166 OUTLINES OF ICHTHYOLOGY. 



scope, each line denoting a successive stage of 

 growth by the addition of a fresh layer the 

 smallest, or top layer, having been first produced 

 and the others added underneath. 



In the Cycloids these circular lines are smooth ; 

 whilst in the Ctenoids they are furnished with 

 minute spiny points, or teeth, which, however, 

 usually wear off on the disc of the scale and 

 remain only on the posterior margin. The Ganoids 

 have a hard, shining, enamelled surface and other 

 peculiarities, and are found in some families of 

 salt-water fish. 



The Placoids, which lack the hard enamel of 

 the Ganoids, are scattered over the Rays and 

 Sharks and one or two other groups. 



Modes of Propulsion. The principal organ of 

 motion in fishes is the tail, assisted by the simul- 

 taneous action of the fins. Progression is effected 

 by the tail striking obliquely right and left against 

 the water ; for which purpose the spine is con- 

 structed to bend sideways, or laterally, instead of 

 upwards and downwards as in most other verte- 

 brate animals. 



Bones. To give in detail an account of the 

 bones of the different kinds of fish would occupy a 

 volume. There are twenty-five in the head of the 

 common Perch. Even in the case of the spine 

 the number of vertebrae composing the spinal 



