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MOLLUSKS: ACEPHALS. 



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, <fcc. 



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 ; d, posterior retractor -, t, lateral teeth ; c, cardinal tooth ; 

 /, lunale ; , umbo ; A, hinge ligament ; s, retractor of siphons 5 #, pallial impression. 



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

 hinge, and held tightly together by one or two strong 



