728 NATPEAL HISTORY OF AQUATIC ANIMALS. 



have been eroded by the action of the carbonic dioxide in solution in the sea-water. The 

 presence of the larval shells in an unimpaired condition on the umbones of the valves of Oysters 

 is therefore an indication that such specimens are young, probably under a year old. 



The third character, alluded to above, which distinguishes the larval shell of the Oyster is 

 the perfect homogeneity of the calcareous matter. Unlike the valves of the spat or translucid 

 flakes from the shell of the adult, they exhibit no prismatic arrangement of the calcic carbonate 

 in a matrix of conchioline. In the valves of the adult and spat, on the other hand, the calcic 

 carbonate tends to assume a prismatic arrangement vertical or at right angles to the plane of 

 the length and breadth of the shell. This distinction is so marked that in very young individuals 

 which have only lately become fixed one may very readily determine with the aid of the micro- 

 scope the line of demarkation along which the formation of the larval shell ceased and where the 

 prismatic calcareous structure of the valves of the spat began to be developed. 



Charaotbes of the larval shell. — The only characters of structure which the larval 

 shell has in common with that of the spat and adult are the lines of growth visible in all three. 

 This shows that the valves grow in extent at all stages by the addition of lime to the edges of 

 the valves, each layer of mineral matter and organic matrix extending over successively greater 

 and greater areas, as in the growth of the shells of moUusks in general, the umbones being 

 the points from which the valves grow in au eccentric manner in consequence of the gradually 

 increasing extent of the mantle — the shell-secreting organ — as the growth of the animal 

 proceeds. Having clearly defined the nature of the larval shell of the Oyster, up to the time 

 when it is ready to begin to build or secrete the shell of the spat, we may next discuss the 

 character of the transition from the one to the other. 



The transition is apparently an abrupt one. The excessive convexity of the valves of the fry 

 contrast strongly with the almost flat lower valve and feebly convex upper one of the spat. At 

 the free edges of the larval shells where they pass directly into the structure of the valves of the 

 spat there is a marked offset or angle marking very distinctly the difference of convexity between 

 the. two stages of shell development. 



Food of the young Oyster. — As already remarked, I have seen food in the intestine 

 of the young Oyster on the second day of development, but how long it may take before the 

 young embryo of this stage of growth shall have taken and appropriated one hundred and 

 thirty-two times its own volume of food material, I am not able to say. This it must do 

 before it can have attained to the size of the larva which is transformed into spat. The food 

 is propelled through the alimentary canal by the action of innumerable vibratory filaments 

 which clothe the inside of the throat, stomach, and intestine as in the adult ; the intestine, 

 stomach, and liver are not, however, as complex as in the full-grown animal. 



Of the method of fixation 1 have as yet learned nothing of value. That this is accomplished 

 by some sort of byssus I have no doubt. The fact that it is the left valve which is always the 

 lowermost and attached one would indicate that the method of fixation was not capricious or 

 haphazard in its nature. 



I would infer from what we learn from the study of other animals that it may require quite a 

 week before an embryo reaches the dimensions of one-eightieth of an inch, but we have no data 

 upon which to base any conclusions of value. Of the later stage of development we know some- 

 thing definitely. The main fact which we have so far decided is the size of the larval shell. 



Eate op growth. — After fixation the growth of spat is very rapid, as may be inferred 

 from the fact that I have found spat upon collectors which had not been placed in position 



