1911] MicJiael: Chaetognatha of the San Diego Region. 113 



the water filtered during hauls made with different nets, or with 

 the same net at different times, cannot be determined and con- 

 sequently the number of specimens obtained in respective hauls 

 cannot be accurately compared. 



Owing to this impossibility of estimating the value of each 

 horizontal haul, there is apparently only one practicable method 

 of quantitatively considering the vertical distribution of the 

 organisms. If hauls in each depth are numerous and distrib- 

 uted over considerable time, those environmental factors affecting 

 the quantity of plankton obtained will not affect every haul 

 alike. In fact their effects will tend to cancel one another. Now, 

 if we consider the average number of specimens per haul 

 obtained from each depth, we are justified in neglecting the 

 effects of these factors. However, the time consumed during 

 each haul has varied, so it is necessary to standardize the aver- 

 age number of specimens per haul with respect to time. A period 

 of one hour is selected for this purpose. In the following dis- 

 cussion, therefore, the average number of specimens per hour- 

 haul is regarded as the unit for quantitative comparison. 



Had the same closing net alwaj^s been used in collecting, this 

 method, while not absolutely accurate, would yield a fair aver- 

 age estimate of the relative abundance of specimens from each 

 depth. However, the Kofoid closing net and the Nansen closing 

 net were used and this necessitates standardizing the hauls made 

 by one net with those made by the other. While the nets are 

 proportionally similar, the orifice of the former is 37.0 cm. in 

 diameter, and that of the latter 109.5 cm. Since -n-r- equals the 

 area of the orifice, it follows that the portion of a given volume 

 of water filtered by the Kofoid net is to that filtered by the Nan- 

 sen net as the square of the radius of the former is to that of 

 the latter. In other words, if r = the radius of the Kofoid net, 

 R = that of the Nansen net, w = water filtered by the Kofoid net, 

 and W =^ that filtered by the Nansen net, then as the nets are 



proportionally similar, w : W : : Trr- : ttR- or W= — ~ — =^ 



7rr- r- 



(54 75)^ 

 Now r= 18.5 cm. and R^ 54.75 cm., hence W= ) — '- — — X w 



( 18.5)' 

 or approximately w X 8. 



