FISHES 



17 



In the sturgeon and many other primitive fishes the backbone takes an 

 upward turn at the base of the caudal fin and continues in the fin to 

 near its hinder end, so that the greater part of the fin is ventral to the 

 backbone; such a tail is called heterocercal (Fig. 2). In most fishes, 

 however, the caudal fin is homocercal, that is, the backbone extends 

 only to the base of the fin, which projects fan-shaped back of it. The 

 anal fin lies in the midventral plane behind the anus; it is in most 

 cases single, but in some species is divided. 



Fig. 2. — Types of tail fins of fishes: heterocercal tail of sturgeon {upper left-hand figure) ; 

 heterocercal tail of garpike {upper right-hand figure); homocercal tail of pike-perch {lower 

 figure) {from Fishes of Illinois). 



The paired fins are the pectoral fins, which are anterior in position, 

 and the ventral fins, which are posterior. The pectoral fins are imme- 

 diately back of the head, either on the side of the body or on the breast. 

 The ventral fins (Fig. 3) lie on the belly in front of the anus in the more 

 primitive fishes, in which case they are said to be abdominal in position; 

 in the more specialized fishes they are situated nearer the head, and are 

 said to be thoracic in position when they are immediately beneath the 

 pectoral fins and jugular when they are in front of the pectoral fins. 



The fins are stiffened and supported by slender bony rods called 

 the fin-rays (Fig. 4). These are of two kinds, the soft rays, which are 

 jointed and have divided or split terminal ends, and the spiny rays, 

 which are stiff and not jointed and have undivided and sharp-pointed 

 ends. 



