122 A COURSE ON ZOOLOGY. 



organization, but its parts are naturally modified to suit 

 the special conditions of the medium in which fishes live. 

 The bones that form the head are very numerous, but 

 those of the limbs are never present except in a rudi- 

 mentary condition, and the posterior limbs are often ab- 

 sent. In some species the ribs also are missing, while in 

 compensation the existence of the median fins necessitates 

 the presence of a large number of supplementary carti- 

 lages or bones for their support. On the other hand, 

 though the bones of most species of fish have a chemical 

 composition analogous to that of the bones of mammals, 

 birds, and reptiles, yet in a large number of species the 

 mineral matter is almost entirely absent, and the skeleton 

 is almost wholly cartilaginous. This difference in com- 

 position coincides with other very considerable organic 

 differences, and it has been deemed necessary to classify 

 fishes in two distinct groups, bony fishes and cartilag- 

 inous fishes. Among the bony fishes are the salmon, 

 the carp, perch, eels, etc. As examples of cartilaginous 

 fishes may be named the shark, ray, sturgeon, and lam- 

 prey. 



The brain, like that of batrachians, is formed of 

 linearly-arranged parts. Two olfactory lobes, often 

 largely developed, are in front of the hemispheres, while 

 behind them the optic thalami, followed by the cerebellum, 

 are easily seen. 



In general the organs of sense are quite well developed. 

 The crystalline lens is spherical, the eye but slightly 

 convex in front, and without eyelids. 



The ear comprises only the internal parts : in it there 

 is a sac containing a calcareous stone, often quite large, 

 which corresponds to a calcareous powder found in the 

 ears of mammals. 



