272 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



fig. 17, aud in plate vii, fig. 18, it is seen in surface view, drawn in between the shells, 

 and with its cilia folded down and at rest, as they are seen when the little oyster 

 lies upon the bottom. 



The two shells grow rapidly, and soon become quite regular in outline, as shown in 

 plate VII, fig. 17, and plate viii, fig. 1, but for some time they are much smaller than the 

 body, which projects from between their edges around their whole circumference, 

 except that along a short area, the area of the hinge upon the dorsal surface, where 

 the two valves are in contact. 



The two shells continue to grow at their edges, and soon become large enough to 

 cover up aud project a little beyond the surface of the body, as shown in plate viii, 

 fig. 1, and at the same time muscular fibers make their appearance and are so arranged 

 tliat they can draw the edge of the body aud the velum in between the edges of the 

 shells in the manner shown in plate vii. fig. 18. In this way that surface of the body 

 which lines the shell becomes converted into the two lobes of the mantle, and 

 between them a mantle cavity is formed, into which the velum can be drawn when 

 the animal is at rest. While these changes have been going on over the outer sur- 

 face of the body other important internal modifications have taken place. We left 

 the digestive tract at the stage shown in plate vii, fig. 16, without any communica- 

 tion with the exterior. 



Soon the outer wall of the body becomes pushed inward to form the true mouth, 

 at a point (plate vii, fig. 17) which is upon the ventral surface and almost directly 

 opposite the point where the primitive mouth was situated at an earlier stage. The 

 digestive cavity now becomes greatly enlarged aud cilia make their appearance 

 upon its walls, the mouth becomes connected with the chamber which is thus formed 

 and which becomes the stomach, and minute particles of food are drawn in by the 

 cilia and can now be seen inside the stomach, where the vibration of the cilia keep 

 them in constant motion. Up to this time the animal has developed without growing, 

 and at the stage shown in plate vii, fig. 16, it is scarcely larger than the unfertilized 

 egg, but it now begins to increase in size. The stages shown in plate viii, fig. 1, and 

 plate VII, fig. 18, agree pretty closely with the figures which the Euj-opean embry- 

 ologists give of the oyster embryo at the time when it escapes from the mantle 

 chamber of its parent. The American oyster reaches this stage in from twenty-four 

 hours to six days after the egg is fertilized, the rate of development being deter- 

 mined mainly by the temperature of the water. 



Soon after the mantle has become connected with the stomach this becomes united 

 to the body wall at another point a little behind the mantle, and a second opening, 

 the anus, is formed. The tract, which connects the amis with the stomach, lengthens 

 and forms the intestine, and soon after the sides of the stomach become folded oif 

 to form the two halves of the liver, as shown in plate viii, fig. 1. Various muscular 

 fibers now make their appearance within the body, and the animal assumes the form 

 shown in plate viii, fig. 1, and plate vil, fig. 18.* 



What follows this stage may be best told in the words of Professor 

 Huxley, who speaks of the European oyster, in which the metamor- 

 phosis from the free-swimming fry to the fixed spat and finally the 

 adult oyster is essentially the same as in our species. 



The young animal which is hatched out of the egg of the oyster is extremely 

 unlike the adult, aud it will be worth while to consider its character more closely 

 than we have hitherto done. 



Under a tolerably high magnifying power the body is observed to be inclosed in a 

 transparent but rather thick shell (plate vili, fig. 2, L), composed, as in the parent, 

 of two valves united by a straight hinge, /;. But these Aalves are symmetrical and 

 similar in size and shape, so that the shell resembles that of a cockle more than it 

 does that of an adult oyster. In the adult the shell is composed of two substances 



*Report Maryland Fish Commission, Annapolis, 1880, pp. 19-25, in part. 



