232 ZOOLOGY. 
but the purest occur in the pearl oyster, Meleagrina 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 
2,000,000 eggs; they are yellow, and 
after leaving the ovary are for the 
near ig Macoma Proxima, most part retained among the gills. 
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 head. 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 larve. 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 
