202 . THE MECHANICAL FUNCTIONS. 



cavities of the body, and especially the alimentary canal, 

 correspond in external form, as well as in many circum- 

 stances of internal conformation, to the Annelida. They 

 compose 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 conside- 

 rable advance has been made in the progress of development. 

 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 frame-work of the 

 Annelida, are here consolidated so as to form two principal di- 

 visions of the body, the one in front, termed the Cephalo-tho- 

 rax, which contains the organs of sensation, and of mastica- 

 tion, and also the principal reservoir of circulating fluids; 

 the other, which is behind, and contains the organs of diges- 

 tion, is termed the abdomen. In the spider (Fig. 136, where 



c is the cephalo-thorax, and a 

 the abdomen) these two por- 

 tions of the body are separated 

 by a deep groove, which leaves 

 only a slender pedicle, or tube of 

 communication between them. 

 There are usually in the male 

 four pairs of legs, constantly articulated with the cephalo-tho- 

 rax; but the female is furnished with an additional pair to ena- 

 ble her to carry her eggs. For the purpose of obtaining an ex- 

 tensive base of support, the feet of the spider are spread out in 

 diverging rays, so as to include a very wide circle. They are 

 divided into several joints, those next to the body being termed 



