FISHES. 285 



.fluid without the necessity of employing any voluntary mo- 

 tion or exertion for that purpose: but as the body of a fish 

 is generally a little heavier than the fluid medium, csj)ecial- 

 ly if it be fresh water, it is necessary for the animal to give 

 its body some degree of motion, in order to prevent its 

 sinking. 



In land quadrupeds, the limbs have to perform the double 

 office of supporting the body, and of eficcting at the same 

 time its locomotion: but as nearly the whole of the weight 

 of a fish is already sustained by the clement in which it is 

 immersed, its instruments of motion may be employed ex- 

 clusively for progression, and the powerful hydrostatic pres- 

 sure, which supports the body on all sides, supersedes the 

 necessity of that cohesive rigidity of frame, which is essen- 

 tial to the safety of terrestrial animals. Hence we find that 

 in one whole tribe of fishes, the skeleton is composed mere- 

 ly of cartilage; and, in all, it exhibits much less of the osse- 

 ous character than in the higher classes. The frame-work 

 of the skeleton, even of osseous fishes, has not the compact- 

 ness possessed by that of quadrupeds or reptiles: the pieces 

 which compose it are joined together less firmly; many of 

 them, indeed, remain in an imperfectly ossified condition, 

 their elementary pieces being detached from one andther, as 

 if the usual process of consolidation had been arrested at an 

 early stage. The texture of the bones of cartilaginous fishes 

 corresponds to this primeval condition; for it is composed 

 merely of granules of calcareous phosphate, interspersed 

 amidst the cartilaginous substance in detached masses, or 

 presenting the appearance of coarse fibres, thinly scattered 

 through the semitransparent bone. Compared with the 

 quantity of gelatin which enters into their composition, the 

 bones of fishes contain but a small proportion of earthy in- 

 gredient, a circumstance which explains the pellucidity of 

 the mass, and the readiness with which the osseous fibres it 

 contains can be distinguished. Another consequence of the 

 want of density in the bones of fishes is, that their articula- 

 tions are less regular and perfect than the corresponding 



