THE FISH 237 



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 other vertebrate animals, the brain and 

 spinal cord of the fish are partially inclosed in bone. The central nervous 

 system consists of a brain, with nerves connecting the organs of sight, 

 taste, smell, and hearing, and such parts of the body as possess the sense of 

 touch ; a spinal cord ; and spinal nerves. Nerve cells located near the out- 

 side 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. 



Skeleton. In the vertebrates, of which the bony fish is an example, 

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

 consists of a bony framework, the vertebral column which protects the 

 spinal cord and certain attached bones, the ribs, with other spiny bones to 

 which the unpaired fins are attached. The paired fins are attached to the 

 spinal column by two collections of bones, known respectively as the 

 pectoral and pelvic girdles. The bones in the main skeleton serve in the 

 fish for the attachment of powerful muscles, by means of which locomo- 

 tion is accomplished. In most fishes, the exoskeleton, too, is well developed, 

 consisting usually of scales, but sometimes of bony plates. 



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 

 food of larger ones. The nitrogen balance was maintained through 

 the excretions of the animals and their death and decay. 



The marine world is a great balanced aquarium. The upper 

 layer of water is crowded with all kinds of little organisms, both 

 plant and animal. Some of these are microscopic in size ; others, 

 as the tiny crustaceans, are visible to the eye. On these little 

 organisms some fish feed entirely, others in part. Such are the 

 menhaden 1 (bony, bunker, mossbunker of our coast), the shad, 

 and others. Other fishes are bottom feeders, as the blackfish and 



1 It has been discovered by Professor Mead of Brown University that the in- 

 crease in starfish along certain parts of the New England coast was in part due 

 to overfishing of menhaden, which at certain times in the year feed almost entirely 

 on the young starfish. 



