898 GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



many " worms," star-fishes and sea-urchins, crabs and 

 shrimps, acorn-shells on the rocks and sandhoppers aniong 

 the jetsam, a few insects about high-tide mark, sea-spiders 

 clambering on the seaweeds, abundant bivalves and gas- 

 teropods, 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 

 diflFerent 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, Chaeto- 

 pods, 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 distri- 

 buted, is said not to be derivable from the shore fauna. In 



