[5] DEVELOPMENT OF THE OYSTER. 783 



scope as transparent objects in tlie living condition, and with the cilliary 

 structures in lively movement and the heart pulsating as though nothing 

 had occurred to injure them. 



The shells of the smallest specimens I obtained during the season of 

 1881 were about one-eighth of an inch in their greatest diameter or 

 about ten times as large as the shell of the fry when it ceases to swim. 

 Since then much smaller specimens have been obtained. Lest any one 

 should suppose that I may have mistaken the young of Anomia, or the 

 "silver-shell," for that of the oyster, let me here remark that they are 

 very readily distinguished even when very young. The valves of 

 young "silver-shells" are lustrous, very smooth, and thinner than those 

 of the oyster ; the shell of the young of the latter is never lustrous, and 

 is almost always marked with bands of a dark or purple color which run 

 from the hinge in a radial manner to the edges of the valves. There is- 

 no mistaking these differences, and only a little experience will enable 

 any one to distinguish the very smallest spat of Ostrea and Anomia 

 apart. 



Another means of distinguishing the spat of Anomia from that of 

 Ostrea is afforded by two other characters not before mentioned. There 

 is no pigment developed in the shell of the former, while almost inva- 

 riably in the young oyster a well-marked deep purple streak runs from 

 the hinge-border of the valves to the free-margin, especially in the up- 

 per or left one. Tuis streak also usually widens as it approaches the 

 margin of the valve and coincides with the radius of the shell in which 

 the great posterior adductor is developed. This streak is, in fact, in 

 great measure due to the fact that the insertion of the adductor in 0, 

 virginica is deeply pigmented throughout life, the deposit of color at 

 first shimmering through the thin translucent valves of the young 

 oyster. It is probable that in the spat of 0. edulis no such purple 

 streak is present on the upper valves, because in that species the inser- 

 tion of the adductor is rarely if ever pigmented. The purple streak in 

 the upper valve of the spat of 0. virginica also serves to distinguish 

 which is the posterior or upper margin of the shell, upon mere super- 

 ficial inspection, inasmuch as when at all well developed it is nearest 

 the posterior margin of the shell. Occasionally spat of 0. virginica is- 

 found in which pigment is almost entirely wanting. 



A second character which distinguishes the spat of J.womm from that 

 of Ostrea is the following : In Anomia, when the shell is forcibly de- 

 tached from the surface to which it is affixed, both the upper and lower 

 valves may be lifted from tbeir nidus j in Ostrea, on the contrary, it is 

 only the upper valve which can usually be removed, the lower one being 

 firmly cemented to its surface of attachment. The lower valve of the 

 spat of Anomia is never cemented to the surface of fixation, the lower 

 valve of Ostrea invariably. The byssal plug of Anomia in its spat stage, 

 as well as in the adult, finally perforates the lower valve, and this is the 

 only attachment of the animal to its fixed basis. As elsewhere men- 



