170 A COURSE ON ZOOLOGY. 



be distinguished by its twisted condition. The respira- 

 tory apparatus is a sort of lung consisting of a respira- 

 tory chamber whose ceiling is traversed by canals filled 

 with blood. 



ACEPHALANS. 



The acephalans have no distinct heads ; the mouth is 

 always hidden at the bottom of the mantle or in its 

 folds ; the gills are striated leaves ; the lower part of the 

 body is ordinarily elongated into a sort of fleshy foot. 

 The shell consists of two valves articulated by a hinge. 

 All these mollusks are aquatic, and most of them inhabit 



FIG. 99. 



DIAGRAM OF INTERNAL STRUCTURE OF OYSTER (the dorsal surface is down- 

 ward, the anterior or head end to the left). a, region where water enters 

 and leaves the animal ; the dark lines indicate where one mantle-flap has 

 been cut away to expose the other structures ; 6, gills ; c, margin of one of 

 the mantle-folds ; d, anterior part of hinge ; e, hood over mouth ;/, position 

 of mouth ; gh, labial palps ; i, end of intestine ; I, the closing muscle of 

 the shell ; m, position of the heart. 



the sea. Many, like the oyster, remain firmly fixed on a 

 rocky moorage ; others drag themselves over the muddy 

 bottom. The class includes an immense variety of species, 



