Blologi cal classifications based upon necessary conditions tar existence are of value 

 for the following reasons: By surveying the stock of eiquaticSj eaten by fish, we leam the 

 inherent characteristics of the pond. This again teaches ua the diversity of aquatic life 

 ■which is reasonably to be expected. 



As regards adaptation to varying calcium contents and variable reactions, we have to 

 differentiate between: 



(1) resistance to lack of calcium and to a low pH content are Corixa, Cyclops, 

 also Cloeon and some Chironomida larvae j 



(2) sensitive to lack in calcium and to low pH content are Amphipodae and 

 mollusks; 



In regard to adaptation to pollution caused by rotting organic matter, leading to loss 

 in oxygen, we have 



(1) sensitive to faint pollution and trifling lack in oxygen the followingi 

 Amphipodae. dragon fly larvae. Ephemera larvae (also "stone-fly" larvae); 



(2) tolerating moderate organic pollution and moderate lack in oxygen are the 

 following: Sty la r la. Valvata . Bith>Til3 . Sphaerium. Daphne pulex . Aaellua 

 aquaticus . Sialis larvae. Mot one eta, water beetles, Tanytarsus and other 

 Chironomidae (also "ilueckenegel" ) 



(3) tolerating very strong organic pollution and great lack in oxygen we find 

 mud dwelling red Chironomida larvae and also "Schlammroshrenwuem^r" (m\Ki- 

 tube worms ) . 



The kind of the chiefly preferred habitat of aquatics frequently determines their 

 discovery on the part of pond fish. Also, the knowledge of their preferred location 

 permits to figure out, beforehand, the cliances of fish planting under certain conditions, 

 respectively will permit to tnspro-ve same. 



A classification as to biological communities has been made, already, when discussing 

 the catabolic cycle of the pond, when we found 



(1) free-water organlsma (animal plankton), ' 



(2) vegetation aquatics, 



(3) bottom dwellers. 



A division, according to the mode of intrusion of aquatics into the pond, during the 

 yearly cultivation brings us to the discussion of the productivity of aquatics during the 

 different seasons. 



After more or less long intervals after cultivatlcai aquatics appear in quantitative 

 vast numbers. 



(1) Through influx with the fresh water and f^o.-n out of remaining puddles in the 

 drained off pcxid (worms, mollusks, Podurae. spiders, mites, large insect 

 larvae and all other forms, including fishes in small numbers), 



(2) Through climbing out of the pond bottom, where they have hibernated (especial- 

 ly at the edges of the frozen layers), as in the case of Chironomua plumosis 

 and mollusks. 



(3) Through development of life forms which are resistant to frost and aridity of 

 which are carried into the pond by the wind ( Cladocerae. some Poduras . leethes 

 and Rotatoriae ) . 



(4) Through eggs, deposited into the cultivated pond by flying insects ( Chircnoml- 

 dae. Ephemerae . dragon flies, Chrysopae ) . 



35 



