44 Lessons in Zoology. 



It is whiter and smoother than the outside. It has a 

 line around it near the edge. It has a broad mark near 

 the narrow end and another near the broad end of the 

 valve (Fig. 4). It has a deep curve between the line and 

 one broad mark. 



One child takes the living clam and finds he cannot 

 open its valves. The teacher then holds up a dead clam 

 in which the muscles have not been cut, and after care- 

 fully pushing back the fleshy bag that adheres closely to 

 the edges of the shell, she severs both mjscles. The 

 children see that when they are cut, the valves come apart 

 everywhere, except at the beak. One child now lays the 

 valve back over the body of 

 the clam, and finds that the 

 two broad marks on the inside 

 (Fig. 1, OM and pa) exactly 

 cover two firm, white organs, 

 which are the muscles that held 

 the valves together. In this ''"' * 



way see that the marks are the impressions of the 

 muscles, and were made by them. 



The same process shows us that the line near the edge 

 was made by the thick, muscular part of the fleshy bag, 

 or mantle, that covers the clam, while the deep curve, or 

 sinus (Fig. 4), is the impression of the siphon muscles. 



We have noticed that the dark skin covering the mar- 

 gins of the shell, seems to be connected with the mantle, 

 and, indeed, it is formed by the mantle border. This 

 same border of the clam's cloak is a hard worker, con- 

 stantly laying down new deposits of lime around the edges 

 of the shell. The mantle is closed except at the siphon 

 and the foot openings. 



After finding the little furrow in the lower edge of the 

 mantle and cutting through it from the siphon to the for- 



