FISHES 



245 



to be used as a digestive gland. Many fishes have outgrowths, like a series 

 of pockets, from the intestine, called the pyloric cceca, which aid in digestion. 

 The intestine ends at the vent, which is usually located on the under side of 

 the fish, immediately in front of the anal fin. 



An organ of unusual significance, called the swim bladder, occupies the region 

 just dorsal to the food tube. The size of the swim bladder can be changed by 

 contraction or expansion of its walls. The fish uses this organ to make changes 

 in the space it occupies, so that the 

 water displaced will equal its own 

 weight. Thus the weight of the fish 

 is supported no matter at what depth 

 it wishes to remain. In some fishes 

 (the dipnoi, page 246) it is used as a 

 lung. 



In the fishes the heart is a 

 two-chambered muscular organ, a 

 thin-walled au'ride, the receiving 

 chamber, leading into a thick-walled 

 muscular ven'tride from which the 



blood is forced out. The blood is pumped from the heart to the gills ; there 

 it loses carbon dioxide and receives oxygen ; it then passes on to other parts 

 of the body until it reaches very tiny tubes called cap'illaries. From the 

 capillaries the blood returns, in tubes of gradually increasing diameter, toward 

 the heart again. (See figure, page 171.) Blood supplies the body cells with 

 food and oxygen and carries away wastes. During its course around the 

 body some of the blood passes through the kidneys and is there relieved 

 of nitrogenous waste. 



As in all vertebrate animals, the central nervous system of the fish consists 

 of a brain and a spinal cord; and there are spinal nerves and other nerves. 



The true skeleton, or endoskeleton, is under the skin, as in all vertebrates. I* 

 consists of a skull, the vertebral column, which protects the spinal cord, the ribs, 

 and other spiny bones to which the unpaired fins are attached. In most fishes 

 the exoskeleton, too, is well developed, consisting usually of scales, but some- 

 times of bony plates. The skin secretes mucus, a slimy substance which helps 

 the fish to go through the water easily. 



Most fishes lay very many eggs. There are about 15,000 species. 



A fish opened to show H, the heart; 

 G, the gills ; L, the liver ; S, the stomach ; 

 I, the intestine ; O, the ovary ; K, the 

 kidney ; B, the swim bladder. 



Elasmobranch Ganoid 



Teleost 



Fishes. 



H. NEW CIV. BIOL. — 17 



Dipnoan 



