GENERAL OUTLINES OF ICHTHYOLOGY. 27 



other peculiaritieSj and are found in some families of salt- 

 water fish. The Placoids, which lack the hard enamel of 

 the Ganoidsj are scattered over the Rays and Sliarks and 

 one or two other groups. 



Mode of Propulsion. 



The principal organ of motion in fishes is the tail, 

 assisted by the simultaneous action of the fins. Progres- 

 sion 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 vertebrate animals. 



Bones. 



To begin with the spine. The number of vertebrse com- 

 posing the spinal column varies greatly — from 16, for in- 

 stance, in the Sun-fish, (and fewer in other species,) to 162 

 in the Conger Eel, and upwards of 230 in the Electric 

 Eel. The shapes of the vertebrse also vary, though com- 

 monly their centra, or bodies, are more or less narrowed 

 in the middle like an hour-glass. The ends are cup- 

 shaped, and the cups filled with a bag of gelatine enclosed 

 in a strong membrane, which, being united round the 

 rims, forms a very elastic joint. 



The aimexed engraving will show the names and posi- 



these, according to some aiitliorities, the age of fish may be acciirately 

 ascertained— each layer representing one year's growth. 



c 2 



