Comparison of Habitats IB 



and readily enter them. For example, nematodes have an impervious 

 cuticular covering and this permits certain of them to enter plants 

 or animals as parasites. 



Comparison of Ocean, Fresh Water, and Land 



''Das Land ist das Reich der Gegensdtze, das Wasser das Reich 

 des Gleichmasses" (Simroth, 1891). Water habitats in general are 

 more or less stable; land habitats as a rule present extreme varia- 

 bility. An animal that lives in water is in no danger of death by 

 loss of water from its body fluids, but may be easily killed by 

 soluble poisons in the surrounding medium. Water is comparatively 

 stable because it absorbs and loses heat slowly and cannot circulate 

 rapidly. Shallow water may be subject to violent wave motions, 

 and small stagnant bodies of water may be deficient in oxygen. Land 

 animals have a dependable oxygen supply. The land is subject to 

 wind storms and to sudden and extreme changes in atmospheric 

 temperature and moisture. As water is a dense medium compared 

 to air, aquatic animals move slowly, while land animals may be 

 speedy. 



The ocean is in general more stable and uniform than bodies of 

 fresh water because of its vast extent. Its high and varied salt con- 

 tent and relatively uniform temperature make it a favorable me- 

 dium for protoplasmic activity. It is comparatively simple as an 

 environment for plants and animals (Bigelow, 1931). It contains 

 no nitrifying organims except near shore. A wide variety of algae 

 are present in the sea, but only about thirty species of spermato- 

 phytes occur (Buxton, 1926) . All phyla of animals are represented 

 in the ocean; echinoderms, brachiopods, and tunicates are found 



Fig. 1. Pool that has been cut off from the ocean 

 at Dry Tortugas and is gradually growing less saline. 

 (From Pearse. 1932e.) 



