General Distribution of tne Crustacea. 443 
depths. About 15 per cent, of the Co-pepoda 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 -J- 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 Copepoda 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 I) 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 thq 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. 
