CHAPTER XLI 



ANNELIDS IN GENERAL 



The most striking advance shown by annehds 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 annehds 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 



Oral , 

 tentacles 



Bye. spots 



Mouth 



Pro ton ephriolium 

 eanoflion 



Mouth' x.^r.^^x .^--^ ^^^/ 



tentacles^ 

 Protonephrt- >5^ ^^^ -^ / 



^Anus -^^^^ 



A B ^ 



Fig. \b'6.— Polygordius sp., showing two stages in the development of the trochophore 

 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 some 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 gangha or 

 nerve tracts, shows the development of metamerically arranged ganglia 

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



246 



