annelida

• from Latin, “annelus” which means “little ring.”
• Unique annelid head and paired epidermal setae present (lost in leeches, sipunculans, and some echiurans); flaps called parapodia can be present in the ancestral condition
• bilaterally symmetrical, metameric often with distinct head; metamerism reduced or lost in some, especially echiurans and sipunculans
• Coelom (schizocoel) well developed and divided by septa in all segmented forms, except in leeches; coelomic fluid functions as hydrostatic skeleton
• Triploblastic body
• The annelid body typically has a two-part head, composed of a prostomium and a peristomium, followed by a segmented body and a terminal Most members have bodies divided into similar segments (also called metameres) arranged in linear series and externally marked by circular rings called annuli. portion called the pygidium bearing an anus
• Digestive system complete and not segmentally arranged
• Asexual reproduction by fission and fragmentation; capable of complete regeneration
• Hermaphroditic or separate sexes; larvae, if present, are trochophore type; asexual reproduction by budding in some; spiral cleavage and mosaic development
• ERRANTIA - Marine annelids; parapodia with prominent lobes supported by internalized chaetae and ventral cirri: palps well- developed.

  
• SEDENTARIA - Marine, freshwater, and terrestrial annelids; parapodia with reduced lobes or parapodia lacking; setae associated with the stiff body wall to facilitate anchoring in tubes and burrows; palps reduced.
• Annelids belong to the protostome group