THE CLAM AND OTHER BIVALVED SHELL-FISH. 65 



within can be named by reference to Fig. 65. The atten- 

 tion of the student should be drawn to the byssus, which 

 arises from the base of the foot (//). The gills (i, j) and 

 large feelers (y) should also be noticed. 



The pearl-mussel should be noticed here, and every 

 school museum should have a specimen. It is very large, 

 with a straight, broad, thick hinge-edge, but with no 

 teeth. The shell is rough enough outside, but within is 

 lined very thickly with mother-of-pearl, which is cut into 

 paper-cutters, pen-handles, and various ornaments. 



Pearls are often produced in these shells by particles of 

 sand getting in between the mantle and the shell. This 

 irritates the soft flesh and deforms the shell, so as to give 

 rise either to a little bunch on the surface of the shell, or 

 the particle remains free from but next to the mantle, and 

 thus a round mass or pearl is formed. This may occur in 

 almost any bivalve shell, but the largest and most perfect 

 are formed in the pearl-mussel or pearl-oyster of Ceylon and 

 Panama, and other places in the tropical seas, where exten- 

 sive pearl-fisheries are carried on. The largest pearl known 

 is two inches long, four round, and weighs 1800 grains. 



Now we come to the oyster; and while the animal is de- 

 licious to the taste, the shell is thick, solid, rough, and 

 homely, and of various shapes, though usually oval. The 

 hinge is without a tooth, and, like the scallop, has a hollow 

 in each valve containing the ligament. There is a single 

 large dark muscular impression. The edge of the shell is 

 sometimes scalloped, though usually plain. One valve is a 

 little smaller than the other. 



The grown-up oyster cannot go about in the world. It 

 lies attached by one valve to some stone, or its fellow-oys- 

 ters. Fixed immovably to the bottom, it opens the smaller 

 upper valve, so as to admit the sea-water bearing its food. 

 It therefore needs no siphon, and has none; neither does it 

 have a foot, as it needs none. The oyster gets along well 

 enough without these organs; they would be in its way. The 

 animal is thus adapted to a mode of life quite different from 

 5 



