295 



recurrent pulses, which, in turn, are the cause of the similar and 

 often coincident volumetric fluctuations. 



Attention should be directed to the fact that without any im- 

 portant exceptions this recurrent movement pervades all the 

 organisms of the plankton which are eulimnetic, such as Scenedes- 

 mus, Melosira, Trachelomonas, Codonella, Synchtzta, Daphnia, and 

 Cyclops, and often those which at certain seasons become tempo- 

 rary planktonts, such as Difflugia and Hydra, but not with any 

 regularity the tycholimnetic organisms, such as bdeltoid. rotifers or 

 nematodes. It affects the more highly organized Rotifer a and 

 Entomostraca with slower growth, longer life, and consequent 

 greater cumulative function as well as the algae, diatoms, and 

 flagellates, where rapid multiplication, brief existence, and non- 

 cumulative (in the individual) function prevail. The large share 

 which the young (eggs and immature stages) play in the pulses of 

 Rotifera and Entomostraca will be seen in Table I., and repeated 

 attention has been called to this in the discussion of species. The 

 prevalence of breeding females and of eggs or young during the rise 

 of the pulse, and of eggless, moribund, or dead individuals or their 

 skeletons during the decline, is a common phenomenon in all well- 

 defined pulses. No species of plankton organisms appears to escape 

 the operation of this recurrent movement in production. 



The proportion of individuals surviving from one pulse to the 

 next is subject to great variation, being often least when the ampli- 

 tude of the pulses is greatest, and largest when the pulses culminate 

 at slight amplitudes. As a result of periods of minimum develop- 

 ment, it follows that the possible length of life of most plankton 

 organisms, even of the Rotifera and Entomostraca, in the plankton 

 must fall within rather narrow limits of a few days or a fortnight 

 at the most. Since the contrasts between minimum and maximum 

 numbers are relatively greater among the chlorophyll-bearing 

 organisms, it follows that the survival proportion is less in these 

 groups. 



The duration and amplitude of the plankton pulses will vary 

 within certain limits according to the method of delineation. The 

 volumetric minima and maxima present the total product in cubic 

 centimeters, and the pulses thus marked cat have been described 

 in Part I. They may also be delineated by statistical data of the 

 total plankton or of its larger groups of organisms, or by the domi- 



