CHAPTER XI 

 PHYLUM ANNELIDA 



THE annelids (Lat. annellus, a little ring) can, in most cases, 

 be distinguished from other worms, like Planaria (Fig. 97) and 

 Ascaris (Fig. in), by the fact that the body is divided into a 

 number of similar parts called segments, metameres, or somites; 

 these are arranged in a linear series and are visible externally 

 because of the grooves which encircle the body. The earth- 

 worms and leeches are well-known examples. Annelids live in 

 fresh water, salt water, and on land; some are parasitic upon 

 other animals. 



The ANNELIDA form three classes : 



(1) CLASS ARCHIANNELIDA (Gr. arche, beginning; Lat. annel- 

 lus, a little ring) (Fig. 162), without setae (Fig. 153, set) or para- 

 podia (Fig. 164, para)', 



(2) CLASS CH^TOPODA (Gr. chaite, bristle; pous, foot) (Fig. 

 163), with setae; and 



(3) CLASS HIRUDINEA (Lat. hirudo, a leech), without setae 

 or parapodia, but with suckers (Fig. 169, i, 2). 



i. THE EARTHWORM LUMBRICUS 



The earthworm has been for many yeaf s and is still a favorite 

 type for illustrating the anatomy and physiology of annelids, and 

 for teaching general zoological principles,. The common earth- 

 worm, Lumbricus terrestris, lives in the ground where the soil is not 

 too dry or sandy; it comes to the surface only at night or after a 

 rain. In many parts of this country the species Allolobophora 

 ( Helodrilus) longa or one of the species of Diplocardia are more 

 abundant in cultivated soil than L. terrestris. 



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