Horizontal Distribution — Swarms. 



371 



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.— To tal Crustacea, May — July, 1896. 



No. obser- 

 vations. 



May 1-15.. 

 May 16-31. 

 June 1-15 . 

 June 16-30 

 July 1-15 . 

 July 16-31 



Average. 



Maximum. 



Minimum. 



2,398 



2,966 



1,615 



1,901 



2,963 



1,177 



845 



1,977 



561 



1,265 



1,908 



890 



1,314 



2,332 



1,005 



795 



1,266 



511 



9 



9 



6 



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, ff.) 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 



