CHARACTERISTICS OF FISHES 7 



scales consist of bony plates covered by ganoin (ganoid scales). The 

 protective covering in sharks is in the form of scale plates, each con- 

 taining a small spine which stands upward or curves sHghtly back. 

 The scale-like features of the sharks have a dentine base and a spine 

 normally capped with hard enamel. They are formed in a similar manner 

 to teeth and may be considered as minute teeth set into the skin^. The 

 scales of sharks are known as placoid scales, and the minute teeth-like 

 structures are referred to as denticles. 



Eyes 



The eyes of most elasmobranch and teleost fishes follow the normal 

 vertebrate structure. They are provided with a lens capable of throwing 

 an image upon the photosensitive retina, and according tofNicoP^ 

 show the following layers: a felt-work of optic nerve fibers, two layers 

 of ganglion cells separated by a synaptic layer, a basal layer of rods and 

 cones, and the photoreceptors proper. 



Fishes generally have their eyes positioned on the sides of the head. 

 However, in some they may be placed in a superior position (on top of the 

 head). Most of the bony fishes are myoptic (adjusted for near vision), 

 and the eyes at rest are set for near vision. It has generally been considered 

 that fishes do not have acute vision. However, there is increasing evi- 

 dence that the sharks have distant vision (hypermetropia) and may be 

 able to distinguish between the shapes of small objects at a considerable 

 distance. 



Of the two types of photosensory cells, the rods and the cones, the 

 rods are most effective at low intensities and the cones, being less sensi- 

 tive to light, are able to function over a range of higher intensities. 



Bony fishes which inhabit areas where wide ranges of light intensities 

 occur have both rods and cones in the eyes. Forms which inhabit dimly 

 lighted areas at great depths may contain pure rod retinas^^. The eyes 

 of fishes inhabiting dimly lighted regions are considered to be the most 

 sensitive in existence ; however, in some of the very deep-water fishes the 

 eyes may be degenerate and the fishes become blind. 



Considerable debate has occurred regarding the capabihties of fish to 

 detect differences in color. There seems to be sufficient evidence now to 

 indicate that many species of fish do detect color differences ^^ although 

 considerable variation between species may occur in the ranges of colors 

 that can be delineated. 



Smell 



The chemoreceptors in bony fishes are locahzed in the nasal pits, for- 

 ward of the eyes on the dorsal surface of the snout, and in sharks and 



