natural 



232 ZOOLOGY. 



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

 ritifet'ti (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 

 . i7i. Hacoma '.proximo., most part retained among the gills. 



al size. Aftur Morse. 



In America the oyster spawns irom 

 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 germma. 



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 "Veligcr"' 



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



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



the gastropod larva?. 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. 



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



