CHAPTER 15 



Phylum Annelida 



1 1 6. General Features of the Annelid Worms 



The Annelida are segmented worms, the body wall and coelom of 

 which are divided into a longitudinal series of rings or segments. The 

 epidermis, circular muscle, longitudinal muscle, coelom and peritoneum 

 are all arranged in segments. 



Some of the phyla considered previously have structures that look 

 like segments. The tapeworms, for example, might be said to be seg- 

 mented, with new segments forming in the scolex and the older segments 

 moving to the posterior end as proglottids. Each segment of the tape- 

 worm is eventually shed, however, and is only a temporary part of the 

 body. Many rotifers and a few nematodes have a superficial segmenta- 

 tion, which involves only the cuticle and a part of the musculature. Most 

 of the musculature of the kinorhynchs is segmented and their cuticle is 

 deeply segmented. Young kinorhynchs have few segments, and add new 

 ones at the posterior end as they grow. Most zoologists do not consider 

 these animals to be truly segmented as are the annelids, arthropods and 

 chordates. 



True segmented animals exhibit metamerism, a repetition of a 

 structure or organ from segment to segment. The annelid body is made 

 of a series of metameres or segments, each of which has the same funda- 

 mental structures as all the others. The nervous, circulatory, excretory 

 and reproductive systems of the annelids are metameric in structure. In 

 fact, only the digestive tract of annelids shows little or no metamerism. 

 Thus, segmentation is much more fully developed in the Annelida than 

 in any of the other groups that have been considered. Young annelids 

 usually have few segments, and add new segments as they grow by sub- 

 dividing the terminal segment. 



In annelids the mouth lies between the first and second segments, 

 forming one preoral segment or prostomium. The brain originates in the 

 prostomium, and develops a pair of circumpharyngeal commissures that 

 reach around the pharynx to join the ventral cord, which appears as a 

 chain of ganglia, one pair in each segment. The first segment behind 

 the mouth is often different from the rest, and is called the peristomium. 

 In counting segments, the prostomium is ignored, and the peristomium 

 is counted as segment one. 



Annelids are covered with a thin cuticle secreted by a simple epi- 

 dermis. Each segment has a ring of circular muscle fibers that can con- 



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