Phylum Annelida

Phylum Annelida takes its name from the Latin annulus, a little ring, referring to the externally visible rings, or segments, into which the whole body — both the outer body wall and, correspondingly, many of the internal organs — is divided; this repetitive segmental plan is called metameric segmentation, and it appears for the first time, in the sequence of phyla studied in this chapter, in the annelids. Annelids may be aquatic (marine or fresh-water) or terrestrial (living in damp soil), free-living or, less commonly, parasitic. The body is bilaterally symmetrical, triploblastic, and — importantly — truly coelomate: the body cavity is completely lined by mesoderm on both its outer (parietal) and inner (visceral) faces, so that this phylum is the first, in the classification sequence usually followed, to possess a true coelom, and organisation is at the organ-system level.

Physiologically, annelids possess a closed circulatory system, in which blood is confined throughout to vessels rather than bathing the tissues directly (unlike the open system of most arthropods and molluscs); the paired, segmentally arranged excretory organs are called nephridia, and they perform the combined job of excretion and osmoregulation. A well-developed nervous system consists of a nerve ring (a pair of cerebral ganglia connected by nerves that encircle the anterior gut) continuing backward as a solid, ventral nerve cord with segmental ganglia. …