[13] DEVELOPMENT OF THE OYSTER. 791 



latter are twice as large in diameter as those of the two first meutioned, 

 and, as a consequence, their embryos are only about one-eighth as large 

 in volume as those of the last named when they have attained the free- 

 swimming or veliger stage of development. The relatively very small 

 size of the embryo American oyster is therefore unfavorable to its sat- 

 isfactory study ; indeed, it is very reasonable to suppose that its in- 

 vestigation, for this reason, would be vastly much more difficult than 

 that of the young of 0. edulis. 



The following summary of the changes suffered by the young oyster 

 in its metamorphosis into the condition of the spat may be appended : 



1. The mouth in the larval oyster is nearly ventral in position, while in 

 the adult it opens more nearly in the direction of the hinge or towards 

 tbe afltero-dorsal region. 



2. The retractor muscles of the velum probably atrophy at the end of 

 the larval period ; if they are to be regarded as the musculature of the 

 primitive mantle organ, they are replaced in the spat and adult by the 

 radiating and marginal i)allial muscles. 



3. The intestine of the larva is a simple tubular organ ; in the spat it 

 lias an internal ridge developed on one side, which finally becomes a 

 pronounced induplicature in the intestine of the adult. 



4. The anterior adductor muscle of the larva is replaced by a per- 

 manent posterior adductor in the spat and adult. (Huxley.) 



5. The heart and gills are wanting in the larva; they are developed 

 as post-larval organs. The gills are at first represented by only two 

 folds, the outer pair are developed later, and apparently from before, 

 backwards, or dorso-ventrally. 



6. The connective tissue of the spat and adult, including the organs 

 derived therefrom, seems to be almost entirely developed during post- 

 larval life. 



7. The blastocoel is mostly obliterated by the development of the 

 connective tissues, 



8. The liver is represented by a pair of diverticula which grow out 

 laterally from tbe walls of the stomach of the larva; its subsequent 

 development and subdivision into a A^ast number of follicles is accom- 

 plished during post-larval life. 



9. Some time after fixation the larval oyster seems to lose the straight 

 binge border of its valves, which then acquire umbones ; the valves 

 retain their symmetry up to the time when the spat shell begins to be 

 formed, and it is probable that most of the larval characters of the 

 animal have disappeared when tbe formation of the spat shell begins; 

 in other words, the veliger stage is past and is at once replaced by a 

 structural condition of the soft parts which approximates that observed 

 in the adult. 



