242 THE lMi:CHANICAL FUNCTIONS. 



§ 2. Annelida. 



The lowest division of articulated animals compre- 

 hends those which have a vermiform shape, and 

 which compose the class of Annelida, or Annulose 

 animals ; of which the earth-worm 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 developement, through which nature 

 appears to pass in attaining the higher and more 

 perfect conformations. 



It may be remarked that, in effecting the tran- 

 sition from Zoophytes to the new model of con- 

 struction here presented, nature seems to have 

 wholly abandoned that radiated disposition of parts 

 and those star-like forms, so characteristic 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 symmetry, 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 platie. As a 

 direct consequence 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 reflected 

 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 



