Chap. 33 VERTEBRATES LOWER CHORDATES AND FISHES 673 



not depend upon paired fins to pull or push greatly, or to carry weight. Its faults 

 are in its use by other animals. It started a pattern in evolution and millions 

 of years later, the human shoulder blades (scapulas) and collar bones 

 (clavicles) slip about, or break too easily and often. 



Muscle. The important muscles of a fish are arranged on each side of the 

 body in the V-shaped blocks that are familiar on the dinner plate. The body 

 muscles are wholly responsible for the alternate swimming movements, con- 

 traction and bending to the right and to the left, repeated over and over. In 

 fishes, each group of muscles acts locally within a small area. The muscles that 

 control the fins are concerned with piloting, the tactics of locomotion. 



Fishes can cut straight down or straight up through the water; they can 

 hang motionless, as if suspended in it; and they can maintain themselves facing 

 into a current with the merest flicker of their pectoral fins. Most of this is due to 

 the lift of water, to its density which makes it a support. A fish that in air 

 would weigh about 20 pounds is estimated to have a pressure or equivalent 

 weight of about one pound in salt water. 



Digestive System. The majority of fishes have a large mouth and numerous 

 teeth on the jaws, on the roof of the mouth, the pharynx, and on the almost 

 immovable tongue (Fig. 33.11). All of these are used for gripping and strain- 

 ing food; fishes do not chew and they have no salivary glands. Most of them 

 are carnivorous and their prey is swallowed undamaged until it reaches the 

 stomach. There are great variations from the typical teeth. Some vegetarians 

 like the carp have no teeth in their mouths; in the parrot fish the front teeth 

 are fused into a beak with which it nibbles seaweeds; but both of these fishes 

 grind their food with their pharyngeal teeth. 



The sac-shaped stomach is highly extensible and provided with gastric juice 

 that dissolves bones and shells. As in other animals, the intestines of carnivo- 



LOBES 



tEREBRUM 



EYE 

 SOCKET 



NOSTRILS 



TONGUE 



PHARYNX 



Fig. 33.11. Main internal structures of a bony fish. (Courtesy, MacDougall and 

 Hegner: Biology. New York, McGraw-Hill Book Co., 1943.) 



