General Distribution of tne Crustacea. 443 



depths. About 15 per cent, of the Copepoda were found in the 

 first observation below a depth of 20 m. and about 32 per cent, 

 between 10 and 20 m. On Aug. 31 the proportion found in the 

 deeper levels was much increased, the upper 10 m. containing 

 less than 20 per cent, of the Crustacea while more than ^ were 

 between 20 and 40 m. In the second observation no specimens 

 seem to have been found at a depth below 20 m. If these few 

 observations represent the average summer conditions in lake 

 Plon, it is obvious that the Cvp>epoda extend to far greater 

 depths than in lake Mendota, since deeper hauls here would have 

 added few or no Crustacea to the number obtained in the upper 

 10 m. No reason can at present be assigned for this difference 

 in distribution, but the fact of the difference is sufficient to show 

 that each lake demands careful individual study, if we are to 

 form any correct idea of the actual condition and distribution of 

 life in it. This fact is still more clearly indicated by France's 

 observations on the diurnal movement of the plankton of lake 

 Balaton, to which more extended reference is made in another 

 part of this paper. 



No diurnal movement is clearly disclosed by our results. This 

 question is discussed in detail for each of the Crustacea, and the 

 general conclusions are stated in connection with comments on 

 France's paper. 



In the following tables, A to D show in the columns headed 

 " No. " the average number of Crustacea caught in each period 

 from each level. The column headed "per cent. " shows the per- 

 centage which that number constitutes of the total number of 

 Crustacea obtained from that level, or, in the bottom line, the 

 percentage of the total catch which the particular species fur- 

 nished. The last columns show the total number from each 

 level, and the per cent, of the entire catch which the number 

 from each level constituted. Table E shows the same facts 

 for the entire month. Tables F and G- bring together the per- 

 centile relations shown in the preceding tables. 



