Nemertinea

NEMERTINEA

Ribbon worms

Thin, elongate, unsegmented worms with a soft elastic body that varies greatly in length as the worm stretches or contracts. A few are only 5mm long, many are a few centimeters in length and one species may stretch to 30m. The 650 species are variously red, brown, yellow, green or white (some are solidly coloured, others are striped or cross banded). Most species in this phylum are marine, living closely coiled beneath rocks, among algae, or in burrows (usually in shallow waters, but some are inhabitants of deep water and a few are pelagic. Ribbon (or Proboscis) worms are carnivorous, feeding on small crustaceans and worms captured by a long, flexible proboscis shot out from a special cavity in the head, unique to this phylum.

Characteristics:

1. Symmetry bilateral. Triploblastic.

2. Its body is slender, soft, highly contractile and unsegmented.

3. An eversible proboscis seperate from the digestive tract in a fluid-filled cavity, the rhynchocoel.

4. The digestive canal is straight and complete.

5. No coelom (body cavity) or respiratory organs. The body spaces are filled with mesenchyme.

6. Circulatory system with two main pulsating longitudinal vessels. There often exists additional vessels. There is no heart. There are two lateral excretory canals, branched with flame cells.

7. A nervous system is present with two anterior ganglia and a pair of lateral longitudinal nerves. Some species also have middorsal and midventral nerve trunks.

8. The sexes are usually seperate, with multiple pairs of gonads. Development is direct or via a pilidium larva. Asexual reproduction occurs by fragmentation.

9. Body wall with circular and longitudinal muscle bands in 2 or 3 layers.