Nemertea
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The phylum Nemertea (also Rhynchocoela, Nemertina, Nemertinea or Nemertini) contains the ribbon worms or proboscis worms, which are a group of unsegmented marine invertebrates. There are about 1200 species distributed among two classes: Anopla and Enopla. The Nemerteans are named for Nemertes, one of the Nereids, sea-nymphs of Greek mythology.
Nemertean worms are distinguished by their possession of an eversible proboscis, which in one major group of species is armed with a poisonous stylet. The proboscis is used in the capture of prey. Ribbon worms are carnivorous, feeding mostly on small invertebrates, but these can be several times the normal thickness of the worm and are often swallowed whole (much as snakes do). Those without a stylet wrap the proboscis around prey; those with a stylet use it to make a puncture wound through which neurotoxin is delivered. Most Nemertea live in shallow seas and coastal waters, with a few inhabiting freshwater or damp soil. They range in length from 1 mm to as much as 30 metres (100 feet) in the case of the bootlace worm, Lineus longissimus. Some texts quote lengths of up to 60 m, in which case, L. longissimus would be the longest animal alive (female blue whales reach about 34 m).
The proboscis lies in a fluid-filled coelom, which is the reason for one zoologist naming the phylum Rhynchocoela (but most specialists use the older name, Nemertea).They also possess a closed circulatory system, which may use hemoglobin, and a one-way intestine. Members of the phylum Nemertea have often been considered as modified flatworms, but their placement has been extensively revised in recent years, and they are now placed in the Lophotrochozoa, along with so-called more advanced animals, such as molluscs and annelids. Similar to flatworms, Nemertea are hermaphroditic and reproduce sexually.
[edit] References
- Thollesson, Mikael & Norenburg, Jon L. (2003). Ribbon worm relationships: a phylogeny of the Phylum Nemertea. Proceedings of the Royal Society of London Series B 270: 407–415.
