ENVIRONMENT AND DISTRIBUTION 361 



two or more communities. The classification of communities is 

 primarily a classification of physical environments of which there 

 are three subdivisions: (1) that of the ocean (halobios); (2) that 

 of the fresh water (limnobios) ; and (3) that of the land (geobios). 



Ocean Communities. — Animals found in the ocean are capable 

 of living in an aqueous medium containing certain concentra- 

 tions of inorganic salts, oxygen, and some organic matter, as well 

 as plant life. Some marine animals are so completely adapted 

 to the salinity of the ocean that they die when placed in fresh 

 water. The ocean offers a variety of aquatic environments 

 differing in depth, salinity, oxygen content, light, temperature, 

 etc. 



Littoral Animals. — The water lapping the shore of the ocean 

 forms the littoral zone. Owing to its slight depth, it has the 

 greatest amount of light and wave action and is subject to 

 maximal variations in temperature. Since the ocean tides 

 vary from a few inches to many feet, great changes in depth of 

 the shore water are common. When the tide is out, many 

 marine animals are uncovered for hours. The community of 

 shore animals includes coelenterates, echinoderms, worms, 

 arthropods, and molluscs. At low tide some of these lie buried 

 in sand, while others such as oysters, barnacles, some echino- 

 derms, and corals remain exposed to the air. 



Animals of the open ocean may be divided into three groups, 

 viz., (1) those of shallow water, (2) those living at the surface, 

 and (3) those at great depths. 



Shallow-water Animals. — Shallow water includes depths of 

 450 to 500 ft. At these depths in addition to those animals 

 found in the littoral zone, free-swimming stages of littoral 

 animals occur. Such forms are never uncovered by the tide, are 

 less subject to wave action, and are adapted to a greater pressure. 



Pelagic Animals.— This group includes the free-swimming 

 forms such as fishes, and also floating forms such as jelly- 

 fishes, small Crustacea, many larval forms, and Protozoa. The 

 transparency of the bodies of some of these plankton animals 

 makes them almost invisible. The Portuguese man-of-war is 

 a common floating type provided with a large gas-filled sac 

 (pneumatophore) . 



Animals of Great Depths. — Animals living at great depths of 

 the ocean, 5,000 to 15,000 ft., bathybic animals, are adapted to 



