OLIGOCHAETA 



257 



opening into the interior part of the alimentary canal ; whether they 

 have a digestive function is not known. On the other hand they may 

 unite to form a longitudinal duct (or ducts) which discharges into 

 the hind end of the intestine. Whether there is any physiological 

 meaning for the variations in the segmental organs of the earthworms 

 is entirely unknown. 



ect.^ 



^end. 



Fig. 189. Embryo of Lumhricus foetidiis. After E. B. Wilson. A, Lateral 

 view of an embryo in which the mesoblast is imsegmented. B, Ventral view 

 of the same embryo. C, Longitudinal section of a later embryo a little to one 

 side. D, Transverse section of ventral part of the same embryo along the 

 line Xy in C. hrn. brain; coe. coelomic cavity of mesoblastic somites; ect. 

 ectoderm; end. endoderm; ent. enteron; M.t. mesoblastic teloblast; npb. 

 nephroblasts ; nrb. neuroblasts; nep. nephridia; sep. septa; std. stomodaeum. 



There is a well- developed blood circulation. Blood flowing through 

 the parietal and dorso-intestinal vessels of each segment is collected 

 in the dorsal vessel. It is prevented from returning by an elaborate 

 system of valves (Fig. 190). Waves of peristaltic contraction beginning 

 at the hind end of the dorsal vessel and continued by the ''hearts" 

 press it forwards and ventralwards into the ventral vessel which is the 

 main distributing channel. 



Bi 17 



