[5] DEVELOPMENT OF THE OYSTER. 895 



appear two tufts of vibratile cilia. Finally, the shell is formed from 

 two thickeiiiugs of the epiblast, resembling a pair of watch-crystals of 

 great transparency^, attached on each side to the dorsal dei)ression. 

 The two halves in growing approach each other, finally join, and form 

 the hinge. This, therefore, is not the first to appear. 



The central and opaque part of the embryo is then detached from the 

 peripheral i^art, first at the dorsal side and finally at the ventral side, 

 except in two places, namely, the places occupied by the mouth and by 

 the anus. At this latter i^oint the central part is again joined to the 

 peripheric layer, or epiblast, by a cylindrical stem, which elongates and 

 finally is transformed into the intestines. The stomach is formed in 

 the upper portion of the central mass, and the liver in the lower portion. 

 Two groups of vibratile cilia encircle the pole opposite the shell like a 

 wreath, which is the beginning of the trochal disk, and in the center 

 of which the mouth forms. The intestine and the stomach form a 

 distinct cavity, which is lined with ciliated epithelium. Soon the shell 

 has grown so large that ic inclc.cs the entire body; in front of the 

 anus there is found an appendage resembling a rudimentary foot. 

 Although the trochal disk principally serves as an organ of locomotion, 

 it has also something to do with respiration, and probably also with 

 the taking of food. Even in the more developed larvae neither branchia 

 nor heart could be observed, nor any movement of the trochal disk, 

 as stated by Davaiue. 



2. He succeeded once in keeping the larvae alive, outside the mother 

 shell, for thirty, and at another time for forty-three days. In order to 

 follow the development of the intestines, the larvae were fed on different 

 colored substances. The mouth seems to be placed between the trochal 

 disk and the foot-shaped appendage in front of the anus ; it is a long 

 funnel lined with vibratile cilia, the upper lip beiug formed by the disk 

 itself, and the lower lip by the appendage in question. A tubular or- 

 gan which is found in the center of the trochal disk has, in less developed 

 larvae, been mistaken for the mouth. The stomach is narrower in the 

 middle, in the place where the intestines are attached. The intestines, 

 forming a loop, turn toward the left side of the stomach. Then the 

 lobes of the liver become hollow, and the characteristic granules com- 

 mence to show themselves in the parenchyma. The streaks of fibrous 

 appearance change to bundles of muscles, and attach themselves to the 

 trochal disk, which they force back into the shell. He has never seen 

 the trochal disk, or velum, detach itself spontaneously, but it sometimes 

 happens that the shell in closing abruptly either cuts it olf wholly or in 

 part The hinge of the shell is provided with teeth, as in young mussels. 

 Finally, the otoliths also make their appearance in the form of vesicles 

 which inclose granules, agitated by a very lively movement and placed 

 under the mouth at the base of the lower labium or lip. 



Coste^ (1861), the zealous advocate of fish-culture in France, describes 



'^ Voyage cVexi^loratlon stir le litloral dc la France et de Vltalie, 2tl ed., 1861, p. 93. 



