MR. R. GURNEY ON THE LIFE HISTORY OF THE CLADOCERA. 55 
earlier in 1903 than in 1904. As the temperature curves 
show, the first three months of 1903 were much warmer than 
those of 1904. The bitterly cold April does not seem to have 
had any appreciable effect, except to have delayed such 
species as had not already hatched — such as Ceriodaphnia 
megalops. 
So far as observations have gone during the present year 
they show the probability of an early hatching like 1903. 
For instance, Sida crystallina and Peratacantha truncata 
have already been seen (March 15). Now the mean tempera- 
ture of the first fifteen days of March this year is 43.9 (from 
observations at Sutton Broad Laboratory), so that it may 
be assumed that the “ hatching temperature ” for the 
majority of species is a mean of about 43 0 F. 
But, just as the species appeared earlier in 1903, so they 
disappeared on the whole earlier in that year. Eight species 
disappeared earlier and three later. Such a difference is not 
so easy to explain. It was, perhaps, due to the generally 
colder summer of 1903, since there was practically no difference 
in temperature in October and November. On the other 
hand it is quite possible that each species tends to multiply 
only for a certain definite number of generations, so that if 
it appeared earlier it would also disappear earlier. 
The occurence and intensity of the sexual periods in the 
First Record. 
Last Record. 
Name of Species. 
1903. 
1904 . 
*903. 
1904. 
Sida crystallina 
DiArHANOSOMA brachyurum - 
Scapholf.beris mucronata 
Ceriodaphnia reticulata 
,, megalops 
,, quadrangula - 
,, pulchella 
Camptocercus rectirostris 
Lynceus costatus - 
,, rostratus - 
Graptoleberis testudinaria - 
Peratacantha truncata 
Polyphemus pediculus - 
March 22 
April 25 
April 9 
Feb. 26 
June 23 
Feb. 24 
May 15, 
May 10 
March 12 
April 4, 
March 16 
March 12 
March 22 
May 26 
June 16 
May 31 
April 17 
May 26 
March 3 
May 3 
May 26 
March 13 
April 14 
March 14 
June 4 
April 17 
Nov. 19 
Oct. 24 
Oct. 24 
Nov. 29 
Nov. 8 
Dec. 4 
Dec. 18 
Nov. 17 
Nov. 17 
Nov. 13 
Nov. 19 
Dec. 1 
Nov. 17 
Dec. 7 
Oct. 13 
Nov. 8 
Dec. 6 
Nov. 11 
Nov. 23 
Nov. 17 
Dec. 6 
Dec. 7 
Nov. 29 
Dec. 7 
Nov. 29 
Nov. 29 
Table No. 2. 
