151 



the distribution of the pulses with reference to the floods and the 

 appearance of pulses during rising water suggest the operation of 

 other factors than the one arising from contribution from back- 

 waters. 



The pulse must be dependent to a large extent upon food supply 

 of the organism, and a correlation between its periods of multiplica- 

 tion and the pulses of its food, the chlorophyll-bearing organisms, is 

 to be expected. A comparison of the seasonal distribution in 1898 

 (Table I.) and the pulses of chlorophyll-bearing organisms (PL II.) 

 reveals the fact that three of the A. cockle aris pulses coincide with 

 those of the plants constituting their food, and the other three 

 coincide in part only, the remainder of the chlorophyll-bearing 

 groups reaching their culmination a week prior to that of the rotifer. 

 In 1897 the three pulses of A. cochlearis which lie in the common 

 period (PI. II.) all culminate a week (in one case in part in fourteen 

 days) after the maximum of the plants in question. In 1896, three 

 pulses coincide and three follow in the subsequent collection; and 

 in 1895, two coincide and two follow. Collections at daily intervals 

 would be necessary to follow the correlation more accurately. It is 

 probable from these juxtapositions and sequences in the A. 

 cochlearis-algse pulses that we are dealing with a food relation. 

 Multiplication of algae leads to increase of Anur&a, which, in turn, 

 reduces the algae, and then itself declines until the food planktonts 

 again increase. 



Anurcsa cochlearis is exceedingly variable in the length of the 

 posterior spine, in the development and degree of curvature of the 

 anterior spines, in the arrangement of the areas of the lorica, and in 

 the degree of its ornamentation by small spinules. The separation 

 of these varieties where every individual must be assigned to some 

 one of them, is a matter of some difficulty owing to the presence of 

 intergrading individuals. The characters which signalize var. 

 hispida Lauterborn and var. irregularis Lauterborn are not quickly 

 recognized under the conditions of rapid plankton enumeration, and 

 no effort was made to trace their seasonal distribution in our plank- 

 ton. Lauterborn's var. macracantha was included with the type 

 form his var. typica in our records. These two include those 

 individuals with medium-sized and longer posterior spines. In our 

 waters the variety macracantha is relatively rare, at least as figured 

 by Lauterborn ('98). Indeed, both the type and this variety consti- 



