THE VERTEBRATE ANIMALS 



279 



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 tin- heart again. During its course some of the blood 

 passes through the kidneys and is there relieved of part of its nitroge- 

 nous waste. (See Chapter XXVII.) 



( in -illation of blood in the body of the fish d - 



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 other vertebrate 

 animals, t he brain and spinal cord of the fish are 

 partially inclosed in a series of bony structures 

 railed r r/> l>nr. The central nervous system con- 

 of a,brain, with nerves leading to the organs 

 of ri'^ht. tar 1. -.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 

 usations. Cells of the central nervous sys- 

 tem, in turn, send out messages which result in 

 the movement of muscles. 



Skeleton. In the vertebrates, of which the 

 bony fish is an example, the skeleton is under 

 the skin, and is hence called an endoskelrlun. 

 It consists of a bony framework, the vertebral 

 column, and certain attached bones, the ribs, 

 with other spiny hones to which the unpaired fins 



are attached. The paired fins are attached to the spinal column by two 

 collections of hones, known respectively as the pectoral and pelvic girdles. 

 The bones serve in the fish for the attachment of powerful muscles, by 

 means of which locomotion is accomplished. In most fishes, the exo- 

 skeleton, too, is well developed, modifications appearing from scales to 

 complete armor. 



Plan of circulation in fishes : 

 a, auricle ; b, ventricle ; c, 

 branchial artery; e, l>ran- 

 fhial veins, bringing blood 

 from the gills, d, and unit- 

 ing in the aorta, /; g, vena 

 cava, returning blood to 

 heart 



XXXV (Optional}. A study of some of the relations 

 of fishes to tJieir food supply. (Laboratory Manual, Prob. 



Food of Fishes. We have already seen that in a balanced 

 aquarium the balance of food was preserved by the plants, which 

 furnished food for the tiny animals or were eaten by larger ones, 

 for example, snails or fish. The smaller animals in turn became 



