THE LIFE OF DEVIDS LAKE 45 



ficance in the study of the plancton distribution. They will be 

 considered individually in the annotated list. 



Only two phyla of animals have any considerable representa- 

 tion ill the plancton. These are the Crustacea and the rotifers. Of 

 the former there are three important genera of one species each, 

 and of the latter two genera and three species. 



Of the Crustacea Diaptomus sicilis is of most importance, both 

 because of its numbers and its presence at all seasons of the year. 

 At times it is out-numbered by Cyclops and Moina, but these occur 

 in considerable numbers for comparatively brief periods, while 

 Diaptomus is always more or less common. 



The nauplii are mainly those of Diaptomus, but no attempt has 

 been made to differentiate between them and those of Cyclops. 

 The latter are probably too few in general to affect the distribution 

 curve to any extent. In general the maxima of the nauplius curve 

 alternate with the minima of that of Diaptomus as might be 

 expected. The number of nauplii is, in general, much greater than 

 those of Diaptomus and Cyclops combined, which undoubtedly indi- 

 cates a high mortality in the larval stage. In Lake Mendota also 

 Birge (1897) reaches a similar conclusion regarding their mortality. 



Contrary to his experience, on the other hand, I find the 

 nauplii apparently absent in Devils Lake in winter, tho gravid 

 females and eggs of Diaptomus occasionally occur at this time. 



Birge (1. c.) finds the nauplii more abundant in sununer near 

 the thermocline, while, with the disappearance of this layer, their 

 distribution becomes more uniform. In Devils Lake nauplii are 

 somewhat more numerous near the bottom than the surface in 

 summer, the difference not being evident in spring and fall. The 

 reasons for this difference in distribution is not clear. Nor does 

 Birge offer any satisfactory explanation. 



The curves for the various species will be considered in con- 

 nection with each individually. In the case of the zooplanctou, 

 especially the rotifers, the numbers and abruptness of the waves 

 indicate a brief life cycle and rapid development, but we have as 

 yet few data regarding the life span of any planctont. Needham 

 and Lloyd (1916) state that "the rotifer, Hydatina is said to have 

 a length of life of some thirteen days," but give no authority for 

 their statement. Steuer (1910, p. 269) gives the ages of a few 

 copepods based on the investigations of Burckhardt (1900) and 

 Ekman (1904), the average being 13 months. 



This figure appears high for the average life span of fresh 

 water copepods. Judging from the curves for Devils Lake and 

 for Lake Mendota and Green Lake, Wis., as given by Birge (1897) 

 and Marsh (1898), the average life span is very much less than 



