ORGANIC EVOLUTION 



167 



FIG. 104. Diagrams of worm structure, a, cross 

 section of the body ; e, food tube or enteron ; 

 c, coelom; n, nephridia; v, dorsal blood ves- 

 sels; m, ventral nerve cord, with sub intestinal 

 vessel above it and subneural vessel beneath 

 it; s, s, s, s setae, b, cross section of the body 

 wall; h, hypodermis or epidermis, with cover- 

 ing cuticle; i, circular muscle layer; j, longi- 

 tudinal muscle layer; &, peritoneum, c, cross 

 section of a bit of the wall of the enteron ; I, 

 chloragogue cells (modified peritoneum); m, 

 isolated longitudinal muscle fibres; , circular 

 muscle layer; o, blood spaces; p, digestive 

 epithelium (endodevm). 



of circular muscles 

 and the huge tracts 

 of longitudinal fibres 

 shown in this diagram 

 of circular and longi- 

 tudinal fibres in the 

 walls of the alimen- 

 tary canal, and of a 

 set of slender fibres 

 attached to the base 

 of each seta. 



Organs of reproduc- 

 tion and sex cells. In 

 the region of the 

 ninth to fifteenth seg- 

 ments of the body, 

 the most conspicuous 

 parts seen are the re- 

 productive organs 

 (fig. 105), which are 

 of remarkable com- 

 plexity. The largest 

 of these are the sperm 

 receptacles, three 

 large white paired or- 

 gans in segments ten 

 to twelve, increasing 

 in size posteriorly, 

 the pair of the twelfth 

 segment extending 

 backward at its free 

 upper end into one or 

 two of the segments 

 behind ; these are sep- 



