FRESH WATER. 797 



the sea-weeds, abundant bivalves and gasteropods, sea- 

 squirts in their degeneracy, besides fishes, a few reptiles, 

 numerous shore birds, and an occasional mammal. The 

 shore fauna is thus very representative, rivalling in its range 

 that of the open sea, far exceeding that of the abysses. 



The conditions of life on the shore are in some ways the 

 most stimulating in the world. It is the meeting-place of 

 air, water, and land. Vicissitudes are not exceptional, but 

 normal. Ebb and flow of tides, fresh-water floods and 

 desiccation under a hot sun, the alternation of day and night 

 felt much more markedly than on the open sea, the endless 

 variations between gently lapping waves and blasting 

 breakers, the slow changes of subsidence or elevation, - 

 these are some of the vicissitudes to which shore animals 

 are exposed. The shore is rich in illustrations of keen 

 struggle for existence and of life-saving shifts or adaptations, 

 such as masking, protective coloration, surrender of parts, 

 and " death feigning." We may think of it as a great school 

 where many of the primary lessons of life, such as moving 

 head foremost, were learnt. 



Fresh water. Perhaps the most striking fact in regard 

 to the animals which live in fresh water is their uniformity. 

 The number of individuals in a lake is often immense, but 

 the number of species is relatively small, the number of 

 types still smaller. In widely separated basins and in 

 different countries the same forms occur. 



We may distinguish a littoral, a surface, and a deep- 

 water lacustrine fauna. The deep-water forms are chiefly 

 Rhizopods, Turbellarians, Nematodes, Leeches, Chaetopods, 

 Amphipods, Isopods, Entomostraca, a few Arachnids, some 

 insect larvae, and molluscs, and the general opinion is that 

 these are derivable from the shore fauna of the lake, which 

 includes similar forms, along with a few others, such as the 

 fresh-water sponge and Hydra. On the other hand, the 

 surface lacustrine fauna, consisting of water-fleas, Rotifers, 

 Infusorians, etc., widely and uniformly distributed, is said 

 not to be derivable from the shore forms. In transparency, 

 in gregariousness, in nocturnal habit, and in other ways, 

 they present a marked analogy with the marine Plankton. 

 How are we to account for their origin and wide distri- 

 bution ? 



