No. 588] POPULATION OF « BLANKET -ALG2E " 761 



prising to find some of the pools totally devoid of this 

 kind of vegetation in spring. Pools K, M, x, x 1 , x z , x s , x 4 

 showed this peculiarity. Their " blanket-algae" did not 

 reappear until May. This disappearance of surface 

 vegetation may have been due to spring freshets, as the 

 pools mentioned are in the flood-plain of Cascadilla Creek, 

 although not in the stream-bed. 



The Natural Balance. — Like other societies, the popu- 

 lation of the "blanket-algae" has its producers and its 

 Consumers, its hunters and its hunted, each readily ex- 

 changing roles as occasion demands. The synthetic or- 

 ganisms include with the phytoplankton a few chlorophyl- 

 bearing organisms of the zooplankton ; that is, forms like 

 Euglena, Phacus and Distigma, which, in sunlight, have 

 the holophytic method of feeding (Stokes, 1895). Dia- 

 toms require nitrates, silica and some salts to make their 

 dainty and beautifully marked shells. Since they are 

 comparatively heavy, they sink slowly, but are brought to 

 the surface during the spring and fall circulation of the 

 water. In spring they multiply rapidly near the surface, 

 since they need oxygen and sunlight. 



Many of the tiny creatures, including ciliates, Clado- 

 cera, rotifers and nymphs and larvae of some insects are in 

 search of diatoms. These animals eat other tiny food 

 particles as well as diatoms. The rhizopods, Arcella and 

 Arneba, ingest diatoms, desmids, small protozoans and 

 even rotifers. Vampyrella consumes the cell-contents of 

 algae. Actinophrys prefers the spores of algae, but takes 

 small protozoa. Actinosphcerhim is omnivorous (Stokes, 

 1895). Many of the ciliates eat diatoms and other ciliates. 

 The food is drawn into the oral opening by means of cur- 

 rents of water which are directed toward the opening by the 

 constant motion of cilia. One ciliate, Chilodon, has a pe- 

 culiar method of feeding. It protrudes a broad flexible 

 lip-like expansion of the anterior end and gathers up 

 food particles with a sweep of this organ. 



Turbellarian worms feed on rhizopods, ciliates and 

 rotifers. Rotifers eat diatoms and some nibble algae, 



