194 THE MECHANICAL J'UNCTIONiJ. 



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ber of Mollusca, are limited by the constitution of tbeir 

 system, to an aquatic existence. But in following the series 

 of Articulated animals, we very soon emerge from the 

 waters, and find structures adapted to progression on land. 

 For this we see that preparation is early made in the de- 

 velopment of the nascent structures. A farther design, also, 

 soon becomes manifest; and instruments are given for ele- 

 vating the body above tlie ground, and for traversing with 

 rapidit}^ the light and scarcely resisting atmosphere. This 

 prospective design may be traced in the whole system of in- 

 sects; every part of which is framed with reference to the 

 properties of the medium through which these movements 

 are to be performed. 



§ 2. Annelida, 



The lowest division of articulated animals comprehends 

 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 ex- 

 ample. In the series of structures which constitute this di- 

 vision of the animal kingdom, we may trace remarkable gra- 

 dations of development, 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 dispo- 

 sition of parts, and those star-like, forms, so characteristic 

 of the beings which are placed on the confines of the ani- 

 mal kingdom, and which still retain an analogy with vege- 

 table structures. She now adopts a more regular law of 

 symmetry; by which all the parts are referrible to one lon- 

 gitudinal axis, and also to a vertical plane passing through 

 that axis, and which has been termed the mesial plane. 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. 



