62 OUTLINES OF NATURAL HISTORY. 



direction along the intestine, taking with it all undigested 

 food, till it reaches the second breathing-tnbe, from which 

 it is expelled in an outgoing current. On the other hand, 

 when the animal is disturbed, or when the sand in which 

 it lives is left bare by the retreating tide, the breathing- 

 tubes are partly drawn into the shell, and these currents 

 cease. 



We may now shortly examine the shell of Mya. As 

 before said, the shell consists of two j^ieces or valves, 

 which, as regards the Animal within, are right and left. 



The two valves are like each other in general form, 

 but the left valve is slightly the smallest. The valves 

 are hollow or concave, and they are applied to one another 

 along their concave aspects. They do not, however, fit 

 quite closely, but leave an opening for the breathing- 

 tubes, so that the shell is said to " gape." Each valve is 

 furnished along its ".dorsal" margin with a prominent 

 process or "beak" (fig. 26, h), and the valves are so 

 applied to one another that the beaks are nearly in con- 

 tact and are opposite one another. Between the beaks is 

 a mass of horny fibres which are compressed when the 

 valves are closed, so that when the animal relaxes the 

 adductor muscles (by which the shell is closed), the 

 valves are elastically forced apart. These horny fibres 

 constitute what is called the "ligament" and "cartilage," 

 and they are carried in part by a spoon-shaped process of 

 shell developed below the beak of the left valve (fig. 

 26, s). 



Externally, the shell exhibits numerous fine lines, 

 which run concentrically round the beaks, and which 

 mark the stages of the growth of the shell. The shell is 

 also covered with a thin, brown, or reddish-brown mem- 

 brane, which gives it its colour. 



The form of the shell is an oval, broader at one end 

 than the other. The beak is not placed quite in the 

 middle of the shell, but somewhat to one side. This side 

 of the shell is the broadest and shortest, and is the side 

 on which the mouth is situated (the " anterior " or front 

 side, fig. 26, a). The side of the shell from which the 



