1.- Unique annelid head and paired epidermal setae present (lost in leeches); parapodia present in the ancestral condition.
2.- Marine, freshwater, and terrestrial.
3.- Most free-living, some symbiotic, some ectoparasitic.
4.- Body bilaterally symmetrical, metameric, often with distinct head.
5.- Triploblastic body.
6.- Coelom (schizocoel) well developed and divided by septa, except in leeches; coelomic fluid functions as hydrostatic
7.- Epithelium secretes outer transparent moist cuticle.
8.- Digestive system complete and not segmentally arranged.
9.- Body wall with outer circular and inner longitudinal muscle layers.
10.- Nervous system with a double ventral nerve cord and a pair of ganglia with lateral nerves in each segment; brain a pair of dorsal cerebral ganglia with connectives to ventral nerve cord.
11.- Sensory system of tactile organs, taste buds, statocysts (in some), photoreceptor cells, and eyes with lenses (in some);
specialization of head region into differentiated organs, such as tentacles, palps, and eyespots of polychaetes.
12.- Asexual reproduction by fission and fragmentation; capable of complete regeneration.
13.- Hermaphroditic or separate sexes; larvae, if present, are trochophore type; asexual reproduction by budding in some; spiral cleavage and mosaic development.
14.- Excretory system typically a pair of nephridia for each segment; nephridia remove waste from blood as well as
15.- Respiratory gas exchange through skin, gills, or parapodia.
16.- Circulatory system closed with muscular blood vessels and aortic arches (“hearts”) for pumping blood, segmentally
arranged; respiratory pigments (hemoglobin, hemerythrin, or chlorocruorin) often present; amebocytes in blood plasma.
Integrated principles of zoology / Cleveland P. Hickman, Jr. ... [et al.]. – 14th ed. p. cm.