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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.[5][9] 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[5] and does not molt[15] - on the other hand arthropods' cuticles are made of the more rigid α-chitin,[5][16] and molt until the arthropods reach their full size.[17] Most annelids have closed circulatory systems, where the blood makes its entire circuit via blood vessels.[15]

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12y ago

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