1911] Michael: Chaetognatha of the San Diego Region. 109 



in open net hauls abo\'e these depths. However, how do we 

 know that, in the hauls from below 110 fathoms containing »S', 

 serratodentata and in those from below 75 fathoms containing 

 iS'. Iy7~a, these species were not obtained from 5 or 10 fathoms, 

 or even from the surface? Because they were not near the sur- 

 face during 20 or even 10,000 hauls made above 25 or 75 fathoms 

 does not ivarrant the assumption, that they were not near the 

 surface during haids made from, heloiv these depths. This will 

 be readily acknowledged when we learn that 8. serratodentata 

 was obtained in 25 out of 148 surface hauls, and in closing net 

 hauls from 10 and 15 fathoms, and that »S'. lyra was obtained 

 in 8 out of 148 surface hauls, and in closing net hauls from 5, 

 25, and 50 fathoms. 



These data mean that conclusions based on open net hauls 

 relate only to the usual manner in which each species is vertically 

 distributed. The significance of this, however, depends on a 

 knowledge of the unusual manner in which each is distributed. 

 In other words, a correlation between the vertical distribution 

 of each species and all varying conditions affecting this dis- 

 tribution cannot be made unless we know how the vertical dis- 

 tribution of each species varies. Herein lies the value of the 

 closing net. It enables us to know the exact depth from which 

 every specimen is obtained, and thus supplies knowledge con- 

 cerning the unusual as well as the usual manner in which each 

 species is distributed. 



The first desideratum, then, for an adequate interpretation 

 of the facts of vertical distribution is the frecpient use of closing 

 nets. Used in connection with Ekman's reversing bottles, and 

 accurate deep-sea thermometers, the closing net supplies data the 

 value of which can scarcely be exaggerated. However, in order 

 to solve the problems of vertical distribution, the following pre- 

 cautions are necessary : 



1. The area explored must be as small as possible, or the 

 observations will be too scattered for comparison. 



2. All conditions affecting the quantity, composition, and 

 distribution of plankton in the region studied must be ascer- 

 tained, and their effects determined so far as possible. 



