210 FISHES 



four are borne singly in the middle plane of the body and 

 are therefore called median fins. The first pair of fins is 

 borne, one on each side of the body, just back of the gill 

 openings. These are the pectoral fins and correspond to 

 the fore Hmbs of a mammal. The second pair, the pelvic 

 fins, is placed a short distance behind the pectoral fins and 

 on the ventral side of the body. These are homologous to 

 the hind limbs of a mammal. There are two dorsal fins 

 borne in a middle line on the back, one just behind the other. 

 There is also a single fin, the anal fin, borne on the ventral 

 side of the body in front of the tail fin. Finally, the tail 

 terminates in a single wide fin, the caudal fM (Fig. 146). 



A fin is simply an expanded fold of the skin with a sup- 

 porting framework of spines, or rays. Some of the rays 

 are segmented and are known as soft rays while others are 

 stiff, unsegmented spines. 



The gills. — On each side of the perch's head is a flapUke 

 organ, the gill cover, or operculum. The posterior margin 



Fig. 147. — Head of fish, with gill cover cut away to show gills. 



of each gill cover is free and in a living fish, a stream of 

 water is constantly flowing out through the opening between 

 the gill cover and the body. Beneath each gill cover are 

 four red comblike gills. Each gill consists of a double row 



