362 GENERAL ZOOLOGY 



enormous pressures, low temperature, and varying degrees of 

 darkness. Fishes taken from a depth of 1 mile show no visible 

 structural adaptation to temperature, which is not surprising 

 since fishes are poikilothermous and even shallow-water forms 

 can withstand freezing temperatures. As to pressure, some 

 withstand the transition from a depth of 1 mile to the surface 

 without disintegrating. Adaptations to darkness, however, 

 can be recognized and these take two general forms: (1) the 

 development of long attenuated, highly sensitive tentacles, or 

 (2) the development of luminescent organs. The most abundant 

 forms of deep-sea life are shrimps, prawns, and related crus- 

 taceans, particularly copepods. Many are luminescent. 



Fresh-water Animals. — Fresh-water communities are classi- 

 fied according to the form of the body of fresh water, viz., pond, 

 lake, or stream. Fresh-water environment differs from the 

 ocean in a greater variability in temperature, lower salt content, 

 and greater liability to contamination arising from erosion. 

 Many of the common marine forms such as echinoderms, bar- 

 nacles, and oysters are not found in fresh water. There are 

 fresh-water sponges, some coelenterates (Hydrozoa), and a few 

 species of polychaete worms. The common fresh-water forms 

 are fishes, amphibians, oligochaete worms, leeches, arthropods, 

 molluscs, and protozoa. These, as in the case of marine forms, 

 may be divided into swimmers, floaters, and bottom dwellers. 



Land Communities. — Terrestrial animals may be divided into 

 two general classes: (1) those that live in the soil or in caves, and 

 (2) those that live above ground. 



Subterranean Animals. — The habitats of subterranean animals 

 resemble aquatic habitats in that sudden changes in temperature 

 and moisture do not occur. Oxygen is less abundant than on the 

 surface. Common soil-dwelling forms include protozoans, earth- 

 worms, arthropods, amphibians, reptiles, rodents, and moles. 

 Darwin found earthworms at a depth of 6 ft.; the burrows of 

 prairie dogs may reach a depth of 8 or 9 ft.; but the majority of 

 soil animals are found nearer the surface. Temperature and 

 moisture are remarkably constant throughout the year in caves. 

 The common terrestrial cave dwellers include spiders, insects, 

 and salamanders. Owing to the uniformity of temperature and 

 moisture in caves, hibernation is rare among permanent cave 

 dwellers. Cave animals in general have degenerated eyes and 

 pigment, and highly developed tactile sense organs. Since caves 



