[19] DEVELOPMENT OF THE OYSTER. 909 



It is remarkable that the lower valve does DOt show tbc reticular 

 design of the upper valve, but undulating striie and points joined 

 closely to each other (Fig. 21) ; this different appearance of the two 

 valves seems to be due solely to the circumstance, that the calcareous 

 particles of the lower valve do not have the regular prismatic structure 

 Avhich they assume in the ui^per valve, and that thej^ are smaller and of 

 irregular shape. This difference of structure can be traced more or less 

 distinctly throughout the entire life of the oyster ; for while the upper 

 valve has a linear structure and is composed of layers which do not 

 adhere very firmly to each other and where the prismatic tendency can 

 immediately be recognized, the lower valve, on the contrary, presents 

 a denser appearance and seems to be composed of layers closely joined 

 to each other. 



As regards the difficulty of recognizing the young oyster soon after 

 its fixation, I think that I have overcome it by using a tile covered 

 simidy with hydraulic cement, instead of a mixture of lime and sand. 

 The numerous small uneven places caused by the grains of sand make 

 it difficult to see the young oyster shell on the collector, even to the ex- 

 perienced eye of the oyster cultivator. In order to gvt as even a sur- 

 face as possible, I used a glass tile instead of a common tile ; but I do 

 not consider this absolutely necessary. After this tile remained in the 

 current for eight days, some small oysters became attached to it, the 

 largest of which measured 0.85 millimeter and the smallest only 0.57 

 millraeter in height ; and still even these latter could be distinguished 

 by the naked eye. 



I must, in conclusion, say a few words in regard to a j^robable enemy 

 of the oyster. I had in my aquarium a mother oyster which from time 

 to time ejected large numbers of larvae. There were in the same aqua- 

 rium two Actinice, such as are found in large quantities on the shell of 

 of the oyster. I found that the larvae diminished rapidly ; and in en- 

 deavoring to ascertain the cause I found floating in the water a cer- 

 tain number of small bluish balls, about 2 millimeters in diameter ; 

 and at the same time I saw a similar ball issue from the mouth aperture 

 of an Actinia. In examining these small balls under the microscope I 

 discovered that they were composed of empty oyster shells pressed 

 closely against each other, and that in fact they were the remnants of 

 the repast of the Actinia. Although I do not think that, under ordinary 

 conditions, the Acti^iice can easily seize the larvae, they might neverthe- 

 less destroy a large number, if they multiplied to any great extent 

 among the oyster pares. 



shell holes which were temporarily closed by meaus of thin pieces of glass, on which 

 the couchyoline could be deposited. Mr. Osboru arrived at the conclusion that the 

 shell is formed by the crystallization of carbonate of lime in the chitiuous membrane. 

 The opinion of Hessling (with whose researches Mr. Osborn does not seem to be 

 acquainted) that the carbonate of lime is deposited in the prismatic cavities, does 

 not, according to Mr. Osborn, seem to be borne out by the facts. 



