during the colder months of the winter drills return to the bottom and many assume 

 positions just below the surface. This appears to represent a reversal of the 

 response to gravity at low temperatures, 



Light 



Federighi (1931c) under his experimental conditions was not able to detect 

 a response to light on the part of the drill. However Sizer (1936) under different 

 circumstances determined that direct sunlight has a pronounced effect on them. 

 He placed drills in the center of the bottom of a covered aquarium in direct sun- 

 light . The walls of the container were painted black except for the lower half of 

 one side The temperature of the water was kept below 25* C . About 70% of the 

 drills in the experiments moved away from the source of light, and the remainder 

 exhibited no definite response . When the intensity of the light was reduced on a 

 cloudy day and by placing a sheet of white paper in front of the tank, the drills 

 continued to move away from the light source . As the intensity of the light was 

 reduced to that of a 75 watt bulb the drills moved slightly toward the light , At 

 weaker intensities the tendency of the drills to orient toward the light increased; 

 in dim light the phototactic response was lost completely. 



Sizer saw a similar response in the field: in shallow water up to four feet 

 deep on sunny days Uro salpinx were found attached to the under side of oyster 

 clusters . On cloudy days they occurred on the top of the clusters, and in turbid 

 waters of Delaware Bay at depths of about 15 feet more drills were taken on the top 

 of the clusters than on the lower surfaces . Stauber (1943) rarely found drills on 

 the most exposed portions of the reef on the Delaware Bay Cape Shore, and believes 

 that the value of the negative response of drills to all but low intensities of light is 

 more apparent here than in the deeper water of Delaware Bay . 



Cole (1942) confirmed these observations . In a study of the rate of move- 

 ment of the English Urosalpinx m a wooden trough filled with sea water he 

 observed that until the intensity of sunlight was reduced by shading, the drills 

 tended to move out of the sun into the shady side of the trough in bright weather . 

 The writer has repeatedly observed this response in drills confined in aquaria 

 over long periods of time . The influence of the length of day on the activities of 

 Urosalpinx has not been investigated 



Ectocrines 



The aqueous environment in which the oyster drill lives is enriched by an 

 ecologically important but relatively unexplored complex of chemical substances 

 which are released by living organisms or result from decay after death . Lucas 



77 



