CHAPTER VII 



Annelida or Annulata 



The older terminology included the Molluscoidea, the Platyhel- 

 minthes, the Trochelminthes, and the Nemathelminthes with the 

 Annelida under the general term Vermes. For several decades, 

 however, the Phyla have been separated. 



The Annelida (Lat. annellus, a little ring) are the most highly 

 developed of the worms, with regularly segmented bodies, which in 

 most cases indicate by external annulations the metameric arrange- 

 ment within, which is such that the internal organs are repeated in 

 each segment. The head usually has a " prostomium " in front of 

 the mouth. (See Figure 45.) 



There is usually a well-developed coelom, and an extensive 

 series of blood vessels. Hair-like or comb-like gills function in 

 respiration in some forms, while in others minute capillaries in the 

 skin aerate the blood. The excretory organs, called nephridia, are 

 segmentally arranged and the nervous system consists of dorsal 

 cerebral ganglia, and a ventral nerve cord with segmentally arranged 

 ganglia. 



Classification 



I. Class Archi- Annelid a (Gr. arche, beginning; Lat. anne//us, a little 



ring) without setae or parapodia. 

 1. Class Chaetopoda (Gr. chaite, bristles; pons, foot) with setae. 

 3. Class Hirudinea (Lat. hirude, a leech) without setae or parapodia, 



but with suckers. 



Characteristics 



1. Segmented worms in which the segmentation is in most cases 



visible externally. 



2. Appendages paired, not jointed. 



3. Setae are present in the body wall. 



4. The coelom usually communicates with the exterior by paired 



nephridia, and pores. 



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