14 Vertebrata. 



exoskeleton in the form of median fins. Of these one 

 extends along the upper or dorsal edge, and is named 

 F the dorsal fin, consisting of a suc- 



cession of soft and branched or 

 spiny and hard fin-rays connected 

 by membrane. The other is pre- 

 sent on the under or ventral side 

 of the body behind the terminal 

 opening of the intestine ; this is 

 called the anal fin. These median 

 fins, though apparently single and 



Ctenoid scale. > . * 



central, are in reality composed of 

 two lateral layers placed in close apposition. 



11. Sense-organs of the lateral line. Along the 

 line of greatest convexity of each side of the body 

 of a fish there is a lateral line, extending from be- 

 hind the eye to the side of the tail. This con- 

 sists of a row of scales, each pierced by a minute 

 tube leading into a small simple or branched sac 

 filled with a gelatinous material, in which the extremity 

 of a nerve is embedded. These are organs of sense, 

 and are probably capable of being impressed by several 

 forms of vibration. 



1 2. Backbone and tail. The vertebral column of 

 fishes usually consists of a chain of biconcave vertebral 

 bodies, bearing on their upper surfaces neural arches 

 which are surmounted by long neural spines. On the 

 under side the vertebrae bear ribs towards the front, 

 and V-shaped bones towards the hinder part of the 

 body. The hindmost of the tail vertebrae may either 

 gradually diminish to a point, as in the African mud- 

 fish (fig. 14), or they may undergo modification, being re- 

 placed by a rodlike bone which turns sharply upwards, 



