160 ANIMAL FORMS 



girdle is placed behind the skull, leaving room for a distinct 

 neck ; strong bars of cartilage bear the gills ; others form jaws 

 to carry the teeth ; and a complex skull protects the brain 

 and sense-organs, which are of a relatively high state of devel- 

 opment. Throughout life the skeleton is of cartilage, with 

 perhaps here and there a little bone where greater strength 

 is required. Besides these, there are numerous minor 

 characters which the student will readily find for himself. 



The sharks and skates or rays live chiefly in the sea, 

 and some reach an enormous size, the largest of all fishes. 

 Some are very ferocious and voracious ; others are very mild 

 and weak, and the development of teeth is in direct pro- 

 portion to their voracity of habit. In earlier geologic times 

 there were many more species of them than now exist. 



153. The lung-fishes. The lung-fishes (Dipnoi) are pe- 

 culiar forms living in some of the rivers of Australia and 

 the tropical regions of Africa and South America. In these 

 the air-bladder is developed as a perfect lung. During the 

 wet season they breathe like other fishes by means of gills, 

 but as the rivers dry up they burrow into the wet mud and 

 breathe by means of lungs which are spongy sacs of which 

 the air-bladder of other fishes is a degenerate representative. 

 As we shall see, they resemble in this respect the tadpoles 

 and some adult Amphibia (frogs and salamanders). The 

 paired fins are also peculiar in structure, having an elongate 

 jointed axis, with a fringe of rays along its length, a struc- 

 ture almost as much like that of the limbs of a frog as that 

 of a fish's fin. In fact the Dipnoi must be regarded as an 

 ancestral type, an ally of the generalized form from which 

 Amphibia and bony fishes have descended. Only four liv- 

 ing species of dipnoans are known, but great numbers of 

 fossil species are found in the rocks. 



154. The bony fishes (Teleostei). The bony fishes, or 

 Teleosts, are distinguished by the bony skeleton, the sym- 

 metrical tail, and by the development of the air-bladder as 

 a more or less completely closed sac, useless in respiration. 



