790 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [12] 



their bases. In cross-sectious through the soft parts of the young spat 

 the margin of the mantle is grooved the same as in the adult. The two 

 marginal ridges are evidently the rudiments of the two rows of tenta- 

 cles in the adult. 



The extent to which the right and left mantle leaves are united dor- 

 sally I have not made out accurately, but it is evident that they are 

 joined together for at least the length of the straight hinge of the larval 

 shell. The development of the cucuUus ventrally, and the velamen dor- 

 sally, evidently must occur after the transformation of the larva into the 

 condition of spat. 



Uavaiiie makes the statement that the velum appears to drop off 

 some time about the end of the larval period. Gerbe, on the other hand, 

 asserts that the velum is transformed into the palps. There are 

 marked discrepancies between the figures of oyster embryos which have 

 been published by various autliors; for instance, in Fig. 1, from Huxley, 

 the intestine arises from the end of the gastric dilatation of the ali- 

 mentary canal, but in Horst's figure of the same stage it is shown to 

 arise from the middle ventral part of the stomach. In other respects 

 the figures of these authors agree pretty closely ; Mobius also repre- 

 sents the intestine as arising from the middle of the under side of the 

 stomach in the embryo, and approximates the mouth and vent more in 

 his figure than do the two preceding authors. lie also indicates the 

 presence of a third strong retractor muscle which takes its origin from 

 near the hinge and is inserted into the body-wall near the vent. Da- 

 vaine's interpretations of the form, course, and relations of intestinal 

 canal again differ somewhat in detail from all of the foregoing, as is 

 shown by his figures. The figures given by Coste and Gerbe agree 

 pretty closely ; both represent the intestine as arising from the posterior 

 extremity of the gastric dilatation, but again differ from the other au- 



thors in the way in which they represent the insertion of the retractor 

 muscles into the velum. 



Fig. 3 reproduces the interpretation of the structure of the embryo 

 of Ostrea eduUs as given by the two last-mentioned observers. 



The study of the embryos of 0. virginica and 0. angulata is attended 

 with much more difticulty than that of 0, cduUs, because the eggs of the 



