' ANNELIDA. 



Body elongated, generally divided into somites. Coalom 

 usually extensive. Appendages when present form parapodia. 



CLASS 1. Archi-annelida. 



Without setae or parapodia. Nervous system not 

 separate from the epidermis. (Polygordius.) 



CLASS 2. Chaetopoda. 



With numerous, distinct somites that are pro- 

 vided with seta3. 

 Subclass 1. Archi-chsetopoda. 



Setae retractile. Nervous system not separate 

 from the epidermis. (Saccocirrus.) 

 Subclass 2. Polychaeta. 



With numerous setse. With a great variety of 

 structure. (Nereis, Autolytus, Lepidonotus, 

 Diapatra, Chaetopterus, Cistenides, Spirorbis, 

 Clymenella, Sabella, Hydroides, Arenicola, Am-* 

 phitrite.) 

 Subclass 3. Myzostomida. 



Disk-shaped. Without external segmentation. 

 Parasites on Echinodermata. (Myzostoma.) 

 Subclass 4. Oligochaeta. 



Without parapodia. Setse 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 external 

 annuli than there are somites. With sucking 

 mouth and posterior sucker. 

 78 



