252 thp: mechanical functions. 



whole body, and when the head is fixed, the hinder 

 sucker can be brought forwards by the contraction 

 of the body, and applied to the ground near to the 

 head, and preparation may thus be made for taking 

 another step. 



Most of the parasitic animals which inhabit the 

 interior cavities of the body, and especially the ali- 

 mentary canal, correspond in external form, as well 

 as in many circumstances of internal conformation, 

 to the Annelida. They compose an order denomi- 

 nated the Enlozoa. As they, in general, remain 

 fixed to the part from which they draw their nou- 

 rishment, their powers of progressive motion, if 

 they possess any, are very limited. 



§ 3. Arachnida. 



In passing from the Annelida to the Arachnida, an 

 order which comprehends all the species of Spiders, 

 together with animals allied to them in conforma- 

 tion, we find that a considerable advance has been 

 made in the degree of developement. The frame- 

 work of the body is more consolidated, and the 

 instruments provided for progressive motion are 

 shaped into longer and more perfect levers, are 

 united by a more refined system of articulation, 

 and are moved by more distinct and more powerful 

 muscles; so that the body is elevated from the 

 ground, and enjoys a greater range of action, and a 

 wider sphere of perception. 



The rings, which always compose the framework 

 of the Annelida, are here consolidated so as to form 

 two principal divisions of the body ; the one in 



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