6 MOLLUSC A. 



valve will presently appear. If all the observations 

 can be made by the scholars themselves, the next step 

 will be to determine whether the valves are open all 

 around. It being determined that they are fastened 

 together, and closed at one end, the nature of this 

 fastening can be investigated. Lead them to try first 

 by pressing on the larger end, and then on the pointed 

 end of the smaller and flatter valve. By this tilting 

 they ought to be able to tell the teacher how wide the 

 oyster can open the valves, and that they are held 

 together by some bond (muscle) existing inside near 

 the broad ends, as well as by a sort of hinge at the 

 closed or pointed ends. 



Let them determine for themselves what experiment 

 would be sufficient to prove all these facts. Then, fol- 

 lowing their suggestions, take off the upper valve and 

 show the position and action of the large muscle 

 (Fig. I, ad), which acts only to close the valves and 

 hold them together in the Hving animal. It is best for 

 the teacher to have at his disposal an oyster with the 

 muscle alone cut through ; and this he can use at this 

 point to show how the hinge can act when not re- 

 strained by the muscle. It will be at once seen, that, 

 when tilted, the beak of the upper valve soon comes 

 in contact with the lower, and thus limits the extent 

 of the opening of the valves at the opposite or broad 

 end. 



The operation of cutting off the upper valve requires 

 care ; but I know from experience that workingmen 

 can be taught to do it successfully in a few minutes.* 



* When time is an object, and the scholars numerous, it is 

 advisable to have the muscle cut in all the specimens before- 



