282 THE 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 Cep halo- 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 



