94 



by current, waves, seines, boats, and fish make relatively larger 

 contributions at low-water stages to the diversification of the 

 plankton. In addition to these factors, however, there is abun- 

 dant indication that many individuals assume during the warmer 

 months a eulimnetic habit, and that some of the Rhizopoda 

 become, for the time being at least, typical, though temporary, 

 planktonts. 



It naturally follows that in so far as the plankton is concerned, 

 the Rhizopoda exhibit a seasonal preference for the warmer months 

 above 60! Maximum numbers were attained only at the higher 

 temperatures save in those instances where they attend winter 

 floods. In a measure the seasonal distribution of the Rhizopoda in 

 the plankton reflects that of the group in its normal habitat ; but at 

 the best the picture is incomplete. 



The Rhizopoda have important relations in the economy of the 

 plankton. They feed upon diatoms, desmids, the smaller algae, and 

 even the chlorophyll-bearing Mastigophora such as Trachelomonas 

 and Carteria. Their occurrences in the plankton do not exhibit any 

 striking correlation with those of the groups named. The great 

 pulse of September 7, 1897, for example (PI. II.), lies in a depression 

 of the diatoms and coincides with pulses of Chlorophycece and 

 Mastigophora, and that of August 10 (68,400) exhibits a similar 

 relation, the diatoms rising the following week as the Rhizopoda 

 fall. In 1898 the pulse of Rhizopoda on June 28 of 37,000 (Table I.) 

 culminates a fortnight after that of the diatoms and Chlorophycece 

 and a week after that of the Mastigophora. It thus is intercalated 

 between the June and July pulses of these chlorophyll-bearing 

 organisms (PI. II.). The Rhizopoda pulse of July 19 (28,800), on 

 the other hand, occurs with the coincident pulses of the three 

 groups named (PL II.). The immediate diluent effect of flood 

 waters upon the plankton combined with their tendency to increase 

 the number of adventitious Rhizopoda results at times in the 

 intercalation of their pulses with those of the chlorophyll-bearing 

 organisms whose relative numbers are reduced by the dilution. The 

 data evidently do not afford any adequate solution of the inter- 

 calations of the Rhizopoda with other organisms. 



The Rhizopoda are very frequently found in the digestive tract 

 of limnetic rotifers, but I have never noted the Entomostraca feed- 

 ing upon them. They are important elements in the food of young 



