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Palaeonemertea may be a para- or polyphyletic Order, consisting of 3 to 5 clades and totalling about 100 species. These worms have several apparently simple features and, as their name suggests, they are often considered to be the most primitive nemerteans. The primary body-wall musculature consists of an outer circular stratum overlying a longitudinal stratum. The group includes genera such as Cephalothrix in which the nerve cords are inside the body-wall longitudinal muscle, and Tubulanus, in which the nerve cords are between the outer circular muscle and the epidermis. Tubulanids are commonly encountered in rocky areas of intertidal zones in the northern hemisphere. They are often bright orange or have very distinctive banding and or stripes and can be many metres long, although only a few millimetres thick.
[edit] References
- Thoney, Dennis A. and Schlager, Neil (eds.) (2004) "Anopla (Anoplans)" Grzimek's Animal Life Encyclopedia: Volume 1 - Lower Metazoans and Lesser Deuterostomes (2nd ed.) Thomson-Gale, Detroit, pp. 245-251 ISBN 0-7876-5777-8;
- Gibson, Ray (2002) The Invertebrate Fauna of New Zealand: Nemertea (Ribbon Worms) (NIWA Biodiversity Memoir No. 118) National Institute of Water and Atmospheric Research, Wellington, New Zealand, ISBN 0-478-23249-7 ;
- Sundberg, Per; Turbeville, J. McClintock and Lindh, Susanne (2001) "Phylogenetic Relationships among Higher Nemertean (Nemertea) Taxa Inferred from 18S rDNA Sequences" Molecular Phylogenetics and Evolution 20(3): pp. 327–334;
