34 University of California Piihlications in Zoology [Vol. 13 









TABLE 11 











Distribution 



OF Pleurobrach 



ia AT THE 



Surface, 



BY Two-Hour Periods 



Through 









OUT 



THE Day, 



Summer 



Months 









Time of 

 Day 



No. 

 Total. 

 A 



of hauls: 

 Success. 

 B 



No. of 

 Total. 

 C 



hours: 

 Success. 

 D 



No. 

 Total. 

 E 



of animals: 

 Per hour. 

 F 



Frequency : 



Haul. Time. 



G H 



A.M. 



















1; 6- 8 



31 



8 



20.4 



7.2 



754 



37. 



27 



35 



2. 8-10 



24 



3 



22.1 



2.5 



430 



17. 



12 



11 



3. 10-12 



19 



3 



12.4 



1.5 



72 



5.8 



16 



12 



P.M. 



















4. 12- 2 



16 







13.3 



0. 







0. 











5. 2- 4 



10 



4 



6.2 



1.5 



524 



84.5 



40 



24 



6. 4- 6 



10 



3 



6.2 



1.8 



36 



5.5 



30 



27 



7. 6- 8 



27 



9 



15.2 



4.2 



544 



35.4 



35 



28 



8. 8-10 



7 



3 



3.9 



1.4 



15 



3.1 



43 



36 



9. 10-12 



1 







2.8 



0. 







0. 











A.M. 



















10. 12- 2 



2 







2. 



0. 







0. 











11. 2- 4 



10 



2 



8. 



1.8 



75 



9.4 



20 



22 



12. 4-6 35 8 26. 6.8 5065 195. 23 26 



The average number of animals per hour (column F) is greatest 

 between 4 and 6 a.m. (line 12) and the highest frequencies are found 

 in line 8. While the table as a whole does not show clearly that 

 diurnal migrations occur, it at least makes clear that during the slim- 

 mer the optimum conditions are found at sunrise or shortly after. 

 The large number of animals taken from 4-6 a.m. is due almost en- 

 tirely to August hauling, since all but 15 of the 5065 specimens were 

 obtained during that month. Of the total number recorded for 6-8 

 A.M. all but 14 were in August hauls. The somewhat irregular re- 

 sults in the table do not allow more than the statement that the time 

 of the surface plurimum for August is from 4—6 a.m., but it is not 

 unlikely that this is true for the entire summer. According to Mayer 

 (1912, p. 12) the ctenophores are very sensitive to rough water, and 

 it might be that the early morning plurimum found here is due to 

 the quietness of the sea at that time. It hardly seems probable that 

 this is the explanation but the possibility should not be disregarded. 



It is worth noting that in table 11 there are no successful hauls 

 from 12 to 2 p.m., and some of these are August hauls. Another time 

 of unsuccessful collecting is from 10 a.m. to 2 p.m., but this may not 

 have much significance on account of the small number of hauls. The 

 implication in such results is that the animals leave the surface at the 

 times when surface hauls are unsuccessful and return to the surface 

 later. 



