270 



CCELHELMINTHES 



closely related that they must be included under the common head; the 

 missing characters have been lost during evolution. 



FIG. 251. Diagram of annelid somites (brig.), am, acicular muscles; c, ccelom; 

 cm, circular muscles; cv, circular blood-vessels; d, dorsal blood-vessel; i, intestine; Im, 

 longitudinal muscles; m, mesentery; n, nerve cord; na, nephridium; ne, no, neuro-'and 

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

 typhlosole. 



Sub Class I. Chcrtopoda. 



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

 distinguished by the segmentation. Deep circular constrictions (fig. 252) 



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 into as many 

 chambers as there are metameres, 

 while a longitudinal mesentery, also 

 double, separates the ccelomic pouches 

 of the right side from those of the 

 left (figs. 251, 253). The alimentary 

 canal has a terminal anus, while the 

 mouth is ventral and is overhung by 

 the preoral segment, the prostomium. 



Nervous system, blood-vessels, and 

 excretory organs are influenced by the 

 segmentation. The nervous system 

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



FIG. 252. Earthworm, side view 

 and anterior end enlarged (after Vogt 

 and Jung). i, first segment with 

 mouth and prostomium; 15, male 

 sexual opening; 33-37, clitellum. 



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

 from which the cesophageal commissures pass around the oesophagus 



