274 



ZOOLOGY 



persists throughout h*fe, but which in other fish becomes closed, a thin, 

 fibrous cord taking its place. The swim bladder aids in giving the fish such 

 a volume that it displaces a weight of water about equal to its own weight. 

 The walls of the organ are richly supplied with blood vessels, and it thus 

 undoubtedly serves as an organ for taking in oxygen. It has been compared 

 to a lung of the higher vertebrates. 



Circulation of the Blood. — In the vertebrate animals the blood is said to 

 circulate in the body, because it passes through a more or less closed system 

 of tubes in its course around the body. In the fishes the heart is a two- 

 chambered muscular organ, a thin-walled auricle leading 

 into a thick-walled muscular ventricle. The blood is 

 pumped from the heart to the gills ; there it loses some of 

 its carbon dioxide ; it then passes on to other parts of the 

 body, eventually breaking up into very tiny tubes called 

 capillaries. From the capillaries the blood returns, in 

 tubes of gradually increasing diameter, toward the heart 

 again. During its course some of the blood passes through 

 the kidneys and is there relieved of part of its nitrog- 

 enous waste. (See Chap. XXX.) 



Circulation of blood in the body of the fish is rather slow. 

 The temperature of the blood being nearly that of the 

 surrounding media in which the fish lives, the animal has 

 incorrectly been given the term cold-blooded. 



Nervous System. — As in all vertebrate animals, the 

 brain and spinal cord of the fish are inclosed in a series 

 of bony structures called vertebrae. The central nervous 

 system consists of a brain, with nerves leading to the 

 organs of sight, taste, smell, the ear, and to such parts of 

 the body as possess the sense of touch ; a spinal cord ; and 

 spinal nerves. Nerve cells located near the outside of the 

 body send in messages to the central system, which are 

 there received as sensations. Cells of the central nervous 

 system in turn send out messages which result in the 

 movement of muscles. 



We have already learned something of the senses of a 

 fish. That of hearing is poorly developed, the ear being 

 largely an organ of balancing. Along each side of almost every species of 

 fish is found a line, which consists of a series of tiny pits each connected with 

 its neighbor.^ This lateral line, as it is called, is believed to have to do 

 with the sense of touch. 



Skeleton. — In the vertebrates, of which the bonj'' fish is an example, 

 the skeleton is under the skin and is hence called an endoskeleton. It con- 

 sists of a bony framework, the vertebral column, and certain attached 



1 This line is plainly visible in some fishes because of its dark color. 



Plan of circulation 

 in fishes; a, auri- 

 cle; b, ventricle; 

 c, branchial 

 artery; e, bran- 

 chial veins, 

 bringing blood 

 from the gills, d, 

 and uniting in 

 the aorta, /; g, 

 vena cava, re- 

 turning blood 

 to heart. 



