374 ANIMALS IN INLAND WATERS 



The subterranean origin of many inhabitants of springs is especially 

 noticeable among the fishes which are found in the springs and artesian 

 wells of the Sahara ; 47 these belong to 6 different genera. These fishes 

 seem capable of living at times in subterranean basins with which the 

 springs are in communication; because of this they are often in great 

 numbers in very small springs and sometimes appear suddenly at the 

 mouths of newly drilled wells. In some springs there are no living 

 forms other than the fishes, and these are so underfed that they even 

 attack bathers. 



The fauna of springs is everywhere sparse because of the small 

 amount of available nutritive material; the forms represented are 

 small and insignificant, and if they have a rather wide distribution, 

 e.g., Gammarus pulex or Limnaea ovata, the individuals in springs are 

 smaller than others of their species. Besides the scarcity of food 

 materials, space limitation itself may be a determining factor in pro- 

 ducing this size relation. 



Animal communities of thermal waters. 48 — Increased tempera- 

 ture of water, like increased salt content, forms a handicap for the 

 development of animal life. Although algae can grow in water up 

 to 80°, the maximum limit for protozoans is somewhat above 50°, 

 and for Metazoa it is almost as great. The Protozoa of hot water 

 basins are primarily the shelled rhizopods* and ciliates such as Cycli- 

 dium glaucoma. Dwarf specimens of the ciliate, Nassula elegans (27^, 

 elsewhere up to 200/x), and a few Amoeba Umax (12-15/* in diameter) 

 are found at 50-52°, and the rhizopod Hyalodiscus lives at a tempera- 

 ture of 54°. Few metazoans can endure so high a water temperature. 

 Of rotifers, which are most numerous in very warm waters, Philodina 

 roseola shows the greatest power of resistance. A number of mollusk 

 species are also able to live in fairly warm water; Limnaea peregra, 

 a very adaptable form, occurs in the thermal waters of the Pyrenees 

 up to a temperature of 45°; Paludestrina aponensis also tolerates 45° 

 in the thermal waters of Abbano in the Euganean Hills of north Italy, 

 but its optimum range is from 32° to 36°. Species of Melanopsis, 



vitta, the annelid Haplotaxis gordioides, the cave isopod Asellus cavaticus, and 

 the cave amphipod Niphargus puteanus; and of the snails the species of Lartetia 

 in numerous local varieties, these being often found only as empty shells in the 

 Jura and Muschelkalk regions. 48 To these are added the ordinary inhabitants of 

 creeks such as the snail, Ancylus fluviatilis; the amphipod, Gammarus pulex; and 

 many insect larvae. There are also a number of ubiquitous forms such as varieties 

 of the small bivalve. Pisidium (on muddy substratum), and snails. Limnaea 

 truncatula, L. ovata, and Bythinia tentaculata. 



* Centropyxis aculcata, Difflugia. Trinema, Quadrida. 





