454 



BIOLOGY AND HUMAN LIFE 



and flounder, and there are great differences in size, from little 

 minnows, gobies, and anchovies, two inches or less in length, to 

 the giant sharks, reaching from thirty to forty feet. While all 

 fishes are water animals, they live under a great variety of 

 conditions— in swift streams and quiet pools, near the surface 

 of the sea and at great depths, in sunlit waters and in totally 

 dark caverns, in fresh water, in brackish water, and in the sea, in 

 hot, tropical ponds, and in water nearly cold enough to freeze. 



The head is in most species continuous with the trunk, and 

 the latter with the tail, although the end of the tail is usually 





W I ,",i'<-.,-..\,' /■, '■, I' 



. Fig. i86. How the haddock breathes 



In the fishes the gills are arranged on arches along both sides of the pharynx. Water 

 is taken through the mouth and passes over the gills and out again, as indicated by 

 the arrows. In the bony fishes the gill slits are covered by a shieldlike plate, the 

 opcrcuhnn, which is free on the posterior edge. The bony fishes also have a swim- 

 ming bladder, connected with the pharynx (see Fig. 78), which is apparently used 

 partly for breathing (the blood vessels in the wall of the bladder being exposed to 

 moist air) and partly as a means of raising or lowering the body in the water 



expanded into tail fin or fins. The appendages are normally two 

 pairs, pectoral and pelvic fins (see section 225) ; and there is 

 usually also one fin along the back (dorsal) and one back of the 

 anus on the ventral side ; this is usually called the anal or ven- 

 tral fin. ]Most fishes have an exoskeleton consisting of scales 

 or spines, growing out of the skin. The bony fishes have a true 

 jaw, and most species have teeth. The mouth is the only organ 

 among fishes with which food or prey can be grasped, although 

 many of them can fight viciously with their tails, their fins, and 

 various outgrowths of the snout. 



