LAMELL1BRAXCHIATA. 



63 



or a byssus, most are free and locomotive ; in these, the foot 

 (ib. e) is a mnscnlar body developed from the ventral surface 

 of the visceral mass. Where the foot is well developed and 

 the muscular power frequently exercised, the breathing appa- 

 ratus is usually complicated by distinct muscular tubes or 

 "siphons," one (ib. g') for the entiy, the other (g) for the .exit 

 of the respiratory currents of water. One valve of the shell 

 is applied to the right, the other to the left side, of the body, 

 the valves being articulated by interlocking parts called 

 "teeth," and by elastic fibres called the "ligament" at the part 

 of the shell called the "hinge" (fig. 16). The valves are also 

 attached to each other by one or two muscles, called " adduc- 

 tors," because they pull the valves together and close the 

 shell. In this ope- 

 ration they squeeze 

 the ligament, which, 

 by its elasticity re- 

 opens the shell on 

 the relaxation 

 the muscular action. 

 Each valve is a 

 cone, shewing every 

 grade of depth from 

 the flat plate of the 

 Placuna to the pro- 

 duced and spiral 

 cavity of Isocardia 



and Diceras; it is commonly shallow, with the apex or 

 umbo (fig. 15) turned to one side and directed forward. 

 Place a bivalve shell in the position of the Cytherea (fig. 

 15), and the direction of the umbo determines A as the 

 anterior border, and P as the posterior one ; the upper 

 or dorsal, and the lower or ventral border, are as marked 

 in the cut. The length of the shell is taken from A to 



Fig. 15. 

 Cytherea chione. 



