As shown in table 1, the size range 

 of Manila clams (40 to 50 mm.) used 

 for Experiments I and II was the same. 

 The size range used for Experiments I 

 -and II was also the same for Olyinpia 

 oysters (29 to 39 nnm.). The reason 

 for using the saime size range was that 

 for each of these two species a statis- 

 tical "t" test showed that the mean 

 length of the first sample for Experi- 

 ment I was not significantly different 

 from the mean length of the second 

 Scunple for Experiment II. 



Because there were significant dif- 

 ferences in the mean lengths of sam- 

 ples collected for Experiments I and 

 II, size ranges were changed for Jap- 

 anese drills (from 29-37 mm. to 31-38 

 mm.), bay mussels (from 40-50 to 39- 

 50), and Pacific oysters (from 50-60 

 to 58-69). 



Even though the bay mussels were 

 collected from the same area, their 

 mean lengths for the sample used in 

 Experiment II showed a significant 

 decrease of .87 mm. from the sample 

 used in Experiment I. Other species 

 of bivalves increased in naean length 

 between the samples taken for Experi- 



Tank ffl 



Tank #2 



Tank #3 



ments I and II, as might be expected. 

 The mean length of the bay mussels 

 decreased probably because more of 

 the year class "0" were included in 

 the second collection. 



Design of Experiment 



Two experiments, designated Ex- 

 perimient I and Experiment II, were 

 conducted over a period of 70 days 

 following the procedure shown in figure 



3. In Experiment I, July 4 to Septem- 

 ber 12, 1957, the four replicates were 

 designated tanks 1, 2, 3, and 4. In 

 Experiment II, September 13 to Novem- 

 ber 21, 1957, tank 2 was a continuation 

 of Experiment I. Whereas newly col- 

 lected drills and food were placed in 

 tanks 1,3, and 4. 



At the beginning of each experiment, 

 18 randomly selected drills were 

 marked and arranged in a systematic 

 pattern in each tank as shown in figure 



4. For identification and consistency, 

 the color combination for each drill 

 location was the same for all tanks. 

 For example, all Dl drills were red/ 

 red (apex/varices), and all D2 drills 

 were red/silver. 



Tank #U 



70 days 



End 



70 days 



End 



Experijnent I 

 (July U - Sept. 



12) 



70 days 



Experiment II 



(Sept. 13 - Nov. 21) 



End End 



Figure 3.--Design of experiment. 



5 



