270 THE MECHANICAL FUNCTIONS. 



may be taken as the type, and most familiar 

 example. In the series of structures which 

 constitute this division of the animal kingdom, 

 we may trace remarkable gradations of de- 

 velopement, through which nature appears to 

 pass in attaining the higher and more perfect 

 conformations. 



It may be remarked that, in effecting the 

 transition from Zoophytes to the new model of 

 construction here presented, nature seems to 

 have wholly abandoned that radiated disposition 

 of parts, and those star-like forms, so character- 

 istic of the beings which are placed on the 

 confines of the animal kingdom, and which still 

 retain an analogy with vegetable structures. 

 She now adopts a more regular law of sym- 

 metry ; by which all the parts are referable to 

 one longitudinal axis, and also to a vertical 

 plane passing through that axis, and which has 

 been termed the mesial plane. As a direct con- 

 sequence of this law, we shall find that in the 

 forms which are hereafter to pass under our 

 review, as far as the external organs and general 

 outline of the body are concerned, all that exists 

 on one side is an exact counterpart, like a re- 

 flected image, of what is found on the other side. 

 While in the Star-fish, and Echinus, nothing in 

 point of situation was definite, excepting the 

 upper and the lower surface, and there was no 

 side which could be exclusively denominated 



