370 ANIMALS IN INLAND WATERS 



tion than in solutions containing only one salt. These latter may be 

 called monotonous solutions. Monotonous salt solutions, therefore, con- 

 dition a monotonous fauna, while in waters containing a variety of 

 salts the fauna remains varied until much higher concentrations are 

 reached. 35 For higher concentrations, the number of animal species 

 present decreases with increase in the salt content. 



The inhabitants of salt waters may be divided into various groups. 

 These include the incidentals which are widely distributed in fresh 

 waters, and occasionally or incidentally are found in salt water. 

 "Halocoles" are found regularly in waters of higher salt content but 

 likewise occur in fresh water. As a rule they are widely distributed, 

 often cosmopolitan animals. Finally, "halobionts" are limited to water 

 of rather high salt content, and are more or less salt-tolerant stcnoha- 

 line forms. Wide distribution and abundance in favorable places are 

 characteristic for the latter two types; the elimination of enemies and 

 competitors is favorable to their development. 



Even a comparatively low salt content in water clearly causes 



reduction of the animal life. In the Dortmund-Ems Canal 36 with a 



salt content which fluctuates between 0.3 and 2.1^ c the amount of 



plankton is greatly reduced in comparison with a normal stream of 



the plains (380 individuals against 6800 in an equal amount of 



water). 37 These are purely fresh-water animals except the hydroid. 



Cordylophora lacustris, which inhabits brackish waters. Even at a salt 



content of 25%c a plentiful fauna is present in which the incidentals 



are by far in the majority. The higher the salt content and the greater 



the fluctuations, the fewer the incidental forms ; at 100% c salt content 



incidental species are entirely missing, and only halocole and halo- 



biontic species are found. If the salt content rises beyond 100% the 



halocoles also disappear more and more and specifically salt animals 



are the only ones found; in addition to the brine shrimp Artemia 



(Fig. 106) these include the larvae of Diptera, especially of the genus 



Ephydra (Fig. 107). In a concentration of 120-160%o the fauna is very 



poor in number of species, but the number of individuals of Artemia 



and of Ephydra larvae is not diminished. These are often present in 



countless numbers ; myriads of Artemia give the water a reddish color, 



and the number of Ephydra may be so vast in certain Californian 



salt swamps as to give a brown color to the water. Great Salt Lake in 



Utah supports an immense number of Artemia fertilis, Ephydra 



larvae of 3 species, and a corixid bug. Adult Ephydra flies settle on 



the surface in dense swarms. The waters contain also a considerable 



number of protozoans, including an Amoeba, and of algae, including 



2 species of diatoms, 2 of chlamydomonas, and 1 species of blue-green 



