THE DOG-FISH 197 



I. EXTEENAL CHAEACTEBS. 



Lay the dog-fish on a board, and determine the following 

 joints. 



A. General Appearance. 



The body is greatly elongated : it is broadest about the level 

 of the pectoral fins, in front of which it is flattened dorso- 

 ventrally, ending in the blunt rounded snout. The hinder 

 part of the body is compressed laterally, and tapers gradually 

 to the end of the long slender tail, the hindmost three or 

 four inches of which are bent up at an angle. 



The general colour is grey ; paler on the ventral surface, 

 and marked on the dorsal and lateral surfaces with dark brown 

 spots. The entire surface is covered with small scales, and 

 along each side of the head and body there runs a slight 

 longitudinal groove, the lateral line, 



B. The Fins. 



The fins are flattened outgrowths arising from various parts 

 of the body. Each is supported by an internal cartilaginous 

 skeleton, and is usually triangular in shape, the base or 

 attached border being very commonly firee at its posterior end. 



1. Median fins. 



a. The dorsal fins are two in number, the anterior 



and larger one being placed about the middle of 

 the length of the body, and the posterior one a 

 short distance further back. 



b. The caudal fin forms a vertical fringe around the 



posterior four or five inches of the tail. Its dorsal 

 portion is narrow, its hinder end truncated, and 

 its ventral portion partially subdivided into a 

 small posterior and a large anterior lobe. 



c. The ventral fin is placed opposite the interval be- 



tween the two dorsal fins. 



2. Paired fins, representing the limbs of higher vertebrates. 



a. The pectoral fins are the largest of all the fins, and 

 project horizontally outwards firom the sides of 



