ECOLOGY AND ZOOGEOGRAPHY 



645 



the fresh-water habitat into communities 

 of (1) flowing water and (2) standing 

 water. Then we divide the flowing water 

 into (1) rapidly flowing streams and (2) 

 slowly flowing streams, and the standing 

 water into (1) lakes, (2) ponds, and (3) 

 swamps. Each of these may be further di- 

 vided, for example, the ponds into ( 1 ) 

 those with bare bottoms and (2) those 

 whose bottoms contain vegetation. Each of 

 these types of ponds contains a number of 

 communities, such as those occupying (1) 

 the beach above the water, (2) the shore- 

 line, (3) the shore water, (4) open water 



(plankton and nekton), and (5) the bot- 

 tom. All other habitats may similarly be 

 subdivided. 



Many of the animals we have studied 

 live in fresh-water communities; these in- 

 clude protozoans, rotifers, hydras, planarians, 

 crayfishes, helminthcs, annelids, mollusks, 

 and fishes. In flowing streams we find fish, 

 flatworms, crayfishes, caddisfly larvae, and 

 snails; these are adapted to withstand a cur^ 

 rent by swimming, by clinging to plants 

 and rocks with claws, suckers, etc., and by 

 seeking shelter in crevices or under stones. 

 Standing water contains especially plank- 



Copepod 



Crustacean larva 

 Figure 452. Some types of animals and plants found in plankton. 



Desmid 

 (plant) 



Diatom 

 (plant) 



ton, organisms (Fig. 452) that float or swim 

 feebly and are carried about by wave action 

 and currents; they are mostly small and 

 even microscopic, but some may be of con- 

 siderable size. Larger swimming animals, 

 the nekton, are also present. Animals that 

 live on the bottom constitute the benthos. 

 In swamps and bogs the quantity of oxygen 

 is often reduced, which limits the variety of 

 animals that obtain oxygen from the water; 

 consequently most swamp animals are air 

 breathers. The African lungfish is a notable 

 example of an animal that may carry on 

 respiration by means of either gills or lungs 

 and is therefore able to live in water too 

 foul for fish having only gills. The animals 

 that live in pools are of particular interest 

 because they must resist desiccation when 

 the pool dries up or perish. 



Marine (salt-water) communities 



Animals that live in salt water have a 

 great deal of space to occupy, since 72 per 

 cent of the earth's surface is covered by the 

 sea. Furthermore, animals live at all depths, 

 and 84 per cent of the ocean is deeper than 

 1000 fathoms (6000 feet), and 7 per cent 

 is deeper than 3000 fathoms (18,000 feet). 

 The salinity of the ocean averages about 3.5 

 per cent, whereas that of fresh water is 

 usually less than one per cent. The number 

 of animals is greatest near the surface, where 

 the light penetrates; here they are subject 

 to tides and currents. Along the shores live 

 large numbers of barnacles, sea anemones, 

 and limpets, which attach themselves to 

 rocks; clams, snails, starfishes, and sea 

 urchins, which cling to rocks or hide in 

 crevices; and many species of worms and 



