202 SYSTEMATIC ZOOLOGY. 



lingual ribbon. In the anterior part of the mantle- 

 cavity are a pair of bunches of long threads of unknown 

 function; possibly they are sensory, possibly respiratory, 

 in nature. All of the tooth-shells are marine. 



CLASS IV. ACEPHALA. 



In the Acephala, as the name implies, there is no dis- 

 tinct head. The body 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 sur- 

 face 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,* whence the name Lamellibranchs, often applied 

 to the class. The muscular foot projects from the lower 

 surface of the body. 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 mantle- 



* It is not necessary here to include the gill features of Cuspi- 

 daria, Silenia, etc. 



