50 Lessons in Zoology. 



were made by tabe-bnilding worms. Some little mats with pin- 

 holes in them are the skeletons of tiny creatares, one of which lived 

 in each hole, or cell. Finally, some of the shells have been par- 

 tially riddled by the boring sponge, which makes its tannels by dis- 

 solving away the lime. 



The teacher now cnts the mnsole in her oyster, and as the ohil- 

 dren follow the conrse of the knife in her hand, they see that there 

 is bat one mnscle, and that after catting that, the shell may be 

 opened by breaking the ligament. 



The hinge is not exactly at the beak, but a little dis- 

 tance from it, and between the two is a widening groove 

 containing gome dried remains 

 of ligament (Fig. 1, I). In 

 the young oyster the valves 

 must have been united at the 

 beak, so the groove is the 

 downward path of the liga- 

 ment, as the oyster grew. The 

 inside of the valve is white. 

 There is a birge, dark impres- 

 sion near the center of the valve. A line extends nearly 

 aroand the shell, not far from the margin. This is the 

 pallial line. By laying the valve back over the oyster 

 we see that the large dark spot fits over tbe muscle, and 

 hence is the muscle scar. 



This soar has also traveled downward from the beak. To show 

 this, file the oatside of the thick valve to some depth, and strike 

 it sharply with a hammer, when the inner layers of the shell will 

 split off, disclosing the track of the mnsole. 



The oyster has a mantle that is open all around. The 

 mantle is only a little thickened at the edges. The edges 

 of the mantle are joined together in just one place, the 

 bar (Fig. 1, b), where the convex and the concave sides of 

 the shell meet. In front of the muscle is a clear space con 

 taining the heart (Fig. 1, A.), a little whitish bag. Under 

 the edge of the mantle are the gills, two pairs, as in the 



