326 THE EARTHWORM CHAP. 



pregnatloil arid in forming a case or coccoort for the eggs. 

 On the ventral part of this region are some small 

 glandular swellings, which are more conspicuous in young 

 worms before the clitellum is developed. 



The whole of the body is invested with a delicate, iri- 

 descent membrane or cuticle (p. 306) formed as a secretion of 

 the epiderm or outer epithelial layer of the body (compare p. 

 128). Every segment, except the first and the last, is provided 

 with eight small cuticular spines or setce (Fig. ']S,set) — slightly 

 curved bodies with tapering ends composed of a horn-like 

 substance called cAi'im — each of which is developed in a small 

 sac formed as an involution of the epiderm, and is provided 

 with muscles by means of which it can be protruded and re- 

 tracted. Th3se setse are arranged in couples, forming two 

 double rows along each latero-ventral region of the body, 

 and their points can be distinctly felt on drawing the worm 

 through the fingers : they serve to prevent the animal from 

 slipping backwards as it moves along on the surface of the 

 .ground or in its burrows. 



We have seen that the earthworm takes in its food, 

 together with quantities of earth, by the mouth, and after 

 retaining it for a longer or shorter time in the body, 

 expels it by the anus. It is obvious, therefore, that there 

 must be some kind of digestive cavity into which the 

 food passes by the mouth, and from which effete matters 

 are expelled through the anus. Sections (Figs. 78) show 

 that this cavity is not a mere space excavated in the interior 

 of the body, but a definite tube, the enteric or alimentary 

 canal {^. 23), which passes in a straight line from mouth to 

 anus, and is separated in its whole extent from the walls of 

 the body by a wide space, the body-cavity or ccelome (cxt), as 

 in the frog (p. 20). So that the general structure of the 

 earthworm might be imitated by taking a wide tube, 



