THE CATFISH. 165 



roof of the mouth inside, and observe how the eyeball is 

 affected. 



I. The Mouth. Stretch the mouth wide open, and look 

 into it. Observe a band of numerous sharp, small teeth 

 extending across its margin, above and below. Rub a 

 finger across these teeth to discover in what direction 

 they point. What advantage to the animal in their incli- 

 nation ? 



The bone which forms the border of the lower jaw is 

 called the mandible. The bone of the upper jaw which 

 bears the teeth is called the premaxillary. In other fishes 

 several- bones of the roof of the mouth bear teeth. One 

 of these is the maxillary, which in the catfish is reduced 

 to the pair of rudiments already found supporting the 

 bases of the lateral barbels. 



The tongue is a slight angular elevation on the floor of 

 the mouth. Its boundaries are marked out by correspond- 

 ing grooves on the lower surface of the head. 



The posterior funnel-shaped part of the capacious cavity 

 seen on looking into the mouth is the pharynx. The phar- 

 ynx tapers into the esophagus "in the distance." On 

 the floor of the pharynx are two large prominences cov- 

 ered with teeth. Rub a finger across these teeth to dis- 

 cover their direction. 



On either side of the pharynx are the gill arches, bear- 

 ing the gills on their outer convex edges, and separated 

 by five vertical slits. Pass a pencil in at the mouth, and 

 out through one of these slits to the exterior. On the 

 outside observe that the pencil has lifted a semicircular 

 flap, which is divided by an oblique groove into two por- 

 tions. The lower marginal portion is the operculum, and 

 the upper anterior portion is the prce-operculum. The 

 latter is directly behind the eye, and forms the cheek 

 of the fish. The inner surface of the operculum is cov- 

 ered by a soft lining membrane called the branchiostegal 



