a shell and consume the soft parts 

 were as follows: clams, 7.3 days; 

 Olympia oysters, 6.5; Pacific oysters, 

 5.1 days; and mussels, 4.1 days. 



The average number of days taken 

 by the drills in check tank 3, Experi- 

 ment II, to drill through and consunne 

 the soft parts were as follows: clams, 

 8.1 days; Olympia oysters, 6.2 days; 

 Pacific oysters, 6.0 days; andmussels, 

 4.8 days (table 20). 



The average number of days for 

 drills from tanks 1 and 4 (combined) 

 of Experiment II to penetrate the shells 

 and consume soft parts were as follows: 

 clams, 7.4 days; Olynipia oysters, 6.0 

 days; Pacific oysters, 5.0 days; and 

 mussels, 4.8 days (table 20). 



Effect of Thickness of Prey's Shell 



Table 21 summarizes the correlation 

 and regres-sion analyses between the 

 duration of attack in days and the cor- 

 responding thickness of the perforated 

 shells expressed in ten-thousandths of 

 an inch. No correlation could be demon- 

 strated for the clams; but the data 

 of mussels, Olympia oysters, and Pa- 

 cific oysters showed positive correla- 

 tion between days (Y) and thickness (X). 

 Also, the "t" tests of hypothesis (slope) 

 fi = Q were significant for all three 

 species. The regression lines for the 

 three species are shown in figure 5, 

 and the regression equations are as 

 follows: 



Bay mussels: '^ = 4.1389 + 85,6989 (Xq - .0220) 

 Olympia oysters: Y= 6.5625 + 123,7722 (Xq - ,0418) 

 Pacific oysters: Y= 5.3529 + 103,6502 fX^ - .0316) 



atle 19. — Numtier of organisms attacked and duration of attack for Experiment I 



A : number of organisias attacked 

 B : duration of attack in days 

 C : average duration 



A- B- C 



A- B- C 



B- C 



A- B- 



Manlla clam (C) 20-164-8.2 



Olyinpia oyster (0) 22-157-7.1 



Pacific oyster (P) 13- 70-5.4 



Eay mussel (H) 7- 34-4.9 



Total number of attacks = 165 



Tktle 20. — Duration of attack on each species of food animal 



1-lanila clams 

 Olympia oysters 

 Pacific oysters 

 Bay mussels 



Tanks 

 All tanks Tank No, 3 No. 1 and 4 General 

 Experiment I Experiment II Sxperimeat II indication 



7.3 days 

 6.5 days 

 5.1 days 

 4,1 days 



8.1 days 



6.2 days 

 6.0 days 

 4.8 days 



7.4 days 

 6 . day s 

 5.0 days 

 4.8 days 



7-3 days 

 6-7 days 

 5-6 days 

 4-5 days 



21 



