::70 



CCELHELMIXTIIES 



closely related that they must he incliuled under the common head; the 

 missinLT characters have heen lost durim: e\'olution. 



Fig. ^51. — Diagram of annelid somites (oris;.), am. acicular muscles; c, ccelom; 

 cm, circular muscles; cv, circular blood-vessels; </, dorsal blood-vessel; ?', intestine; Im, 

 longitudinal muscles; m. mesentery; «, nerve cord; iia, nephridium; nc, no, neuro- and 

 notopodia, forming parapodium; 5, septum; so, somatopleure; sp, splanchnopleure; t, 

 lyphiosole. 



Sub Class I. Chcctopoda. 



These, like the Nematoda, are cylindrical worms, but are at once 

 distinguished by the segmentation. Deep circular constrictions (fig. 2^2) 



bound the somites externally. Inter- 

 nally the ccelom is divided by the 

 septa — delicate double membranes 

 which extend from the ectoderm to 

 the alimentary canal — iiito as many 

 chambers as there are metameres, 

 while a longitudinal mesentery, also 

 doulile, separates the ccrlomic pouches 

 of the right side from those of the 

 left (ligs. 251, 253). The alimentary 

 canal has a terminal anus, while the 

 mouth is ventral and is overhung by 

 the preoral segment, the prostomium. 



Fig. 2S2. — Farthworm, side \'ic\\' xt ^ i 1 j t j 



A , ■ 11 1 f, T A'ervous system, blood-vessels, and 



and anterior end enlarged (.alter \ ogt ■ ' 



and Jung). i, first .segment «ith excretory organs are inlluenced by the 

 mouth and prostomium; 15, male segmentation. The nervous system 



se.xual openmg; 33-37, clitelluni. '^ 



is on the ladder plan (p. 113). It 

 begins with a supraocsophageal ganglion ('brain') in the prostomium, 

 from wliich the esophageal commissures pass around the oesophagus 



