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. Jn 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 oe completely closed sac, useless in respiration. 
