THE FISHES 159 



state of balance and to change the direction of movement. 

 There are also median fins consisting of one or more dorsal 

 fins above, and an anal fin on the lower side. In addition 

 to the flexible and frequently branched rays, the fins 

 may be strengthened by sharper and more rigid spines 

 which serve also as organs of defense. Many fish erect 

 these spines when angry or in danger. 



The surface of the body is covered by hard scales which 

 overlap so as to form a uniform protective layer. The 

 scales are embedded in the skin and grow with the growth 

 of the fish. Looked at through the microscope they show 

 a series of concentric lines indicating successive stages 

 of growth. In some kinds of fish (catfish) the scales may 

 be absent, while in others, such as the gar pike, they may 

 be large and very hard, forming a veritable coat of mail. 



On the head of the fish the large eyes are noteworthy 

 on account of the lack of eye-lids, so that they are per- 

 manently "open." The nostrils, of which there may be 

 two pairs in some fishes, do not open into the mouth cavity 

 as they do in higher vertebrates, but they lead to the or- 

 gans of smell. At the side of the head is a large flap, the 

 operculum, which covers over the gills. The jaws of the 

 fish are quite different from our own in that the upper jaw 

 is movable instead of being firmly united to the skull. 

 The jaws are furnished with sharp, conical teeth which are 

 fitted for seizing and retaining prey; for the fish does not 

 take time to chew its food, but swallows it entire. Teeth 

 are frequently present also on the roof of the mouth, and 

 in some fishes in other parts of the mouth cavity and even 

 in the throat. Back of the mouth is the pharynx, the lat- 

 eral walls of which are perforated by four pairs of gill-* 

 slits, between which lie the rows of slender, red filaments 

 which constitute the gills. Water is taken into the mouth, 

 passed through the gill-slits, bathing the gill filaments on 



