Nemertea

Article By:

Gibson, Ray School of Biological and Earth Sciences, Liverpool John Moores University, Liverpool, United Kingdom.

Humes, Arthur G. Department of Biology, Boston University, Boston, Massachusetts.

Jennings, J. B. Department of Pure and Applied Zoology, University of Leeds, Leeds, United Kingdom.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.588300

A phylum of bilaterally symmetrical, nonsegmented, ribbonlike worms, also referred to as the Nemertinea or Rhynchocoela. With one exception (the unusual New Zealand species Arhynchonemertes axi), nemerteans have an eversible proboscis housed in a fluid-filled chamber, the rhynchocoel, which is situated above the gut. Nemerteans have a complete digestive tract with separate mouth and anus. Although nemerteans have long been regarded as acoelomate invertebrates (with no body or coelomic cavity), recent studies have shown that the rhynchocoel is ultrastructurally similar to the coelom of mollusks and other coelomate groups. Mesenchyme (or parenchyma) and the various muscle fiber layers of the body wall fill the area between the ciliated epidermis and the cellular lining of the digestive tract and other body organs (such as the brain and the reproductive and excretory systems). See also: Animal symmetry

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