352 Birge — The Crustacea of the Plankton, 



were any parts of the skeleton of this species found. The legs 

 and similar appendages of Cyclops were not infrequently seen. 

 Large Daphnias have ordinarily a shell so thick that the weak 

 jaws of Leptodora are unable to pierce it, and a very large pro- 

 portion of the Daphnias seized by Leptodora escape apparently 

 uninjured. 



Apstein ('96, p. 175), notes that this animal in the Einfelder 

 See was very large, over 1 cm. long. It is not at all uncom- 

 mon to find specimens measuring 18 mm. in lake Mendota. The 

 average size is dependent apparently on the abundance of food. 

 In Green lake and the Oconomowoc lakes the length is decid- 

 edly less than 1 cm. 



FACTORS DETERMINING THE ANNUAL DISTRIBUTION. 



Our knowledge of the conditions of limnetic life is at present 

 far too fragmentary to permit any complete explanation of the 

 factors which determine the number of Crustacea present in the 

 plankton. Certain provisional results however, may, be reached 

 as a result of this study of the Crustacea. The following factors 

 are present and combine to determine the total number of the 

 Crustacea present at any time and the number of the members of 

 each species. 



1. The food, both in quantity and quality. 



2. Temperature. 



3. Competition. 



Food. 



It is plain that the quantity of available food must set an 

 upper limit to the number of Crustacea. Available food must 

 be carefully distinguished from plant material, since all plants 

 are by no means equally edible by the Crustacea. Gloiotrichia, 

 for example, is present in lake Mendota in considerable num- 

 bers from the latter part of July to the early part of Septem- 

 ber. It is never the dominant alga, as it is apt to be in the 

 plankton-poor lake. But it is often the most prominent alga 

 to the eye, and is present in such numbers as to form on calm 

 days a thin scum on the surface. It does not appear, however, 

 that any species of Crustacea regularly eats it. I have given 

 very careful study to this point during three seasons, and have 



