Exp. II, 9/5/57 in field, pH 7.47 and 

 salinity 28.97 %o 

 9lblbl in aquarium, pH 7.59 

 and salinity 29.53 %o 



Light 



A 100-watt incandescent lamp over 

 each test aquarium was mechanically 

 turned on every morning and off every 

 evening. By varying the voltage gradu- 

 ally, it required 35 minutes for the 

 lamps to reach maximum illumination 

 in the morning (5:05 a.m. to 5:40 a.m.) 

 and 34 minutes to decrease the illu- 

 mination until completely extinguished 

 in the evening (8:30 p.m. to 9:04 p.na.). 

 This was done to approximate sunrise 

 and sunset conditions. 



Shell Thickness 



Thickness of shells perforated by 

 Japanese drills was measured to the 

 nearest 0.0001 inch with a full- jeweled, 

 dial indicator, as shown in figure 1. 



Test Animals 



All test animals were collected from 

 oyster beds of the Olympia Oyster 

 Company, Oyster Bay (Southern Puget 

 Sound), Washington. To obtain a group 

 of test animals that were from approxi- 

 mately the same environmental sur- 

 roundings, all subjects for this study 

 were collected at a single locality 

 within a radius of about 150 yards. 

 To insure uniformity, all drills were 

 collected fronn the surface of Pacific 

 oysters. All test animals for Experi- 

 ment I were collected on June 27, 1957, 

 and for Experinnent II on September 5, 

 1957. 



Since it was not possible to ascer- 

 tain the sex of drills by external 

 characteristics, it was assumed that 

 the sex ratio was equal. Attempts 

 to determine the sex of Ocinebra with 

 the Hargis (1957) rapid live-sexing 

 technique failed. 



Of the four food species, bay mussels 

 were taken from oyster dyke walls; 

 Manila clams were dug from the gravel 

 parts of oyster beds; and Olympia and 

 Pacific oysters were picked from the 

 beds. 



Figure l.--The thickness of a Manila clam shell indicated 

 by dial meter. 



METHODS 

 Conditioning and Measuring 



Prior to the test, drills collected 

 for Experiment I were conditioned to 

 the salt water of the aquarium for 7 

 days, during which time they were not 

 fed. On the first day of the acclimatiz- 

 ing period, the drills were all measured 

 as shown in figure 2. The apex of the 

 whorl was placed against a backstop, 

 and reading in millimeters was taken 

 at the tip of the anterior siphonal 

 canal. Drills which fell within an ar- 

 bitrarily selected size range around 



