44 COMPARATIVE ANATOMY 



in extent and surround the gut do the structures derived from 

 the epiblast, the external skin, the nephridia, and the nerve 

 cord develop in segmental connection with the somites. 

 There is, however, one region of the body which is not 

 segmented — namely, the prostomium with its single prostomial 

 cavity. This region is so much reduced in earthworms that 

 its importance is easily overlooked, but it attains greater 

 development in many other worms, and, indeed, preponderates 

 during larval life. In order to understand better the segment- 

 ation or metamerism of the earthworm, it will be advisable to 

 consider shortly the structure of some of the nearest allies of 

 the earthworms. 



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