4 



HOW THE ANIMALS HAVE CHANCED 



' I HE view that life originated in the ocean has already been set 

 forth. Some great groups have never left the ocean and are 

 confined to it today — Ctenophora, Brachiopoda, Echinodermata, 

 Polychaeta, Cephalopoda, and Tunicata. Three other groups are 

 largely marine; comparatively few representatives of sponges, 

 coelenterates, and bryozoans are found in fresh water. Amphibia 

 are confined almost wholly to fresh water, but a few frogs even 

 breed in brackish water which may contain as much as 2.6% salt 

 (Pearse, 1911). Lung-fishes (Dipnoi) are found only in fresh 

 water. Only two large groups of animals, Myriapoda and Ony- 

 chophora, are exclusively terrestrial. 



No Protozoa are terrestrial. Though many species occur in soils, 

 they are active only when water is present. Some even occur at times 

 on snow. A few flatworms are terrestrial. They are protected from 

 desiccation to some extent by their slimy secretions, but are con- 

 fined to moist situations and are nocturnal. Nematoid worms occur 

 in the soil in great numbers. Other species are found in or on ter- 

 restrial plants. Earthworms are commonly confined to damp soil, 

 but some of them climb trees and lurk in crevices in the bark. Land 

 leeches commonly rest under fallen leaves or other objects along 

 shady pathways and hasten out into the open when a prospective 

 host passes. Among mollusks only gastropods have attained land 

 life. Many marine and river snails are branchiate; littoral and pond 

 snails are commonly pulmonate. Land snails lack gills and use the 

 mantle cavity as a lung; they are protected by a mucous covering 



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