54 NORTHERN FISHES 



or dorsal side, to the belly, or ventral side, is contained in the length 

 of the fish. Eye in head means the number of times that the width, or 

 diameter, of the eye is contained in the distance from the tip of the 

 snout to the posterior edge of the opercle. Eye in snout refers to the 

 number of times that the diameter of the eye is contained in the dis- 

 tance from the tip of the snout to the anterior margin of the eye. 



Externally all fishes are covered by a thin skin containing numerous 

 mucous glands that render the fish more or less slimy. This skin covers 

 the scales and plates. The slime, or mucous covering, is a protective 

 device and prevents the entrance of bacteria and molds. Infections of 

 molds and bacteria frequently result when fishes are handled carelesslj' 

 and the slime is rubbed off. 



Fishes have many sensory structures in the skin. The most conspic- 

 uous of these structures is the lateral line, which appears as a line run- 

 ning from the opercle to the tail fin about midway on the side of the 

 body. This line really consists of a canal with many nerve endings run- 

 ning under the skin. Although present in mo^t Minnesota fishes it is 

 absent or poorly developed in a few species. Various functions have been 

 assigned to this structure, but the best evidence at present seems to 

 indicate that it is concerned with the perception of vibrations and 

 water currents. 



Fishes also have ears, which are entirely internal and are buried in 

 the sides of the skull. Structurally the ear is developed more for the 

 sense of balance than for hearing, but the sense of hearing is present. 



The sense of smell is developed in fishes, and smell organs with well- 

 developed nerve connections are present in the nostrils. There are 

 many small sensory structures with various sensory functions scattered 

 over the skin, and part of the sense of taste may be located in the 

 skin. 



The eye of the fish is well developed and consists essentially of the 

 same structures as the human eye. The eyes of most fresh-water fishes 

 are more or less fixed and can be moved only slightly. Nor can the fish 

 readily adjust its vision for distance. Most fishes have their sight fixed 

 for near vision only and hence are nearsighted. Since the water is 

 usualh^ so opaque that it would be impossible to see any great distance, 

 this nearsightedness is probably not much of a handicap. Although it 

 has been claimed that many fishes are at least partly color-blind, most 

 can distinguish certain bright colors like red. 



The scales are of at least four types. The most primitive type is the 

 placoid scale, found only in the sharks and their near relatives. These 

 scales do not overlap. They have flat bases imbedded in the skin and a 

 conical point or spinelike process protruding through the skin. These 

 processes are similar in structure to a tooth, being composed largely of 

 dentine. There is some controversy as to whether they are tipped with 



