OF NATURAL HISTORY. 91 



Sir dilates, and makes their bodies fpecifically lighter. By 

 this curious piece of mechanifm, the animals have the power 

 of finking to the bottom, or of riling to the furface. Ac- 

 cording to the different degrees of contradlion and dilation 

 of this bladder, fifhes can, at pleafure, keep themfelves high- 

 er or lower in the water. Hence fiounders, foles, fkate, and 

 other fifhes which have no fv^'imming bladder, always grovel 

 at or near the bottom. It is likewife a confequence of the 

 relaxation of this bladder, that dead fifhes which are furnifh- 

 ed v/ith it uniformly rife to the furface. The air-bag, in 

 fome fifhes, communicates, by a duel, with the gullet, and, 

 in others, with the flomach. At the upper end of the air- 

 bag, there are red-coloured glandular bodies conne£led with 

 the kidneys. From the kidneys the ureters proceed down- 

 ward to their infertion in the urinary bladder, which lies in 

 the lower part of the abdomen, and the urethra terminates 

 in the anus, 



Fifhes have a membranous diaphragm, or midrifF, that 

 forms a fack in which the heart is contained. The heart is 

 of a triangular figure. It has only one auricle, one ventri- 

 cle, and one great artery. This artery, inflead of fupplying 

 all the parts of the body, as in the frog, is diftributed entire- 

 ly on the gills. All the branches terminate there, and be- 

 come at lafl fo fmall that they efcape the naked eye. The 

 branchiae, or gills, lie in two large flits on each fide of the 

 head, and are analogous to the lungs of land-animals. The 

 figure of the gills is femicircular, and on each fide of them 

 are immenfe numbers of fibrils refembling fringes. The 

 gills are perpetually fubjected to an alternate motion from 

 the prefTure of the water and the aftion of the mufcles. 

 They are covered with a large flap, which allows an exit to 

 the water necelTarily taken in by the animals every time 

 their mouths are opened. The blood is again collected by a 

 vafk number of fmall veins, which, inftead of going back sl 



