Horizontal Distribution — Swarms. 
871 
The following table gives the variations of the total number 
of the Crustacea during three months of 1896. It will be seen 
the variations are somewhat smaller than are those of the single 
species but are of a similar character, and also resemble those 
of Table XXVI. 
Table XXVII. —Total Crustacea, May —July, 1896. 
Average. 
Maximum. 
Minimum. 
No. obser¬ 
vations. 
May 1-15..... 
2,398 
2,966 
1,615 
8 
May 16-31...... 
1,901 
2,963 
1,177 
8 
June 1-15.... 
845 
1,977 
561 
9 
June 16-30.... 
1,265 
1,908 
890 
9 
July 1-15.... 
1,314 
2,332 
1,005 
6 
July 16-31 ..... 
795 
1,266 
511 
11 
I think that I have given here and in the tables of the ap¬ 
pendix, sufficient evidence to enable the student to undertands 
the extent of the variation in the distribution of the Crustacea. 
I do not know whether the figures will be interpreted as show¬ 
ing an equal or unequal distribution. I judge that Marsh, from 
his discussion of the subject (’97, p. 218, if.) would regard the dis¬ 
tribution as irregular. I think that it is quite as uniform as 
Apstein would expect. For myself, I have never supposed that 
every square decimeter of the surface of the lake covered an 
equal number of Crustacea. I have been surprised that a net 
20 cm. or 10 cm. in diameter should disclose such a uniform 
number as it actually shows, especially in the case of organisms 
so highly organized as the Entomostraca. 
On the other hand, there is clear evidence for swarms in cer¬ 
tain species of Crustacea, and at certain times. (1) The dis¬ 
tribution of Daphnia pulicaria is very irregular, far more so 
than that of any of its congeners. This species in lake Men- 
dota is confined during summer to the region of the ther- 
mocline, and as this stratum works downward through the lake 
in summer, the area inhabitable by the species is contracted 
around the edge of the lake, and the Crustacea as they move out 
from the shore to keep in the cool water, may accumulate in 
swarms. These have already been mentioned in connection with 
