The following classification incorporates the views of several authorities.
- Phylum Annelida (segmented worms)
- Body wall covered with a cuticle secreted by the epidermis and containing an outer circular and inner longitudinal muscle layer; chitinous (tough, complex carbohydrate material) setae usually present, secreted by follicular cells and arranged segmentally; head or prostomium preoral, with or without appendages; closed circulatory system, with blood often containing a respiratory pigment; coelom, of schizocoelic origin, divided segmentally into compartments by walls, or septa; nervous system includes a dorsal, bilobed brain and a pair of connective nerves that encircle the digestive tract and unite to form a ventral nerve cord with 1 ganglion per segment.
- Class Polychaeta (marine worms)
- Paired lateral appendages, or parapodia, bearing chitinous setae; name of group refers to the many setae per segment; head with or without appendages; sexes generally separate with gametes discharged directly into the water, where fertilization and development occur; the free-swimming larva called a trochophore; more than 6,000 living species; free-moving and sedentary (tube-dwelling) forms.
- Order Aphroditamorpha (scale worms)
- Free-moving; dorsally rounded, with flattened pairs of scales more or less alternating with the dorsal cirri (slender projections); head ... (200 of 10,361 words)