CURRENT KNOWLEDGE ABOUT MYZOSTOMES
Eeckhaut, I.
Laboratoire de Biologie Marine, Faculté des Sciences, Université de Mons, Bt. Sciences de la Vie, Av. du Champs de Mars, 7000, Mons, Belgium
The taxon Myzostomida includes 130 species that are obligatory symbionts of echinoderms, mainly of crinoids (a few live on or in ophiuroids and asteroids). It is generally thought that myzostomes were already infesting echinoderms 300 Ma since traces of parasitic activities, similar to those produced by contemporary myzostomes, have been observed on fossil crinoids. The long coexistance between echinoderms and myzostomes has led, in the latter, to the acquisition of a morphology perfectly adapted to a symbiotic way of life. Myzostome morphology and behaviour are unique amongst polychaetes. They are so different that zoologists often have considered the Myzostomida as a taxon separate from and the same rank as the Polychaeta. A summary of the recent studies made on myzostomes will be presented. The main subjects that will be treated are: (1) their symbiotic way of life and the specificity they show towards their echinoderm hosts (2) the morpho-function of their organs and systems, especially their digestive and reproductive systems (3) their phylogeny and taxonomy.