ECOLOGY AND ZOOGEOGRAPHY 



631 



food only, or for a month or more on green 

 food. Certain species, such as the prong- 

 horned antelopes, jack rabbits, and certain 

 ground squirrels, can obtain all the water 

 they require from green food. 



Complete dr}ing kills most animals, but 

 members of some species can withstand 

 considerable desiccation (drying). This is 

 particularly true of small organisms such as 

 the protozoans, rotifers, and minute crus- 

 taceans, which escape death by encysting or 

 by laying eggs with heavy shells that resist 

 evaporation. The water bears (Tardigrada) 

 are famous for their ability to withstand 

 desiccation for several years. Some animals, 

 such as the lungfish, estivate during dry hot 

 weather; while the earthworm and certain 

 other burrowing animals make their way 

 deeper into the soil where more moisture 

 is present. Too much moisture is also a fre- 

 quent hazard; animals such as the earthworm 

 are flooded out of their habitats; they have 

 been "rained up" out of their burrows and 

 not "rained down" as some people suppose. 



Substratum 



Land. The substratum is another impor- 

 tant ecological factor. On land, the charac- 

 ter of the soil is an important factor in 

 determining the nature of the vegetation 

 and the types of animals that can maintain 

 themselves upon it. Animals find different 

 kinds of soil available for their homes and 

 a place in which to escape from enemies, 

 high temperatures, and desiccation. Count- 

 less animals live in the soil, notably proto- 

 zoans, nematodes, worms, insects, burrowing 

 amphibians, reptiles, and mammals. The 

 type of soil, whether it be sand, loam, or 

 clay, has a great influence on the ability of 

 an animal to live in it. 



Animals are adapted in various ways for 

 life on or in the soil. The limbs of land 

 vertebrates are under the body instead of on 

 the sides, as are the fins on animals from 

 which they evolved; this is necessary in order 

 to sustain the weight of the body in a 



medium (air) with a lower specific gravity 

 than water. The legs of insects are derived 

 from biramous swimming appendages, such 

 as those of crustaceans. Sturdy bodies and 

 enormously developed digging appendages 

 with shovel-like claws have been developed 

 by moles, mole crickets, and the nymphs of 

 cicadas. Burrows must be reinforced to pre- 

 vent cave-ins; the earthworm cements the 

 wall with slime, the tiger beetle with saliva, 

 and the trap-door spiders line their bur- 

 rows with silk. 



Water. Many aquatic animals inhabit the 

 region on or beneath the bottom of water, 

 Neanthes, the sandworm, burrows in the 

 sand or mud of the seashore at tide level; 

 the long-neck clam lives in the same type of 

 habitat with its long neck reaching through 

 the sand to the surface of the bottom. Corals 

 and acorn barnacles live on various solid 

 objects such as wood, shells, and corals, 

 since they cannot attach themselves to mud 

 or sand. The edible mussel Mytilis edulis 

 attaches itself to rocks by means of a stringy 

 secretion called a byssus. Oysters are not 

 carried away by currents because they are 

 attached to the substratum by one valve. 



Types of environment 



For purposes of convenience, environ- 

 ments may be classified as fresh-water, ma- 

 rine, terrestrial, and symbiotic. Each of 

 these in turn may be broken down further 

 into subdivisions such as lakes and streams 

 as types of fresh-water environments. Fur- 

 thermore, each environmental type has its 

 own limiting factors. 



Fresh-water environment 



Fresh-water environments mav first be 

 subdivided into standing-water and running- 

 water habitats. Examples of the former are 

 lakes, ponds, and bogs, and of the latter, 

 springs, streams, and rivers. It should be 

 emphasized that there are no sharp boun- 

 daries among t)pes, and that each may be 

 in the process of becoming another, as when 



