338 



Professor Huxley 



[May 11, 



which inheres only in living muscle. Hence a dead oyster is readily 

 known by its persistent gaping. 



We shall see that a certain amount of separation of the valves is 

 necessary for the discharge of all the functions of the living oyster. 

 Hence, it is of advantage to the animal that this condition should be 

 assumed and maintained without any muscular exertion ; while intruders 

 and enemies can be shut out, or at any rate sharply pinched, at any 

 moment, by calling the adductor into action. 



If one valve is removed very carefully, so as not to injure the soft 

 body within, the form of the latter is seen to have a general corre- 

 spondence with that of the interior of the shell. It is therefore 

 flattened from side to side, with the left side convex and the right 

 flattened (Fig. 1). Its contour is oval, with the long axis perpendi- 

 cular to the middle of the hinge ; and there is a short dorsal side 



Fig. 1. 



An Oyster with the right valve of the shell removed. Natural size. 



sh. shell; um. umbo; /. ligament; m. mouth; p. palps; br. branchiae; mt. mantle; 

 add. adductor muscle ; h. heart ; cL cloaca ; ant. anterior ; post, posterior side. 



which answers to the latter, and is excavated in the middle, in corre- 

 spondence with the convex ventral face of the cushion of the ligament. 

 The dorsal half of the anterior edge of the body is convex, that of 

 the posterior edge is nearly straight, or even slightly excavated, 

 while the ventral margin continues back the curve of the anterior 

 edge. The cut end of the thick adductor muscle is a conspicuous 



