REACTIONS OF BIVALVE MOLLUSKS 303 



would cause the valves to close partially, and at the same time 

 eject a stream of water from the exhalant siphon carrying with 

 it a greater or less amount of debris from the mantle cavity. 

 Such activities would recur at short intervals until the mantle 

 chamber was apparently free from the foreign material. Then 

 the responses would become normal again. The presence of 

 the foreign material in the mantle chamber seemed to act as 

 a counter irritant, inhibiting for the time being the reactions 

 toward decreases in light intensity. 



(5) Some of the largest animals proved to be responsive very 

 slightly or not at all, to decreases in light intensity. They would 

 keep their siphons open wide, with vigorous currents passing 

 in and out of the openings, and fail to close them when the 

 light intensity was reduced. These were gravid females, and 

 their non-responsiveness may have been due to the physiolo- 

 gical necessity of maintaining a constant flow of water along 

 the embryo-filled gills. 



b. Variations Among Different Animals 



The differences in sensitivity among different individuals is 

 perhaps largely to be explained likewise by differences in physio- 

 logical states induced by the same factors that were found to 

 be effective in single individuals. Doubtless other unrecognized 

 factors are involved in both cases, and individual peculiarities 

 among the clams are also to be expected. A single example will 

 indicate some of the differences to be met with. Mussel No. 

 203, normally, after stimulation at one-minute intervals for 

 15 or 20 minutes, begins to show irregularities in the responses, 

 doubtless due to fatigue, and these irregularities increase if the 

 stimulation is continued. In one experiment mussels Nos. 

 203 and 205 were placed side by side and subjected to the same 

 conditions. No. 203 failed to open its siphons at all on this 

 occasion, while No. 205, on the other hand, reacted to decreases 

 in light intensity promptly and without any irregularities every 

 m'.nute for 50 minutes, at the end of which time the experiment 

 was discontinued. 



3. Comparative Measurements of Sensitivity 



A sufficient number of experiments were performed on mussel 

 No. 203 to make it possible to recognize a normal type of behavior 



