28 



BtlLLETIN OF THE BUREAU OF FISHERIES 



In experiment B the plankton was formed mostly by minute Naviculse and 

 Glenodinium, while in experiment A such large forms as Coscinodiscus, Rhizosolenia, 

 and Ceratium were present. In both experiments the water discharged by the gills 

 contained a considerable amount of mucus. 



Because of the practical importance of the bacteriological examination of the 

 oyster, it is interesting to determine how many bacteria can be retained by the gills. 

 Three experiments were performed with oysters kept in water to which varying 

 amounts of fresh 24-hour-old cultures of Bacterium coli were added. The experi- 

 ments were carried out in October, 1925, at Doctor Pease's laboratory in New York. 

 The water in the tank to which B. coli was added was well stii-red and the oyster 

 was allowed to filter it for 30 minutes. During this period the small vessel of the 

 tank (fig. 2) was twice emptied and refilled with the water passed by the gills. For 

 counting the number of B. coli 1 centimeter of water was planted on Endo's plates, 

 and for the total number of bacteria the same amount was planted on beef agar. 

 The plates were kept for 48 hours at 37°. The results of the experiments are as 



follows : 



Table 9. — Filtering of water by the gills 



As can be noticed from this table, the water, after it passed the gills, always 

 contained less bacteria than it had before but the difference was not constant. Ap- 

 parently only a small number of bacteria are retained by the gills; the microorganisms 

 are so small that they can pass easily between the lateral cilia and escape back into 

 the surrounding water. 



OPENING AND CLOSING OF THE SHELL 



Feeding and respiration in the oyster is dependent on two distinct functions — 

 the ciliary activity of the gill epithelium and the opening and closing of the shell. 

 The movement of the shell is controlled by the adductor muscle, the relaxation of 

 which causes the opening of the valves, while its contraction brings the valves together 

 and keeps them tightly closed. Because the oyster has no power of locomotion, the 

 contraction and relaxation of the adductor muscle is the only noticeable reaction by 

 which the organism responds to the external and internal stimuli. One should 

 anticipate, therefore, that the opening and closing of the shell is a complex phenom- 

 enon that is controlled by a great variety of factors. No attempts were made in the 

 present paper to study the physiology of the adductor muscle, but it seemed desirable 

 to obtain some data regarding the duration of time the oysters keep the shell open. 



