304 Aquatic Societies 



There are probably many factors determining this 

 annual distribution ; but chief among them would seem 

 to be these three: 



1. Chance seeding or stocking of the waters. 

 Each species must be in the waters, else it cannot 

 develop there; and for every species, there are many 

 vicissitudes (such as famine, suffocation, and parasitic 

 diseases) determining the seeding for the next crop. 



2. Temperature. Many plants and animals, as we 

 have seen, habitually leave the open waters when they 

 grow cooler in the autumn, and reappear in them when 

 they are sufficiently warmed in the spring. They pro- 

 vide in various ways (encystment, etc.) for tiding over 

 the intervening period. Some of them appear to be 

 attuned to definite range of temperature. Thus the 

 Cladoceran, Diaphanosoma, as reported by Birge for 

 Lake Mendota, has its active period when the tempera- 

 ture is about 20° C. (68° F.). For this and for many 

 other entomostraca reproduction is checked in autumn 

 by falling temperature while food is yet abundant. 



3. Available Food. Given proper physical condi- 

 tions, the next requisite for livelihood is proper food. 

 For the welfare of animal planctonts it is not enough 

 that algse be present in the water ; they must be edible 

 algae. The water has its weed species, as well as its 

 good herbs. Gloiotrichia would appear to be a weed, 

 for Birge reports that no crustacean regularly eats it, 

 and it is probably too large for any of the smaller ani- 

 mals. Birge says also ('96 p. 353), 'T have seenDaphnias 

 persistently rejecting Clathrocystis, while greedily 

 collecting and devouring Aphanizomenon." Yet 

 Strodtmann ('98) reports Chydorus sphcericus as feeding 

 extensively on Clathrocystis, even to such extent that 



