Factors Determining the Annual Distribution. 355 



Ceratium offers an instance of an alga which, while not ab- 

 solutely unavailable as food, is far less rapidly eaten than other 

 species. So far as my observations extend, the adult Cyclops 

 devour it more freely than do any other species of Crustacea. 

 Cyclops, indeed, is the most omnivorous of the plankton Crustacea. 

 It seizes and devours rotifers, nauplii, and other small animals, 

 as well as plants. I have seen it pounce upon and devour Cer- 

 atium several times, while I have never seen Diaptomus do the 

 same, and have only very rarely found fragments of Ceratium 

 in the intestine of Diaptomus. During 1895 I did not find in 

 a single instance Ceratium within the shell of Daphnia, but 

 in 1896 I found it in a very few cases. Ceratium is a prominent 

 alga during the summer, and at some time ordinarily becomes 

 the dominant form, so that there is fairly a Ceratium period. 

 In 1895 this period fell from the middle of June to the middle 

 of July, and for a week on each side of the first of July, Ceratium 

 constituted more than 90 per cent, of the plankton algae. In 

 1896 this period was later, coming in August and early Sep- 

 tember. It was present in large numbers from the early part of 

 the summer, but seemed to be hindered in its development by the 

 great numbers of Aphanizomenon , which were present in the 

 water. For nearly a month it seemed doubtful whether there 

 would be a Ceratium period at all, but finally in August, 

 Ceratium predominated decidedly over Aphanizomenon, although 

 a considerable quantity of the latter species and Anabaena was 

 always present. Ceratium, like Aphanizomenon, occupies the 

 upper strata of the water, and its presence there is a hindrance 

 to the development of the young Crustacea, since it is so large 

 and its shell is so hard that it cannot be eaten by them. The 

 Ceratium period in 1895 marked the beginning of a decline in 

 the numbers of the Crustacea. The same was true to a less 

 marked extent in 1896. I have no doubt that the presence of 

 this alga in great quantity is one of the factors which influences 

 the late-summer minimum in the numbers of the limnetic Crus- 

 tacea. In 1894, Ceratium was present, but its numbers were 

 always far inferior to those of of Lyngbya. 



The quantity of food also exerts an influence on the number 

 of the Crustacea. In a lake in which the plankton is so abun- 



