66 THE MICROSCOPE. ' May 



lie blue median line, Fig. 1, 1. The head is about one-fifth 

 the entire* length of the body. The jaws are large and 

 well supplied with teeth which point back toward the 

 throat. The teeth are not set in sockets. The tongue 

 and palate are also armed with teeth. 



A thin long flap, the " operculum." at the back of the 

 head covers the gill chamber, Fig. l,o. This as well as 

 the fins and scales point back, and in this way afford no 

 resistance to the passage of the fish through the water. 

 The sharks and some other fishes have no operculum. In 

 the case of the shark there are on each side, just back of 

 the head, five vertical slits. 



The eye is round and without lids; the pupil large and 

 the iris silvery. Near the tip of the nose on the dorsal 

 side of the head, are two pits covered with a thin mem- 

 brane. These are the nostrils, they do not open into the 

 mouth, (Fig. 1, n). 



The fins are divided into two classes, paired and me- 

 dian. The paired fins are the Pectoral and Pelvic fins. 

 The Pectoral fins (Fig. 1 p. f.) are just behind the gill- 

 chamber, while the Pelvic fins are further back and more 

 ventral in position (Fig. 1, v. p). In many fishes these 

 fins are close to each other, the Pelvic fin being some- 

 times almost immediately under the Pectoral, or, as in 

 the Sole, a little ahead. 



The median fins are four in number, viz., the Dorsal on 

 the dorsal line and about half way back, the Adipose fin ; 

 dorsal in position and back of the Dorsal fin ; the Anal 

 fin, ventral, and just back of the anus; the Caudal fin at 

 the extreme end of the body. The back bone does not 

 extend into the caudal fin (homocercal). 



The adipose fin is a fleshy, rayless fin peculiar to the 

 SalmonidsB. All the other fins are rayed, and each fin 

 has always a certain number of rays. Thus the number 

 of rays in a fin helps to determine the species of the fish. 



The shape and arrangement of the fins varies much in 



