[13] DEVELOPMENT OF THE OYSTER. 903 



On the contrary, the description given by Mr. Hatschek of the origin 

 of the shell of the Teredo agrees perfectly with what I have observed 

 in the Ostrea ; and I think we are justified in considering- as certain 

 that the shells of all mollusks originate in the same manner. This is, 

 doubtless, as the last-mentioued naturalist justly remarks, a very im- 

 portant argument in favor of the theory of the monophyletic descent 

 of the mollusks, a theory which has been violently attacked by Von 

 Jhering. 



Meanwhile the ectoderm has become detached from the entoderm 

 along the entire circumference of the embryo, so that for the first time 

 a peritoneal cavity may be observed; a crown of vibratile cilia has 

 developed above the mouth, and the surface of the velum is covered by 

 deep cylindrical cells (Fig. 13). The entoderm has also grown consid- 

 erably, and is now composed of a spacious stomach cavity, below which 

 there is a diverticulum, still temporarily closed, but which later will 

 communicate with the ectoderm and form the posterior part of the ali- 

 mentary canal. 



On the following day (Fig. 14) the shell, which has grown very much, 

 covers a large portion of the body ; it already contains carbonate of 

 lime, as is shown when treated by acids. After steeping it for some 

 time in acetic acid nothing remains but a thin membrane of conchyo- 

 line. The ectodermic cells, which are found on the surface of the shell, 

 have become very thin and transparent, so that the outlines cannot be 

 distinguished, but only the refringent nuclei. The larva (Fig. 15) takes 

 food continually, moves about in a lively manner in all directions, and 

 grows considerably. The velum now forms a very distinct part of the 

 body, which is almost entirely covered by the shell. The surface of 

 the velum, surrounded by a wreath of vibratile cilia, already shows in 

 its center a thickening which is the beginning of the cephalic disk. 

 An esophagus, in the shape of a funnel, leads to a large pear-shaped 

 stomach, communicating with the outside through the intermediation 

 of an intestinal canal. 



Owing to the development of pigment at several points ou the body 

 (as on the cephalic surface, esophagus, gastric pouch, &c.), the larvue 

 gradually acquire a gray or bluish color. The shell now (Fig. IG) mea- 

 sures 0.16 millimeter in height. Its form is almost round, except the 

 hinge, which is straight. During this period it is already furnished 

 with small teeth, as De Lacaze-Duthiers has observed. From time to 

 time the body of the larva may be observed to be withdrawn entirely 

 into the shell. This is done by means of a dorsal muscle (ds) and a 

 ventral muscle {vs) originating on the edge of the hinge in a ramified 

 extremity, while their other extremities are inserted into the lower part 

 of the velum. These muscles originate from mesodermic cells which, 

 with their ramifications, cross the internal cavity of the body at several 

 points ; some of these cells which cross the dorsal part, from the left 

 to the right valve, are gathered in a group, and form the adductor 



