86 INTRODUCTION TO ZOOLOGY. 



the order Ganoidei, and the Australian, African, and 

 South American lung fishes belong to the order Dipnoi. 

 The Selachians have a skeleton that is cartilaginous 

 throughout, while the skeleton of the Ganoids and of 

 the Dipnoi is partly cartilaginous and partly bony. 



The tail is the principal organ of locomotion. The 

 other fins serve mainly to balance the fish, keeping it 

 upright and guiding it in its course through the water. 

 The pectoral and ventral fins are homologous to the 

 fore and hind limbs of a dog or other mammal. 

 Usually the pectoral fins are in front of the ventral 

 fins, but this is not always so. When the ventrals 

 are anterior to the pectorals, their ventral nature can 

 be recognized by their being closer together and 

 nearer the median line of the fish than the pec- 

 torals. 



The vertebrae are concave at both ends. The spaces 

 between the ends are filled with a cartilaginous sub- 

 stance which represents the notochord. The noto- 

 chord is a cartilaginous rod which is found in the 

 early life of every vertebrate, occupying the place of 

 the backbone. In the sturgeon, and in other fishes 

 with cartilaginous skeletons, the notochord persists 

 throughout life ; but in the bony fishes generally, as 

 well as in all other vertebrates, bony vertebrae take 

 the place of the notochord. 



Usually, the tail fin is two-lobed and the lobes are of 

 equal size. The spinal column ends at tlie place in 

 which the tail fin begins. Such a tail is called 

 homocercal. But in the sharks, and in some other 

 fishes, the backbone seems to run out into one of the 

 lobes of the tail, making that lobe much larger than 

 the other. Such a tail is called heterocercal. 



The fish breathes by gills, as the crawfish does. 

 The water, however, goes over the gills from the front, 

 instead of from behind. It enters the mouth when 

 the mouth is open, and the closing of the mouth forces 



