ANNELIDA 



Body elongated, generally divided into somites. Coelom 

 usually extensive. Appendages when present form parapodia. 



Class 1. Archi-annelida. 



Without setae or parapodia. Nervous system 

 not separate from the epidermis. (Poly- 

 gordius.) 

 Class 2. Chaetopoda. 



With numerous, distinct somites that are pro- 

 vided with setae. 

 Subclass 1. Archi-chaetopoda. 



Setae retractile. Nervous system not separate 

 from the epidermis. (Saccocirrus.) 

 Subclass 2. Polychaeta. 



With numerous setae per segment. With a 

 great variety of structure. (Amphitrite, 

 Arenicola, Autolytus, Chaetopterus, Clymen- 

 ella, Diopatra, Hydroides, Nereis, Pectinaria, 

 Polynoe, Sabella, Spirorbis.) 

 Subclass 3. Myzostomida. 



Disk-shaped. Without external segmentation. 

 Parasites on Echinodermata. (Myzostoma.) 

 Subclass 4. Oligochaeta. 



Without parapodia. Setae few and simple. 

 (Tubifex, Lumbricus.) 

 Class 3. Gephyrea. 



No segmentation. With or without setae. 

 With introvert or proboscis. 

 Order 1. Inermia. 



With introvert. Anus dorsal. No setae. (Phas- 

 colosoma.) 

 Order 2. Armata. 



With proboscis. Anus posterior. Setae few. 

 (Echiurus.) 

 Class 4. Hirudinea. 



Somites constant in number, with more exter- 

 nal annuli than there are somites. With suck- 

 ing mouth and posterior sucker. 



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