298 D. H. WEN RICH 



mussels, Anodonta fluviatilis. 1 I am indebted to Professor G. 

 H. Parker for advice and assistance during this part of the 

 work. Observations on marine forms were renewed the follow- 

 ing summer while the writer was occupying a table at the U. S. 

 Fisheries Laboratory at Woods Hole. Here special attention 

 was given to the reactions of Pecten to moving objects. 



II. RESULTS WITH ANODONTA 



A. Methods 



In the experiments on this fresh- water mussel, the following 

 arrangements were made use of for the control of the lights 

 and for making the observations. The vessel in which the 

 animals were placed for experimental study was a rectangular 

 museum jar of a capacity of about 6 liters. Coarse sand was 

 placed in the bottom to a depth of about 5 centimeters, and 

 enough water was added to make a depth of from 10 to 15 centi- 

 meters above the sand. The mussels were always placed near 

 one end of the jar with their siphons toward that end, and with 

 their long axes parallel to the longer horizontal axis of the jar. 

 A wooden box was painted black inside and provided with a 

 black cover through which a small observation hole was made. 

 In one end of the box another hole, 5 centimeters in diameter, 

 was bored about 7 centimeters from the bottom, so that its 

 height would correspond to that of the siphons of the mussels 

 when the jar containing them was placed in the box. 



Electric lights (incandescent bulbs) were employed as sources 

 of illumination. Two sources were placed on opposite sides of 

 a line which coincided with the long axis of the mussels in the 

 jar and the center of the hole in the end of the box. The lights 

 were so placed that their rays made equal angles with this line. 

 A screen was arranged so that the light from one of the bulbs 

 could be suddenly cut off without interfering with the rays from 

 the other bulb. The screen was not allowed to set up vibra- 

 tions by touching the floor. Further precautions against vibra- 

 tions, to which the animals are very sensitive, were taken by 

 supporting the experimentation-box by another box on the 

 cement floor of a basement room, and then placing wads of 



1 I am indebted to Mr. W. F. Clapp for advice in the determination of the species 

 mentioned in this paper. 



