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H 



(Columns) 



K 



M: Bay mussels 

 0: Olympia oysters 

 P: Pacific oysters 

 C: Manila clams 



D+Arabic numerals: Japanese drills 

 h: Initial orientation of siphonal canal of drill 



Figure 4. --Arrangement of food organisms and position of each drill at time of release for Tank 1. 



The siphonal canals of all drills in 

 each column were pointed a certain di- 

 rection at release as shown in figrure 

 4. The direction of drills in each 

 coliunn differed by 60 degrees from 

 those in adjacent columns. This was 

 done in each tank to check the initial 

 movement of the drills toward the four 

 species of food offered (directioned 

 variable). Eighteen food einimals of 

 each species were used in each tank. 

 The only difference between the four 

 tanks in Experiment 1 was the position 

 of the food animals. Each species of 

 food animal was shifted clockwise one 

 place from tanks 1 through 4. For 

 example, the following squares repre- 

 sent the position of food animals in 

 each tank of Experiment I: 



M O 



M 



The individual food animals were 

 placed at equal distances from each 

 other (4.5 inches) with the aid of 

 small yellow dots of synthetic paint, 

 painted on the bottom of the four test 

 aquaria before the experiment. 



In figure 3, "replacement of dead 

 food" means that when a food animal 

 was drilled and eaten, it was taken 

 out and replaced by another animal 

 of the same species. The food animal 

 was not removed and replaced until 

 the drill had finished its attack and 

 moved away. The credit for this mor- 

 tality, in number of days, was given to 

 the drill first noted on the dead or- 

 ganism. The shell of the drilled and 

 eaten food animal was then placed in 

 an envelope and filed away for later 

 use. At the end of Experiment I, all 

 shells collected were measured for 

 thickness. 



