True Fishes 273 



forms such as the sea fans and pens, the sea anemones, 

 sponges, and corals. In the bottom ooze are miUions of 

 small organisms: bacteria, protozoa, worms, molluscs, and 

 crustaceans comparable to the bacteria, protozoa, worms, 

 and insects found in earth on land. 



Over the whole ocean bottom as over the whole land 

 mass hovers an atmosphere containing the vital element 

 of life, oxygen. In the sea the oxygen is dissolved in the 

 water, while in the air it is mixed with various other gases. 

 It has been found that a given amount of air has over 30 

 times as much oxygen as the same quantity of sea water. 

 Further, water is about 800 times as dense as air at the 

 same temperature. 



As a group, fish have adapted to the relatively low con- 

 centration of oxygen in water both by requiring less of 

 this element in respiring as compared with the land verte- 

 brates and by the development of very efficient respiratory 

 organs, the gills. To overcome the impediment to move- 

 ment imposed by the dense water medium, faster-swimming 

 fishes have developed streamlined bodies. 



The physiology and shape of the body, the form of the 

 mouth, teeth, and fins, and other external as well as internal 

 structures have been modified in the various species of 

 fishes for adaptation to a particular type of environment. 

 As a result, these fishes are not distributed throughout the 

 ocean waters but are restricted to a particular area or 

 habitat. 



In most fishes the eggs are fertilized after extrusion and 

 develop and hatch in the water. However, in some species 

 the eggs are fertilized while inside the fish, as for example 

 in the guppi and other "live-bearers," and are carried by 

 the female throughout development. Depending on the 

 species and the size of the fish the number of eggs released 

 may vary from less than a hundred to millions. In the 

 species that protect their eggs in some manner, such as 

 the "live-bearers," nest builders, and mouth breeders, far 

 fewer eggs are produced by each female than in species 

 that broadcast their eggs throughout the water. 



