CHAPTER XIX 

 PHYLUM ANNELIDA 



The annelids are more completely metameric than the 

 animals in any other phylum. The flatworms and nema- 

 todes do not have any indication of segmentation in their 

 bodies, the rotifers are only imperfectly segmented, but the 

 annelids are always made up of a chain of very similar 

 parts. The phyla which rank above the annelids in 

 structural complexity have the metamerism obscured by 

 the outgrowth of the limbs and other appendages (Arthro- 

 poda, Chordata), or lack any trace of segmentation (Mol- 

 lusca). This phylum therefore shows metamerism as it 

 appears nowhere else in the animal kingdom. As a rule 

 the ccelom of annelids is large and well developed; the body 

 is therefore of the tube-within-a-tube type (Fig. 16). The 

 alimentary canal possesses two openings. Annelids live 

 in the ocean, in fresh water, and on land; some are even 

 parasitic upon other animals. There are two important 

 classes and one or two of less general interest, which will 

 not be considered. 



Class 1. Choetopoda. Annelids with setae, or bristles, along the 

 sides of the body. 



Class 2. Hirudinea. Without setae, but with suckers at either end 

 of the body. 



CLASS CILETOPODA 



The chsetopods (chaite, bristle; pous, foot) are abundant 

 in most parts of the earth and are easily recognized by the 

 setae along the sides of the body. Sometimes the setse pro- 

 trude in great brush-like tufts from each metamere, in 

 other instances they are small and almost completely buried 



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