244 



SYSTEMATIC ZOOLOGY 



muscles which hold the two valves of the shell together. These 

 are called the adductor muscles, and there may be an anterior 

 and a posterior, or only a large one, which then corresponds 

 to the posterior muscle. These muscles serve to close the 

 valves of the shell, which are opened by an elastic ligament 

 attached to them at the hinge. 



The mantle consists of two lobes, a right and a left, attached 

 to the dorsal side of the body and completely lining the two 



valves of the shell. It is the 

 mantle which forms the pearls 

 used as ornaments. The edges 

 of the mantle may be entirely 

 free from each other, but fre- 

 quently there are two slight de- 

 pressions at the posterior end 

 in each mantle, so that when 

 they are approximated, two slit- 

 like openings are left. Through 

 the ventral opening the water 

 is drawn into the mantle cavity 

 and forced out through the 

 dorsal. In other cases the ed°:es 

 of the mantle are fused at the 

 posterior end. leaving two permanent slits, and in some cases this 

 fusion passes farther and farther toward the anterior end, until 

 the mantle forms essentially a sac inclosing the whole bodv with 

 only two openings at the posterior end, and one at the anterior 

 end through which the foot may be protruded. Further, the 

 edges of the mantle about the posterior openings may be drawn 

 out into two long tubes or siphons, the water flowing in through 

 the ventral and out through the dorsal. The two siphons may 

 remain distinct or the two may fuse, the tubes, however, remain- 

 ing separate, and one siphon may be longer than the other 

 (Fig- 253). 



The body lies within the mantle cavity. It is strictly bilater- 

 ally symmetrical and is usually considerably compressed later- 

 ally. The gills lie between the bodv wall and the mantle, and 

 are usually leaflike plates, two on each side of the bodv attached 

 near the dorsal side (Fig. 254). Their surfaces, like the inner 



Fk;. 252. Venus x'"'^'' 1 - Inner surface of 

 left valve, a/, anterior lateral tooth; a.tn, an- 

 terior adductor impression ; c, cardinal teeth ; 

 /, ligament; lu, lunule; p, pallia] line; /./, 

 posterior lateral tooth ; p.m., posterior adduc- 

 tor impression; p.s, pallia! sinus; //, umbo. 

 (From the Cambridge Natural History.) 



