2o6 F I S H E S 1 N GENERAL. 



benefit of a frefh fupply : yet nothing is more difficult to be accounted for 

 than the manner in which they obtain this fupply. Fifh in the water 

 have a motion of their lips and gills, or at leaft of the bones on each fide 

 that cover them. This motion is, without doubt, analogous to our 

 breatl^ing ; yet it is not air, but water, that the fifh adually fucks in. 

 Their manner of breathing is probably thus: The fifh firfl: takes a quan- 

 tity of water at the mouth, which is driven to the gills; thefe clofe, and 

 keep the water fo fwallowed from returning by the mouth; while the 

 bony covering of the gills prevents it from going through them, until 

 the animal has drawn the proper quantity of air from the water thus im- 

 prifoned : then the bony covers open, and difcharge it; the gills alfo admit 

 a frelli quantity of water. Should the fifii be prevented from the free 

 play of its gills, or fhould the bony covers be kept from moving, by a 

 ftring tied round them, the animal would fall into convulfions, and die. 



The air-bladder may ferve to contain this air for vital purpofes; as 

 well as contribute to enabling the fifh to rife and fink in the water at 

 pleafure, as it is dilated or compreflTed. The air-bladder is a bag filled 

 with air, fometimes compofed of one, fometimes of two, and fometimes 

 three divifions, fituated towards the back of the fifh, and opening into the 

 maw or the gullet. Some fifh have an air-bladder that continually crawl 

 at the bottom, fuch as the flounder ; and fome are without it that fwim at 

 eafe in every depth; flich as the anchovy and gudgeon. Its neceffity 

 then is not always equally ftrong. 



" Moft of the diforders incident to mankind,*' fays Bacon, '' arife 

 from the changes and alterations of the atmofphere: but fifhes refide in 

 an element little fubjecl to change; theirs is an uniform exiftencej their 

 movements are without effort, and their life without labour. Their 

 bones alfo, which are united by cartilages, admit of indefinite extenfion; 

 and the different fizes of animals of the fame kind among fifhes is very 

 various. They ftill keep growing ; their bodies, inftead of fufFering the 

 rigidity of age, which is the cauie of natural decay in land animals, flill 

 continue increafing with frefh fupplies; and, as the body grows, the con- 

 duits of life furnifh their flores in greater abundance. How long a fifh, 

 that feems to have fcarce any bounds put to its growth, continues to live, 

 is not afcertained; perhaps the life of a man would not be long enough to 

 meafure that of the fmallefl." 



When a fifh's fcale is examined through amicrofcope, it will be found 

 to confifl of a number of circles, one circle within another, in fome mea- 

 fyre refembling thofe which appear upon the tranfvcrfe fedion of a 



tree, 



