THE PHYLUM MOLLUSCA 



by the mantle, which has a tough, rubbery texture and is very muscular. At 

 what appears to be the posterior end of the animal a pair of horizontal fins 

 extend laterally; the tentacles lie at the opposite end. The mantle encloses a 

 mantle cavity, a space which surrounds the visceral mass and contains the 

 paired, feathery gills. Attached to the visceral mass at the open end of the 

 mantle cavity is the exposed portion of the body, comprising the combined 

 head and foot. The foot is represented by the ten sucker-bearing arms and by 

 the siphon, a nozzle-like structure protruding from the mantle cavity. The 

 head bears the mouth, surrounded by the arms and provided with a pair of 

 horny jaws. On the sides of the head are located the large and well- 

 developed eyes. 



The visceral mass contains the digestive tract, which is essentially U-shaped 

 (Fig. 13.18). A long esophagus leads from the mouth to the stomach; the 

 stomach is connected with a large, sac-like caecum and receives the ducts from 

 two digestive glands. From the point where the caecum joins the stomach, the 

 intestine extends forward, becoming the rectum which opens into the mantle 

 cavity at the anus. The conspicuous ink sac opens into the rectum. 



Fig. 13.17. The common squid, Loltgo pealeii. Note the eight short tentacles and two longer 

 ones, all provided with rows of suction cups, and the highly developed eyes. The photograph 

 shows the chromatophores in the skin, which by continually expanding and contracting change the 

 color and appearance of the animal. (Photograph by George Lower.) 



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