72 PRACTICAL ZOOLOGY. 



Measure along the base of each fin; this is the 

 length of the fin ; extend the fin fully, and measure 

 the length of its longest ray; this is the height of 

 the fin. Compare the length and height of the fin. 

 In some fishes the dorsal fin is single ; in others it is 

 divided, forming two or more dorsal fins. 



The tail-fin is the caudal fin ; when this fin is sym- 

 metrical, or nearly so, the backbone apparently ending 

 at the centre of its base, the tail is said to be homo- 

 cereal, as in most fishes; if the backbone extends 

 into the upper lobe of the fin, making this lobe larger, 

 as in the sturgeon, the tail is called heterocercal. 



The fin in front of and below the caudal, is the 

 anal (being just back of the external opening of the 

 intestine, the anus) ; compare this fin with the dorsal. 



The fins above named, being in the middle line, 

 are called median, or vertical fins. 



The remaining fins are called paired fins; the pair 

 back of the head are the pectoral fins, and are con- 

 sidered as representing the forelimbs of the higher 

 animals ; the lower pair (usually farther back) are 

 the ventral fins, representing the hind limbs of 

 higher animals. Take the ventral fins between the 

 thumb and finger to feel their bony support; rest 

 the fish on its back, and press the thumb and fore- 

 finger of the other hand on the bony structures at 

 the bases of the pectoral fins; move the ventrals 

 about to determine, as far as possible by feeling, the 

 relations between the bones supporting the two pairs 

 of fins. 



3. Open the mouth of the fish by pulling its lower jaw 

 down as far as possible; the bone which forms the 



