292 



THE RISE OF ANIMAL LIFE 



V. -i.'^, f^iiOX. -M 



Fig. 13-15. The long-nosed gar {Lepidosteus osseus), a 

 vicious carnivore which feeds on other fish. This fish 

 inhabits the Great Lakes and most of the streams 

 of the Mississippi Valley. 



gills to four pairs and have covered them 

 with a thin bony cover, the opercukim 

 (Fig. 13-17). Their bodies are covered with 

 large overlapping scales arranged like the 

 shingles on a house. The fins, while in gen- 

 eral of the prototype, are highly variable 

 both in position and in size among the 

 different groups of fish. There is nothing 

 strikingly different about their internal 

 anatomy with the possible exception of the 

 swim, or air bladder, which occupies a large 

 part of the body cavity in many species 

 (Fig. 13-17). We shall discuss its origin 

 later. Its function is to regulate the buoy- 

 ancy of the body. As the fish moves to dif- 

 ferent depths tlie gases (COo, N, and Oo) 

 increase or decrease in the swim bladder 

 automatically, adjusting the specific grav- 

 ity of the fish to the corresponding depth, 

 but if a fish is suddenly pulled from great 

 depths to the surface the expanding blad- 

 der may force the stomach out of the 

 mouth. 



Most present-day fish possess bony skele- 

 tons, a very ancient character. Finally, com- 

 pared to the ancient bony fish, present-day 

 forms show a tendency toward reduction of 

 the massive head bones and toward a re- 

 duction in the number of bones generally 

 through fusion. There are never more, and 

 frequently fewer, bones in later fishes. 



The retention of hard internal skeletons 

 made it possible for the fish to begin their 

 long migration onto land, to a new type of 



life outside of water. Although this move- 

 ment began with the fish, it was not com- 

 pletely accomplished until the advent of 

 the reptiles, many millions of years later. 

 The hard bones made it possible for ap- 

 pendages to become sufficiently strong to 

 support a body in the air, a feat which the 

 degenerate cartilaginous skeleton of the 

 sharks could never have accomplished. 



Among the ancient fish there were some 

 that had a fleshy portion, or "lobe," which 

 extended some distance out into the fin. 

 This contained certain skeletal elements 

 that have been found to correspond di- 

 rectly with similar bony elements in the 

 appendages of true land forms, even to the 

 appendages of man himself. Descendants 

 of these fish undoubtedly were able to 

 migrate onto land at a later time to give 

 rise to the great array of land vertebrates. 

 "Lobe-finned" fish were long thought to be 



Fig. 13-16. The sea horse (Hippocampus kuda) swims in 

 a vertical position by means of its dorsal fin. Note 

 its prehensile tail, used to cling to vegetation. The 

 male has a pouch under the tail where the eggs are 

 brooded until they hatch. 



