328 THE EARTHWORM CHAP. 



nation and in forming a case or cocoon 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, 

 iridescent membrane or cuticle (p. 313), formed as a 

 secretion of the epiderm or outer epithelial layer of the 

 body (p. 128). Every segment, except the first and the 

 last, is provided with eight small cuticular spines or set a 

 (Fig. 81, set) slightly curved bodies with tapering ends, 

 composed of a horn-like substance called chitin (compare 

 p. 232) 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 

 retracted. These setae 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, there- 

 fore, 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 

 (Fig. 81) 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 (p. 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 (ccel), as in the frog (p. 20). So 

 that the general structure of the earthworm might be 

 imitated by taking a wide tube, stopping the ends of it 



