Factors Determining the Annual Distribution. 353 



never seen any evidence that any of the limnetic Crustacea feed 

 upon it. Of course in cases of necessity it may be eaten, but 

 even where other food is comparatively scanty, Gloiotrichia 

 seems to be avoided. It should, therefore, be subtracted from 

 the quantity of available food. 



Glathrocystis and Goelosphaerium appear also to be far less 

 readily eaten than other species. I have made very numerous 

 observations upon Daphnia of all three of the species present 

 in lake Mendota and have uniformly found that while the dia- 

 toms, Anabaena, and Aphanizomenon are greedily eaten, the 

 colonies of the genera first named are uniformly rejected. Dur- 

 ing the autumn and winter of 1894-5, Clathrocystis and Aphan- 

 izomenon were almost the only algae present. The food of Daph- 

 nia was almost exclusively the latter species, and I have seen 

 hundreds of Daphnia persistently rejecting Clathrocystis, while 

 greedily collecting and devouring Aphanizomenon. Daphnia 

 eats freely all of the filamentous diatoms, including Fragillaria, 

 Melosira and Diatoma, while Diaptomus seems to prefer Ana- 

 baena and Aphanizomenon to the diatoms, when all are present 

 in large numbers. Since these preferences for various 

 kinds of food are so strikingly marked among the Crusta- 

 cea, it may easily happen that a period when vegetation is su- 

 perabundant in the lake may be one of scarcity for the Crusta- 

 cea. The most conspicuous case of this sort occurred in the 

 summer of 1894, when my observations on the Crustacea began. 

 In July and early August of that year a species of Lyngbya 

 overgrew all the other species of plants, constituting more than 

 95 per cent, in bulk of the vegetable plankton. It was so 

 abundant as to constitute a thick scum on the surface of the 

 lake during calm weather. The filaments of Lyngbya are large 

 and perhaps for other reasons than size are little available as 

 food. The Daphnias present were carefully examined and hardly 

 a single filament of the species was found in them, nor could I 

 find any evidence that the other species ate it, although the re- 

 mains of diatoms and other species of plants were found in their 

 intestines. The number of every species of limnetic Crustacea, 

 except Diaptomus, was far smaller during this period than in 



other years, as the following table will show: 

 23 



