58 NORTHERN FISHES 



when fishes had more gills than they now possess. Such a gill, known as 

 the opercular gill, is found on the lower inner surface of the opercle of 

 some fishes, such as the gar and the sturgeon. This type of gill has the 

 same blood circulation as a normal gill, receiving venous, or nonaerated, 

 blood. The pscudobranchia is a rudimentary gill found on the upper 

 inner surface of the opercle of certain fishes. It does not have normal 

 gill circulation but receives only arterial, or aerated, blood. It appears 

 as a small patch of red filaments. 



The fifth gill-arch is located next to the shoulder girdle and bears no 

 functional gills. The bones of the upper part of the fourth gill-arch and 

 those of the lower part of the fifth arch are modified and bear teeth, 

 those of the fourth gill-arch being known as the upper pharyngeals and 

 those of the fifth as the loiver pharyngeals. In the perch these pharyn- 

 geal teeth are very numerous and fine and are set in several masses, 

 usually two masses in the upper and two in the lower. It is often neces- 

 sary to know the nature of the lower pharyngeal bones and teeth for 

 identification, especially those of the suckers, minnows, and sunfishes. 

 In the suckers these teeth are arranged in a single row on each side, 

 each row containing numerous teeth, which in some species are all 

 about the same size; in others the lower teeth are very large, gradu- 

 ating to smaller ones as they proceed upward. In the minnows they 

 maj' be in either one or two rows, but in any case the outer row con- 

 tains the largest teeth, of which there are seldom more than five. The 

 teeth in this row may have developed grinding surfaces and in some 

 species they may be hooked. 



The fins of the fish are of two types, the paired fins and the unpaired, 

 or median, ones. The paired fins consist of the pectoral fins and the 

 ventral, or pelvic, fins, which are respectively homologous to the front 

 and hind legs of land animals. The pectoral, or shoulder, fins are located 

 just behind the gill-arch. The ventral fins may be located just anterior 

 to the anus, or vent, in which position they are termed abdominal. Fre- 

 quently they are located anteriorly in close proximity to, but slightly 

 behind, the pectoral fins, in which case they are called thoracic. When 

 they are anterior to the pectoral fins, they are known as jugular. 



The unpaired fins are always median in position and consist of a 

 dorsal fin on the back, a caudal, or tail, fin, and an anal fin located under 

 the tail and just behind the vent. Frequently the dorsal fin is divided 

 into a front, or anterior, part and a hind, or posterior, part. The length 

 of the dorsal or the anal fin is the distance along its base, and the 

 height is the length of its longest ray. The tail, or caudal, fin may be of 

 either the heterocercal or the homocercal type (Diagram 3) . 



The heterocercal type is found only in primitive fishes, such as the 

 sharks, the sturgeons, and the paddlefish. It consists of a fin forked into 

 an upper and lower lobe, the upper lobe being larger. The backbone 

 extends into the upper lobe. In some fishes, such as the gars and the 



