234 A STUDY OF VERTEBRATES 



in shape. Such an eye has been found to be very nearsighted. 

 Thus it is unlikely that a fish is able to perceive objects at any 

 great distance from it. The eyes are unprotected by eyelids, but 

 the tough outer covering and their position afford some protection. 



Feeding experiments with fishes show that a fish becomes aware 

 of the presence of food by smelling it as well as by seeing it. The 

 nostrils of a fish can be proved to end in little pits, one under each 

 nostril hole. Thus they differ from our own, which are connected 

 with the mouth cavity. In the catfish, for example, the barbels, 

 or horns, receive sensations of smell and taste. They do not 

 perceive odors as we do for a fish perceives only substances that 

 are dissolved in the water in which it lives. The senses of taste 

 and touch appear to be less developed than the other senses. 



Along each side of most fishes is a line of tiny pits, provided with 

 sense organs and connected with the central nervous system of the 

 fish. This area, called the lateral line, is believed to be sensitive 

 to mechanical stimuli of certain sorts. The " ear " of the fish is 

 under the skin and serves partly as a balancing organ. 



Food Getting. A fish must go after its food and seize it, but 

 has no structures for grasping except the teeth. Consequently 

 we find the teeth small, sharp, and numerous, well adapted for 

 holding living prey. The tongue in most fishes is wanting or 

 very slightly developed. 



Breathing. A fish, when swimming quietly or when at rest, 

 seems to be biting when no food is present. A reason for this act 

 is to be seen when we introduce a little finely powdered carmine 

 into the water near the head of the fish. It will be found that a 

 current of water enters the mouth at each of these biting move- 

 ments and passes out through two slits found on each side of the 

 head of the fish. Investigation shows us that under the broad, flat 

 plate, or operculum, forming each side of the head, lie several long, 

 feathery, red structures, the gills. 



Gills. If we examine the gills of any large fish, we find that a 

 single gill is held in place by a bony arch, made of several pieces 

 of bone which are hinged in such a way as to give great flexibility 

 to the gill arch, as the support is called. Covering the bony 

 framework, and extending from it, are numerous delicate filaments 



