The Annual Distribution of the Crustacea. 307 



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 brevispinosa, 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 



