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Characterization of nervous system-phylum Annelida

How is nervous system characterized in beings of phylum Annelida? How can one compare cephalization in the annelids to cephalization in platyhelminthes and nematodes?

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Annelids contain a nervous system made up of two ventral chords and one comparatively big nervous cell concentration in its anterior part resembling a primitive brain.

Nematodes contain an anterior neural ring joined to two neural chords, a ventral and a dorsal one, whereas in planarias (or platyhelminthes) there are just two small anterior “cerebral” ganglia from which neural chords divide. Cephalization in annelids therefore is more outstanding than in nematodes or in the flatworms.

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