ANIMALS AND THEIR ENVIRONMENT 303 



phores or the smm bladders of fishes. The air-breathing whales, seals, 

 and turtles are floated by their lungs. Animals having no floating 

 mechanism must actively swim, if they are to avoid settling on the 

 bottom; among vertebrate animals only the powerful sharks and a few 

 bony fishes without swim bladders are capable of the incessant exertion 

 necessary to prevent sinking. 



Fewer groups of animals are represented in the open ocean than on 

 the bottom. There are no sponges, no brachiopods, no bryozoans. 

 Hydroids and other sessile coelenterates are missing, and there are few 

 echinoderms (except larval stages), few worms, few clams and snails. 

 The bulk of the sudmming animals (90 per cent) are copepod crustaceans. 



Ocean currents either come to an end by spreading out and slowing 

 down to zero (Gulf Stream), or they form a closed circuit. The meeting 

 of warm (Gulf Stream) and cold (Labrador) currents of the terminating 

 type causes great mortality of organisms, and adds to the organic detritus 

 used by bottom forms. The larger closed circuits take a year (North 

 Atlantic) or two (South Atlantic) to bring their organisms back to any 

 starting point. In the eddy enclosed by such a circuit there are often 

 accumulations of seaweeds (Sargassum), perhaps torn loose by hurri- 

 canes, and in these weeds is a characteristic animal community (certain 

 fishes, crabs, shrimps, hydroids). An eddy of this sort is known as a 

 Sargasso Sea, and each of the great oceans except the polar ones has one 

 or more of them. 



Coral Reefs. — Coral reefs are built up from the bottom in tropical 

 seas by two different groups of coelenterates, aided by a number of 

 other lime-depositing organisms. They may be developed along the 

 shore line (fringing reefs), out some distance leaving lagoons between 

 them and the shore (barrier reefs), or at any distance from the mainland 

 in the form of a ring or horseshoe (atolls). Various theories to account 

 for reefs, beginning with those of Charles Darwin, have been proposed. 

 The theories postulate the type of habitat in which corals will grow, the 

 possible rise or fall of the land, differences in exposure to the open ocean, 

 and long-time changes in the water level of oceans; but none of the 

 theories is entirely satisfactory. About these reefs there are character- 

 istic communities of other kinds of animals. 



Geographic Areas in the Oceans. — Swimming and floating organisms 

 requiring moderate or relatively high temperatures are limited to their 

 respective oceans, being cut off from other oceans by the continents 

 which the}^ cannot pass around. Yet the animals of the Atlantic have 

 a considerable likeness to those of the Indian and Pacific Oceans. In 

 the coi^epods, even some of the species are identical. This likeness 

 presumably resulted from the connection between the two areas across 

 Central America in Tertiary time. 



