CHAPTER XLI 



ANNELIDS IN GENERAL 



The most striking advance shown by annelids is the appearance of 

 metamerism. A forecast of this may be considered as shown in the 

 transverse groovings on the body of some of the Nemathelminthes, but 

 in that case no internal segmentation corresponded to this external 

 indication. Metamerism in annelids involves not only the division of 

 the body wall into a series of sections but also a metameric arrangement 

 of internal structures, shown most strikingly by the excretory and nervous 

 systems. The irregularly distributed groups of flame cells opening to 



Ganglion 



Mouth 



Protonephrr- 

 olium 



Mouth ^^ 



Pro ton ephriolium 

 Ganojlion 



Anal 

 tentacles 



Oral 

 tentacles 



Bye. spots 



Anus 



Anus 



B 



Fig. 143. — Polygordius sp., showing two stages in the development of the troehophore 

 larva (A, B) and the adult (C). {From Wieman, "General Zoology," after Hatschek and 

 Fraipont, by the courtesy of McGraw-Hill Book Company, Inc.) A and B much magnified, 

 C X about 2. 



the outside through one or a few openings of a common large duct, seen 

 in previous phyla, are replaced in members of this phylum by a series of 

 excretory organs arranged in pairs, one pair to a metamere and each 

 complete in itself. The nervous system, also, instead of showing a 

 general tendency toward the accumulation of cells in a few ganglia or 

 nerve tracts, shows the development of metamerically arranged ganglia 

 connected by a ventral nerve cord. An earthworm is really made up of 

 a series of nervous units. These may act individually but are so related 



241 



