Phylum Annelida

Annelida (Latin annulus, a ring) were the first animals to evolve true segmentation, and the phylum's name reflects the ringed appearance this gives their body. Annelids may be aquatic or terrestrial, and while most are free-living, some are parasitic. They are triploblastic, bilaterally symmetrical, schizocoelomate animals with organ-system organisation.

The defining feature of the phylum is metamerism: the elongated body is divided externally and internally into repeating segments (metameres), separated internally by partitions called septa. The fluid-filled coelom within each segment acts as a hydrostatic skeleton, and works together with longitudinal and circular muscles in the body wall to power locomotion. Aquatic annelids such as Nereis carry paired lateral appendages called parapodia for swimming, while other annelids use small chitinous bristles called setae for traction, sometimes combined with the body-wall muscles for burrowing or crawling locomotion (as in the earthworm, Pheretima). …