I'M THE MliCllANlCAL 1' UNCTIONS. 



the organization of animals, to bestow particular 

 attention on the habits and mode of life for which 

 each respective tribe and species has been destined. 



In the examination of the mechanical functions 

 which will form the first part of this treatise, I shall 

 keep in view, as the leading object of inquiry, the 

 faculty oi progressive motion, noticing its different 

 degrees of perfection as we follow the ascending 

 series of animals; but adverting, also, occasionally, 

 to the other topics which belong to this class of 

 functions. 



It may be observed in general, that the mecha- 

 nical construction of animals constantly inhabiting 

 a watery element is more simple than the construc- 

 tion of those which live on land, and which are en- 

 compassed by a lighter medium. Differing but 

 little in their specific gravity from the fluid in which 

 they are immersed, aquatic animals are necessarily 

 supported, on all sides, by a powerful hydrostatic 

 pressure, which nearly balances the force of gra- 

 vity, and counteracts the tendency of their bodies 

 to descend in the fluid. Many of the obstacles to 

 progressive motion are thus removed ; and there is 

 no necessity for the compactness of frame, and the 

 rigidity and cohesion of substance which are re- 

 quired in terrestrial animals. 



The animals which occupy the lower divisions of 

 the scale can exist only in a liquid element. Their 

 forms present many analogies with vegetables ; 

 and hence they have been denominated Zoophytes, 

 that is, animated plants : but as it is now well 

 ascertained that they possess the essential charac- 

 ters of animals, the term of Phytozoa, or plant-like 

 animals, which has been given to them by some 



