads when 

 laying eggs. The gelatinous egg capsules, about three inches long, are discharged 

 through the opening of the siphon and attached by one end to seaweeds and rocks, 

 usually in clusters. They are commonly washed in upon the shore all along our 

 coasts. 



