646 



COLLEGE ZOOLOCY 



clams which burrow into the sand or mud. 



The animals of the open sea are called 

 pelagic. As in lakes, they may be divided 

 into plankton that float or swim feebly, and 

 nekton that swim about freely. Among the 

 plankton (Fig. 452) are Protozoa, especially 

 the foraminiferans, radiolarians, and dino- 

 flagellates; the medusas; the Portuguese 

 man-of-war; ctenophores; small crustaceans; 

 and pteropod mollusks. The nekton in- 

 clude fish, dolphins, birds, whales, etc. 



Deep-sea animals are particularly inter- 

 esting since they must be able to withstand 

 unusual environmental conditions. The pres- 

 sure is increased to several hundred atmos- 

 pheres, and when a fish is drawn to the 

 surface, the change in external pressure re- 

 sults in swelling of the body, bulging of the 

 eyes, and protrusion of the stomach from 

 the mouth. This is not true of all species 

 because some have special adaptations that 

 make toleration of considerable pressure 

 change possible. Light penetrates to a depth 

 of about 500 fathoms (3000 feet) in clear 

 water; hence animals living below that 

 depth must move about in total darkness or 

 furnish their own luminescence. Many have 

 no eyes. Below a depth of 600 feet only 

 slight variations in temperature occur, the 

 mean being about 2.5° C. Deep-sea animals 

 may feed on each other, but they depend 

 for their primary source of food on plants 

 and animals that die and sink down from 

 above, that is, they are principally scaven- 

 gers. Most of the deep-sea bottom is covered 

 with globigerina and radiolarian ooze. Abys- 

 sal animals that live below 2000 fathoms 

 (12,000 feet) include fish, protozoans, 

 worms, mollusks, sea cucumbers, and star- 

 fishes. The color of these abyssal animals 

 is monotonous, usually dark brown or red. 



Terrestrial communities 



Terrestrial animals may live on the sur- 

 face of the ground, under the surface, or 

 largely in the air. Most terrestrial animals, 

 such as Protozoa, insects and their larvae, 

 nematodes, and moles live within 5 or 6 

 inches of the surface. Others such as rodents 



(gophers and prairie dogs), earthworms, 

 and ants may burrow much deeper. Adapta- 

 tions for a burrowing type of life have al- 

 ready been mentioned. The character of the 

 soil and the amount of water it contains 

 largely determine the types of animals that 

 live in subterranean communities; for ex- 

 ample, the earthworms need moisture. The 

 principal kinds of soil are clay, and sand. 

 Comparatively few species of animals live 

 in pure clay soils; they include insect larvae, 

 isopods, and prairie dogs. Clay soil rich in 

 humus is usually well populated since it 

 contains an abundance of food and is easily 

 penetrated by burrowing. Earthworms ac- 

 tually eat their way through humus. Bur- 

 rows in sandy soil cave in unless the walls 

 are treated; hence fewer species live in this 

 type of soil. Soils that are too rocky are 

 unfit for burrowing, but many animals find 

 the crevices between them a satisfactory 

 hiding place. 



Surface communities 



Surface communities also depend largely 

 on the character of the underlying soil, but 

 even more so on the type of vegetation 

 present. Thus ecologists recognize animal 

 communities in coniferous forests and de- 

 ciduous forests (Fig. 453), among shrubs 

 and low plants, and on deserts. Many sur- 

 face animals such as salamanders and slugs 

 require moisture and hence are to be found 

 only on wet soil or under logs and stones. 

 Animals that move about quickly, like 

 ground beetles, ants, rabbits, deer, and 

 wolves, possess legs fitted for running or 

 jumping. Representatives of some species 

 that live in trees are adapted for boring, 

 climbing, or clinging; these include boring 

 beetles, climbing monkeys, squirrels, and 

 clinging tree frogs with sucking disks on 

 their toes. 



Shrubs are inhabited by insects such as 

 walkingsticks, beetles, and plant lice; by 

 spiders, snails, and by representatives of 

 many species of birds, including catbirds, 

 brown thrashers, and field sparrows. Areas 

 that are covered with low plants, such as 



