Characteristics of Phylum Annelida

 

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

      skeleton.

 

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

       from coelom.

 

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.

 

 

Referense:

Integrated principles of zoology / Cleveland P. Hickman, Jr. ... [et al.]. – 14th ed. p. cm.

 

© 2014 - 2019​

Thomas Heran Arce

Underwater photography,  Chile

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