24 o ECONOMIC ZOOLOGY AND ENTOMOLOGY 



In habits too they differ as much as in size and shape. 

 Some live only in quiet ponds or lakes, others occur in the 

 swiftest mountain streams; some live only in fresh water, 

 others in the brackish waters of the bays or the salt water of 

 the sea. Certain kinds live close to the surface of the water, 

 others at moderate depths and still others in the deepest 

 parts of the ocean. 



General Form and Structure. A typical fish, such as a sun- 

 fish or perch, has the body more or less pointed, the sides 

 somewhat flattened, the head wedge-shaped, and in many 

 other ways shows a body formed for moving rapidly through 

 the water. Most fish have the body covered with scales, al- 

 though many have the skin naked or covered with small scales 

 so hidden in the skin as to be hardly visible. The scales are 

 small horny or bony plates, outgrowths of the skin, which 

 usually overlap each other like shingles. The number, shape 

 and size of these scales are characters that are much used by 

 ichthyologists in classifying fishes. Three regions of the body 

 may be recognized, the head, the trunk and the tail, the latter 

 comprising that part of the body beyond the anal opening. 

 On the head are two usually conspicuous eyes set in protective 

 sockets. There are usually no eyelids, the skin of the body 

 being continuous, but transparent, over the eyes. Fishes are 

 near-sighted and vision is probably not very precise, although 

 the trout and some others seem to possess a very keen eye- 

 sight. In some of the deep sea fishes and in some cave-dwell- 

 ing species the eyes are rudimentary or wanting. 



The mouth in most fishes is comparatively large and trans- 

 verse, and the jaws usually bear numerous teeth that enable 

 the fish to bite or hold their prey. Some fishes, however, 

 have the mouth small and round and fitted for sucking rather 

 than biting, and many have few or no teeth. 



The nostrils are paired openings usually situated in front 

 of the eyes. They end in a pair of nasal sacs and do not 

 open into the roof of the mouth as they do in mammals, and 

 so have no relation to breathing. The sense of smell is rela- 

 tively acute in most fishes. On each side of the head is a 

 flap-like gill-cover, or opercnlum, the posterior margin of which 



