70 FIRST LESSONS IN ZOO LOOT. 



way that the oyster is spread almost all over the world. In 

 its babyhood it is a great traveller. 2. The spat. When 

 the shell becomes thicker and heavier the young oyster 

 sinks to the bottom and settles down to a quiet life. In 

 this stage the spat fixes itself, with the head or hinge end 

 downward, usually to the old oyster-shells ; and thus a nat- 

 ural oyster-bank is formed ; and there may be seen in any 

 bed of oysters four sets of shells, i.e., the dead shells buried 

 in the sand, etc., at the bottom ; a second set partly living 

 and partly dead ; a third set in which all are living, and 

 to whose shells the spat are attached, though some of the 

 spat live attached in the holes between the dead shells. 



At the time the fry is large enough to be fixed (Fig. GO) 

 it is g 1 ^ of an inch in diameter. The shells are now some- 

 what like those of a cjuohog, and both valves are alike, as 

 in that shell (3). As soon as the oyster ceases to be active 

 and becomes fixed it ceases to be symmetrical, the lower 

 shell being unlike the upper, and thus takes a backward 

 step. 



The oyster grows rapidly after it is fixed. Fig. 69, 7, rep- 

 resents an oyster not over twenty days old, and Fig. 69, 10, 

 one not over three mouths old. The oyster gets its growth 

 and lays eggs by the time it is a year old, while most of the 

 oysters sold in the markets have reached the age of two or 

 three years. How many years the oyster can live is un- 

 known, but probably ten or fifteen ; some bivalves are 

 thought to live nearly a century. 



Many animals prey upon the oyster. It also takes in, 

 whether willingly or not, as a boarder or messmate, a little 

 yellowish crab ; the oyster crab, which seeks shelter be- 

 tween its shells. Upon the shell of the crab, at times, grow 

 multitudes of animalcules, whose young are swept into the 

 oyster's mouth, and thus the crab may involuntarily be quite 

 useful in purveying for its host the oyster. 



The two more deadly enemies of the oyster are the 

 "drill" and starfish. The drill is a snail which carries in 

 its head a sort of slender rasp, by which it drills a hole 



