MOLLUSC A. 7 



The knife-blade should be passed between the 

 mantle and the mner side of the upper valve ; and 

 when this has been separated, the muscle can be cut 

 loose by the same process, though very slowly and 

 carefully. If this is well done, the muscle will be the 

 only part which has been lacerated ; the mantle will be 

 entire throughout, and the hinge unbroken (Fig. i).* 



Thus, when the upper valve is Hfted or broken off 

 gently, the hinge will be seen to break just at the edge 

 of the white internal surface. They can then, by prob- 

 ing this and observing it, see that it is not unlike a 

 piece of horn, elastic, and quite capable of acting as a 

 spring to keep the valves open (Fig. i, Ji). 



A piece of bent horn can be used to illustrate this 

 point, though it should not be forgotten that these are 

 distinct, though horny, substances, and derived from 

 far different animals, and that the ligament loses its 

 elasticity when dried. 



Replace the smaller valve, and proceed to the more 

 minute study of the structure of the shell itself, which 

 must be clearly understood before any explanation can 

 be given of how it is built by the soft animal within. 



Notice, in the first place, how each valve is rough- 

 ened and encircled by concentric, projecting ridges. 

 The edges of these may be followed back towards the 



hand; but the hinge had better be left untouched. In this case 

 the action of the hinge in opening the valve, and of the beaks 

 in limiting the amount of this action, can be just as readily 

 taught, and the function and strength of the muscle in holding 

 them together shown by the teacher with a live specimen, 



* It must be noticed, that the lower valve has been removed, 

 and not the upper one, in this figure. 



