232 ZOOLOGY. 



but the purest occur in the pearl oyster, Mehagrina marga- 

 ritifera (Linn.), which occurs at Madagascar, Ceylon, the 

 Persian Gulf, and at Panama. The largest pearl known 

 measures two inches long, four round, and weighs 1800 

 grains. All bivalves pass through a metamorphosis after 

 birth. The development of the 

 oyster is a type of that of most La- 

 mellibranchs. 



A single oyster may lay about 

 3,000,000 eggs ; they are yellow, and 

 after leaving the ovary are for the 

 ■mg. m.-Maeomapmidrm, most part retained among the giUs. 



natural size.— After Morse. ^ p 



In America the oyster spawns from 

 June till September ; during their growth the eggs are en- 

 closed in a creamy slime, growing darker as the "spat" or 

 young oyster develops. 



The course of development is thus : after the segmenta- 

 tion of the yolk (morula stage), the embryo divides into a 

 clear peripheral layer (ectoderm), and an opaque inner layer 

 containing the yolk and representing the inner germinal 

 layer (endoderm). A few filaments or large cilia arise on 

 what is to form the velum of the future heai The shell 

 then begins to appear at what is destined to be the posterior 

 end of the germ, and before the digestive cavity arises. The 

 digestive cavity is next formed (gastrula stage), and the anus 

 appears just behind the mouth, the alimentary canal being 

 bent at right angles. Meanwhile the shell has grown enough 

 to cover half the embryo, which is now in the "Veliger" 

 stage, the "velum" being composed of two ciliated lobes in 

 front of the mouth-opening, and comparable with that of 

 the gastropod larvae. The young oyster, as figured by Salen- 

 sky, is directly comparable with the Veliger of the Cardium 

 (Fig. 172). Soon the shell covers the entire larva, only the 

 ciliated velum projecting out of an anterior end from be- 

 tween the shells. In this stage the larval oyster leaves the 

 mother and swims around in the water. According to 

 Brooks the American oyster becomes a free-swiming larva 

 in six hours after the egg is fertilized. When about .03 mm. 

 in diameter it becomes fixed. The oyster is said to be 



