146 THE MECHANICAL FUNCTIONS. 



fection 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 me- 

 chanical construction of animals which con- 

 stantly inhabit a watery element is more simple 

 than the construction of those which live on 

 land, and are encompassed 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, Avhich nearly 

 balances the force of gravity, 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 co- 

 hesion of substance which are required in ter- 

 restrial animals. 



Tlie animals that occupy the lower divisions 

 of the scale can exist only in a liquid element. 

 Their forms present many analogies with vege- 

 tables ; and hence they have been denominated 

 Zoophytes, that is, animated plants : but as it is 

 now well ascertained that they possess the essen- 

 tial characters of animals, the term of Phytozoa, 

 or plant-like animals, which has been given to 

 them by some modern writers, would appear to 

 be a more appropriate designation. It is, how- 



