Factors Determining the Annual Distribution. 853 
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, Glathrocystis 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 Glathrocystis , 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: 
