ACEPHALA. 



In the Acephala, as the name implies, there is no distinct 

 head. The ootiy is flattened from side to side, and the two 

 sides are almost exact repetitions of each other. On either 

 side of the body there is a strong outgrowth of the body 

 wall, the mantle, which secretes on its outer surface the 

 shell, which is divided in the median line so that two halves 

 or valves result. Between the mantle folds and the body 

 is the mantle-chamber, and into this on either side there 

 usually hangs down a pair of leaf-like gills.* From the 

 lower surface of the body projects a muscular foot. 'With 

 these features the animal presents a marked resemblance to 

 a book in which the valves represent the covers; the mantle, 

 gills, body, and foot, seven leaves. 



Where the two valves are hinged together there is an 

 elastic ligament which tends constantly to open the valves, 

 which are closed by means of adductor muscles extending 

 from one valve to the other. Usually there are two of 

 these muscles anterior and posterior, but the anterior of 

 these may disappear. 



In some, as in the oyster, the mantle edges are free from 

 each other throughout their extent; but not infrequently 

 they become fused in places, leaving openings between. 

 At the posterior end this fusion frequently results in the 

 formation of two tubes or siphons connecting the outer 



* It is not necessary here to include the gill features of Cuspidaria, 

 Silenia, etc, 



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