Myzostomes are a bizarre group of worm-like animals that are all obligate symbionts, mainly with crinoids and other echinoderms. They range from species that are mobile and move freely over the host stealing food, to others that are endoparasitic and live inside the host. Some of these stimulate the host to form cysts or galls around them. While myzostomes occur worldwide they are commonest in reefal areas where crinoids are abundant. The body forms of the mobile species range from flattened ovals to disc-like forms and some are ridged or have elongate extensions that resemble the pinnules on the crinoid arms (Fig. 22.4C, D). Many are coloured to resemble their hosts and they may be quite difficult to spot on the crinoid. The mobile forms basically have a fused body and appear unsegmented, the head is not a distinct structure, there are no eyes and the mouth is a ventral or terminal structure. When dissected, it can be seen that the body is clearly segmented with the five segments separated by septa and bearing five pairs of appendages. Each appendage has a stout emerging hook-shaped chaeta, supported by a single aciculum. These structures are far less obvious in the endopara-sitic species. They are usually protandric hermaphrodites and are initially functional males that then become simultaneous hermaphrodites at maturity. Fertilisation is internal and fertilised eggs are spawned into the water column giving rise to planktonic, non feeding tro-chophores and they settle on a host within 5-8 days. The larvae have two bundles of long chaetae that resemble those found in the larvae of some polychaete families. How the endoparasitic and the gall-producing species reproduce still has not been documented.
The exact relationship of Myzostomes to other worm taxa is still being debated and while some studies suggest that they are embedded within the polychaetes, others support the concept of a separate group—the jury is still out.
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