290 ANIMALS IN INLAND WATERS 



Fresh waters also vary among themselves in the content of other 

 salts, especially of calcium and magnesium. It is this quality which 

 makes them very different from sea water, which contains very little 

 calcium carbonate. We distinguish between soft waters, poor in lime, 

 and hard waters, rich in lime. The calcium carbonate content is low 

 in the waters of granitic or sandstone areas, high in waters flowing over 

 dolomite or limestone. This lime content is a decisive environmental 

 factor in animal distribution. Fresh-water sponges, Bryozoa, and the 

 cladoceran Holopedium gibberum, for instance, are intolerant of excess 

 lime. For that reason we find no Spongillidae or Bryozoa in Montenegro, 1 

 and in west Ireland the mountain lakes are much richer in sponges 

 than the limy lakes 2 ; Holopedium is most commonly found in the 

 mountain lakes of Europe, Iceland, and America, but not in the Jura 

 or in the limestone Alps. A wealth of lime in water is favorable for 

 the development of snails and mussels. Neritina, for example, occurs 

 in the lime-rich lakes of the Aland Islands but is not found in the 

 inland waters of Finland 3 which are poor in lime. The smaller size of 

 snails and mussels in Victoria Nyanza has been ascribed to the want 

 of lime in the water. 4 



In the Highland Lake District of Wisconsin, mollusks are present 

 even in acid water with a pH as low as 5.1 and a calcium content as 

 low as 0.1 part per million. Pisidium, the finger-nail clam, and 

 Campeloma, the gilled snail, are characteristic forms in clear lakes 

 with soft waters which have even less dissolved carbonates than the 

 typical bog-lakes with colored, acid waters. Pisidium has extremely 

 thin shells when growing under such extreme conditions, while in some 

 bog-lakes, with a pH of 5.1 to 6.1 and with 3.0 to 5.0 p.p.m. of dis- 

 solved carbonates, large, well-developed specimens of Pisidium have 

 been taken. Mussels of the genus Anodonta, growing in extremely soft 

 and acid waters, have shells so thin and poorly mineralized that when 

 fresh and wet they can be bent through almost 20° without breaking. 5 



In the same lake region, the distribution of fresh-water sponges is, 

 to some extent, correlated with the mineral content of the water. 

 Spongilla lacustris, although found under other conditions, attains its 

 best development in small lakes of high color and organic content and 

 rather low mineralization. In water in which the content of silicon 

 oxide is below 0.4 mg. per liter and which is also low in total solutes, 

 as the mineral content decreases, S. lacustris shows a progressive at- 

 tenuation of its spicules. If the water is sufficiently soft, the micro- 

 spinal dermal spicules are lost; this is the more interesting in that these 

 spicules have been regarded as an important species character. Simi- 



