Factors Determining Vertical Distribution. 427 



below the ice. This is especially noticeable in the case of the 

 former species. 



The position of D. pulicaria must be controlled by tempera- 

 ture. I have never been able to detect any noteworthy differ- 

 ence between Daphnia pulicaria and Daphnia hyalina in their 

 relation to light, by means of laboratory experiments. Nor 

 have I as yet been able to find any difference in sensitiveness to 

 light between Daphnias brought from a depth of three meters 

 and those from a depth of twelve or more meters. 



The conclusion is, therefore, that in the upper meter and per- 

 haps within a range not exceeding two meters from the surface, 

 light is an extremely important factor in determining the vertical 

 position of the Crustacea. Below this depth, however, there are 

 no effects which can be definitely ascribed to light. I am not 

 at all inclined to deny that, in lakes whose water is more 

 transparent than that of Mendota, light may influence the Crus- 

 tacea to a greater depth. During the summer the water of 

 lake Mendota is always turbid with vegetation, which cuts off 

 the light very rapidly. My brass-topped dredge can rarely be 

 seen to a depth greater than two meters, and frequently disap- 

 pears between one-half and one meter. Vegetation, also, is 

 especially effective in cutting off the violet and blue rays, on 

 which the action of the light chiefly depends. In lakes whose 

 water transmits these rays more freely, light may be a far more 

 important factor in controlling distribution. 



The diurnal movement of the Crustacea, which is clearly 

 present during summer within the narrow limits of the upper 

 meter, is chiefly due to light. Wind or calm alter the condi- 

 tions of movement but during summer can hardly be considered 

 factors in causing it. 



Wind. 



On the whole, wind has only a small influence on the vertical 

 distribution of the Crustacea, although its effect varies greatly 

 with the season and with the condition of the several species of 

 Crustacea. The action of the waves prevents the formation of the 

 dense swarms of young Crustacea which are apt to be near the 

 surface during calm weather. These young Crustacea seek the 



