■210 



FISHES 



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

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

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

 openings. These are the jKctoraL fins and eorres[>ond to 

 the fore limbs of a mammal. Tlie second pair, the jielric 

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

 on the ventral side of the body. The.se are homologous to 

 the hind limbs of a manmial. There ai'c two dorsal fins 

 borne in a middle line on the l)ack, one just behind the other. 

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

 side of the body in fi-(.iut of the tail fin. Finally, the tail 

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



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

 porting framewoik of spines, or rays. tSome of the rays 

 are segmented and ai-e known as soft rays while others are 

 stih', unsegmented spines. 



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

 organ, the g'dl carer, or operculum. The posterior margin 



of each gill covei' is fi'ee and in a li\ing hsh, a stream of 

 water is constantly flowing out through the opening between 

 the gill cover and the bod>'. IVneath each gill cover are 

 four red comljlike gills. Ivich gill consists of a. double row 



