The Annual Distribution of the Crustacea . 807 
Young Cyclops began to appear under the ice, but the condi¬ 
tion of the species in the middle of March resembled that in 
the middle of February in 1896, and the progress of the develop¬ 
ment was in general about a month later. 
The rotifers also show similar differences in reproduction in 
different seasons. Of this group there are regularly present 
during the winter, Triathra , two species of Notholca , Anurea 
aculeata ,, cochlearis , and brevispi?iosa 1 Synchaeta pectinata , and a 
species of Oecistes. All these reproduce more or less actively, 
and become quite abundant before the breaking up of the ice. 
Other species are present in smaller numbers. 
The difference in the reproductive activity of these animals 
in different years seems to depend upon the temperature of 
the water, as will be explained at length in a later section of 
this paper. In all seasons there is an abundance of food. One 
of the chief winter algae is Aphanizomenon, which continues its 
development vigorously throughout the entire winter. Several 
species of the diatoms are also present, and in 1896 Fragilla- 
ria and Diatoma contributed largely to the plankton algae, but 
in 1895 and 1897 were insignificant in quantity, as compared 
with Aphanizomenon. There is no season of the year in which 
the Crustacea fully overtake the food supply, except at the time of 
the spring maximum. During the winter the Crustacea are ac¬ 
tive and fat, but those species which do not reproduce do not 
increase in size. Careful measurements of numerous individuals 
of Daphnia hyalina showed no appreciable increase in the aver¬ 
age size between December, 1894, and April, 1895. When the 
temperature of the water is between 1.5 degrees and 2.25 de¬ 
grees C., Cyclops develops very slowly or not at all from the 
nauplius state to that of the immature Cyclops , but at tempera¬ 
tures above 2.5 degrees the development goes on, although, of 
course, more slowly than at higher temperatures. 
The Crustacea in Spring. 
Lake Mendota has no large affluent, and the breaking up of 
the ice is slow, since it is due to the combined action of rain, 
sun and wind. The date of the disappearance of the ice differs 
greatly in different years. In 1895 the last expedition on the 
