282 T^E MECHANICAL FUNCTIONS. 



the alimentary canal, correspond in external 

 form, as well as in many circumstances of inter- 

 nal conformation, to the Annelida. They com- 

 pose an order denominated the Entozoa. 



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 

 conformation, we find that a considerable advance 

 has been made in the progress of developement. 

 The frame- work of the body is more consoli- 

 dated : and the instruments provided for pro- 

 gressive motion are shaped into longer and more 

 perfect levers, are united by a more refined sys- 

 tem of articulation, and are moved by more dis- 

 tinct 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 frame- 

 work of the Annelida, are here consolidated so 

 as to form two principal divisions of the body, 

 the one in front, termed the Cephalo- thorax, 

 which contains the organs of sensation, and of 

 mastication, and also the principal reservoir of 

 circulating fluids ; the other, which is behind, 

 and contains the organs of digestion, is termed 



