1911] Michael: Chaetognatha of the San Diego Region. 107 



tion, tributary water, various hydrographic factors, and by food 

 supply, and possibly, also, by chemical conditions not directly 

 concerned in nutrition, but the available data fail completely to 

 afford any satisfactory environmental factor or group of factors 

 which stands in correlation, even remotely obvious with this 

 cyclic movement in production. I therefore class this periodic 

 grotvth, these sexual cycles which cause volumetric pulses, under 

 the head of internal factors. The element of pfrioclicity in 

 itself does not seem to be consequent upon any known external 

 factor." On the basis of these facts we must agree with Kofoid 

 (1903) that "If this cyclic movement in production be charac- 

 teristic of the plankton generally, freshwater and even marine, 

 it must follow that scattered and irregular collections, or those 

 at intervals exceeding a week or at most a fortnight, may fail 

 entirely to give adequate representation of the course of plank- 

 ton production or relative fertility of a body of water. Chrono- 

 logical series throughout the whole seasonal range of climatic 

 conditions and at close intervals — of one week or less — are neces- 

 sary for any accurate delineation of production and fertility of 

 water by the plankton method." This is obviously a laborious, 

 time-consuming, and costly method, but it seems to be the only 

 accurate one by which definite conclusions concerning the 

 quantitative distribution of plankton can be obtained. 



Before concluding these remarks on the problems of distri- 

 bution, it is well to emphasize the importance of closing nets in 

 obtaining definite knowledge concerning the vertical distribution 

 of plankton. Their importance has been recognized by many 

 investigators such as Chun, Peterson, Agassiz, Hensen, Apstein, 

 Cori, Bergert, Bruce, Giesbrecht, Albert I of Monaco, and 

 Kofoid, through whose researches and inventions the closing net 

 has been developed to its present state of efficiency. However, 

 excepting the Conseil International, "Siboga, " Biscayan, and a 

 few other expeditions, most have depended mainly upon open 

 vertical nets for collecting data relative to vertical distribution. 

 Important conclusions have been based on evidence thus ob- 

 tained, one of which is that the surface Chaetognatha of the 

 Arctic seas would be found, if at all, in the mesoplankton of 



