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MOLLUSKS- ACEPHAI.S. 



Fig. 428. — Hylea. 

 Atlantic. 



Fig. 429. — Limacina. 

 South Polar Seas. 



Fig. 430. — Clio. 

 Arctic. 



ACEPHALS, OR HEADLESS MOLLUSKS. 



These mollusks seem to have no head, this part of 

 the body being concealed within, and only faintly 

 shown, as in Clams, Oysters, Mussels, &o. 



BIVALVES. 



These are acephals which have a shell composed of 



Fig. 431. — One valve of a Bivalve Shell, showing the names of the parts. 



a, anterior retractor muscle ; 6, posterior retractor -, i, lateral teeth •, c, cardinal tooth j 

 I, lunale ; u, umbo ; A, hinge ligament i a, retractor of siphons ; p, pallial impression. 



two pieces, or valves, joined together on one side by a 

 hinge, and held tightly together by one or two strong 



