Distinguishing Features
No single feature distinguishes Annelids from other invertebrate phyla, but they have a distinctive combination of features. Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called annuli and internally by septa ("partitions") at the same points, although in some species the septa are incomplete and in a few cases missing. Most of the segments contain the same sets of organs, although sharing a common gut, circulatory system and nervous system makes them inter-dependent. Their bodies are covered by a cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen and does not molt – on the other hand arthropods' cuticles are made of the more rigid α-chitin, and molt until the arthropods reach their full size. Most annelids have closed circulatory systems, where the blood makes its entire circuit via blood vessels.
| Annelida | Recently merged into Annelida | Closely-related | Similar-looking phyla | |||
|---|---|---|---|---|---|---|
| Echiura | Sipuncula | Nemertea | Arthropoda | Onychophora | ||
| External segmentation | Yes | no | no | Only in a few species | Yes, except in mites | no |
| Repetition of internal organs | Yes | no | no | Yes | In primitive forms | Yes |
| Septa between segments | In most species | no | no | No | No | No |
| Cuticle material | collagen | collagen | collagen | none | α-chitin | α-chitin |
| Molting | Generally no; but some polychaetes molt their jaws, and leeches molt their skins | no | no | no | Yes | Yes |
| Body cavity | Coelom; but this is reduced or missing in many leeches and some small polychaetes | 2 coeloms, main and in proboscis | 2 coeloms, main and in tentacles | Coelom only in proboscis | Hemocoel | Hemocoel |
| Circulatory system | Closed in most species | Open outflow, return via branched vein | Open | Closed | Open | Open |
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