1911] Michael: Chaetognatha of the San Diego Region. 119 



That there should be lack of parallelism between frequency 

 and abundance in each depth, and why there should be such an 

 abrupt drop in 7-12 fathoms is probably due to the following 

 factors: (1) insufficient number of hauls, (2) impossibility of 

 standardizing the filtering capacities of surface nets with closing 

 nets, and (3) irregular distribution of hauls with respect to the 

 effects of cloudy and foggy days, and moonlit nights. However, 

 whateA^er effects these factors may have cannot offset the fact 

 that a very evident difference between day and night distribu- 

 tion exists. 



The question naturally arises, if this species migrates toward 

 and away from the surface, at what time of day or night does it 

 attain its maximum abundance in each depth? To determine 

 this the specimens obtained from each depth have been segre- 

 gated into groups according to two-hour periods as indicated in 



the following table : 



TABLE 38 

 Number of specimens of S. bipunctata per hour obtained with respect to 



Time of Day. 



Equal intervals of time throughout day and night 



In this table sp. = number of specimens per hour, and min. = number of 

 minutes consumed in hauling. Wherever number of specimens is not recorded 

 no hauls have been made, but wherever hauls were made and no specimens 

 obtained, a indicates this fact. 



From this table we learn that S. bipunctata appears in maxi- 

 mum abundance between 6 and 8 p.m. on the surface and in 

 4-6 fathoms, between 4 and 6 a.m. in 7-12 fathoms, between 8 and 

 10 a.m. in 15-20 fathoms, between noon and 2 p.m. in 25-35 

 fathoms, and between 10 a.m. and noon in 40-75 fathoms. 



