798 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 
acorn-shells on the rocks and sandhoppers among the jetsam, 
a few insects about high-tide mark, sea-spiders clambering 
on the seaweeds, 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, Chzetopods, 
Amphipods, Isopods, Entomostraca, a few Arachnids, some 
insect larvee, 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 fauna. In transparency, 
in gregariousness, in nocturnal habit, and in other ways, 
they present a marked analogy with the marine Plankton. 
