312 Birge — The Crustacea of the Plankton. 



cea. The species present are those which are most easily avail- 

 able as food, so that both in kind and quantity of food, the 

 Crustacea find the most favorable possible conditions from early 

 September to the latter part of November. Temperature is the 

 predominant factor in influencing their development. 



In 1894 and 1896 Ghydorus was present in great numbers. 

 Both of these seasons were characterized by the great abundance 

 of Aphanizomenon. In 1895 and 1897, when the predominant algae 

 were almost exclusively diatoms, the number of Chydorus was 

 extremely small. Diagram 10 shows the number of Crustacea from 

 July to December, after subtracting Chydorus. It will be seen 

 that the form of the curves is strikingly similar in all years, 

 and that the numbers are extremely close for 1895 and 1896, 

 with the exception of a great rise in late October, 1896, which 

 was due to the sudden multiplication of Daphnia hyalina&t that 

 time. 



From the fall maximum the number declines, at first rapidly, 

 and afterwards more slowly toward the winter minimum. The 

 rapidity of the decline depends upon several factors. If a large 

 number of young forms are produced late in the season, many 

 of them die as well as their parents, and the decline in num- 

 bers is correspondingly rapid. The number of the periodic 

 species also exerts a great influence. In 1896, when Daphnia 

 retrocurva was present in large numbers, its sudden disappear- 

 ance at the close of its sexual period aided to cause a rapid de- 

 cline in the total number of Crustacea present. The climatic 

 conditions also exert a great influence. A rapid decline in tem- 

 perature, accompanied by violent storms, causes the numbers to 

 sink more rapidly than a more equable approach of winter tem- 

 peratures. In any case the number of the Crustacea falls off 

 rapidly during November, more slowly during December, and 

 by the middle or last of that month the lake freezes aud the 

 winter conditions are fairly established. 



The different species of limnetic Crustacea enter the winter in 

 very different conditions. Daphnia hyalina produces in the late 

 fall large numbers of young, which serve to carry the species 

 through the winter. The old individuals disappear during No- 

 vember and December, very few lingering into January. Dur- 



