i6 THEPHILOSOPHY 



the blood, and is circulated throwgh the body, for the nourilhmenc 

 of its different parts. It is of no moment, for our prefent purpofe, 

 to be more particular, efpecially as this fubjedl will be afterwards 

 more fully handled. I fhall therefore juft mention, that there are 

 particular organs or glands for fecreting various fluids, which are ne- 

 ceflary to the exiftence of the larger animals, as the kidneys for the 

 fecretion of urine ; the liver for the fecretion of gall ; the ftomach 

 for the fecretion of the gaftric juices ; the faUvary glands for the 

 fecretion of faliva, &c. 



From this fketch of the ftrudure of man and of quadrupeds, very 

 little attention is neceffary to perceive, that Nature purfues a fimilar 

 plan iu the formation of birds and fifhes. 



In that numerous clafs of animals diftinguiflied by the name of 

 in/ens, there is a great variety of form and ftrudure. In many of 

 thefe, Nature feems to depart from her general mode of operation. 

 But, upon a more accurate examination, this feeming departure will 

 appear to be only an extenfion of that univerfal plan which fhe ob- 

 ferves in the formation of all animated beings. Some infedts, the 

 lobfter, and all the cruftaceous and fhell animals, have their bones 

 on the outfide of their bodies. To thefe bones the mufcles and 

 other iuftruments of motion are attached. Many fpecies have no 

 bones ; but their bodies confift of a fuccefTion of rings incafed into 

 each other. By contrading and dilating thefe rings, all the move- 

 ments of this kind are performed. The head, in fome fpecies, 

 changes its form every moment. It contrads or dilates, appears or 

 difappears, at the pleafure of the animal. Thefe motions are per- 

 mitted by the flexibility of the membranes, or coverings of the 

 head. In other fpecies, the form of the head is permanent, owing 

 to the hardnefs of the coverings, which are fcaley or cruftaceous, 

 and approaches nearer to that of the more perfeiS animals. 



3 • Many 



