290 



THE EARTHWORM AND OTHER ANNULATA 



and at the other a nephridio'pore or external opening (Fig. 142). 

 The circulatory system consists of a rather complex arrangement 

 of vessels, of which the principal ones are the large dorsal blood 

 vessel (Fig. 137 C), and the ventral blood vessel, lying beneath the 

 intestine; and the five pairs of transverse vessels, called hearts, 

 which pass from the dorsal to the ventral vessel in segments seven 

 to eleven. Save in the anterior region, where the digestive tract 

 is specialized into a pharynx, esophagus, crop and gizzard and where 







Fig. 140. — Transverse section of earthworm in posterior region (c/. Fig. 141). 



a.c, cavity of the alimentary canal; c, cuticle; cm., circular muscles; coe, coelome; 

 C.V., lateral intestinal blood vessel; d.v., dorsal or supra-intestinal blood vessel; hy, hypo- 

 dermis or epidermis; l.m., longitudinal muscles; n.c, ventral nerve cord; p.e., peritoneal 

 epithelium; s, seta; s.g., setigerous gland; sA.v., ventral or sub-intestinal blood vessel; 

 s.m., retractor muscle of setae; ty, typhlosole. (From Sedgwick and Wilson, "General 

 Biology," copyright, 1914, by Henry Holt and Co., reprinted by permission.) 



the reproductive organs are located, the body is relatively simple, 

 since it is composed of a series of somites, or metameres, all of which 

 possess a similar structure. With this general body plan in 

 mind, we may now examine the special systems of organs. 



Body Wall and Coelome. — The wall of the earthworm's body 

 (Fig. 140) consists of several layers. Externally there is a delicate 

 non-cellular cuticle covering an epidermis of low columnar epithe- 

 lium. Beneath this is a layer of circular muscle fibers supported 

 by connective tissue; a thicker layer of longitudinal muscles, 



