LESSONS IN ZOOLOGY. 295 



out the muscles of the eye, and trace the optic nerve to the optic 

 lobes. Notice the form of the eye, its flat cornea and spherical 

 lens; its freedom of motion and the absence of eyelids. 



Auditory organs are usually present, but it is said that fish 

 cannot hear well, and the senses of taste and touch do not seem 

 to be well developed. The olfactory sense is, perhaps, better 

 developed than any other except the sense of sight. 



Open and shut the mouth and note the motion of the different 

 bones which make up that cavity. Cut away the gill cover on 

 one side and note the motion of the gills corresponding to the 

 motions of the jaws and tongue. Keep minnows or small fish in 

 jars for observation, and notice their movements and habits. 



Fish, in general, are carnivorous; they are voracious eaters 

 and rapid growers. The flesh of most kinds is highly prized for 

 food. The fishing industry is one of the great industries of the 

 world . 



Most fish are oviparous, producing vast quantities of spawn, 

 but their natural enemies are so numerous, and the number de- 

 stroyed by man so great, that many of the valuable fish began to 

 diminish in numbers. This fact led several nations to establish 

 departments looking toward the protection and culture of valu- 

 able food-fishes. 



These departments have gathered an immense fund of informa- 

 tion relating^ to the habits and life history of the more valuable 

 food-fishes; to the habits and history of the various forms of life 

 used by them as food ; to the habits and history of those forms 

 of life that prey upon food-fishes, and to the various forms of 

 disease that attack valuable fish. This knowledge has led to the 

 adoption of rational methods of protection and culture, so that 

 waters formerly barren now yield an abundance of good fish, and 

 other waters are furnishing a largely increased product. The 

 more important food-fishes included in this group are the herring, 

 cod, mackerel, shad, salmon, hake, haddock, halibut, white fish, 

 trout, bass, pickerel, and many others. 



The Dipnoi, are scaly animals breathing by means of gills and 

 lungs ; they have the form and appearance of fish, but their lungs 



