ABSTRACT
Turbellarians are among the commonest entosymbiotes of echinoderms and occur in all major taxa. The intestine and coelom are the principal habitats; others are the tube feet and gonads. Predation of co-symbiotic protozoa is the primitive nutritional pattern but most of the turbellarians are tissue-feeding parasites; some depend on the hosts for digestive enzymes as well as food. Many possess haemoglobins giving them preferential access to the available oxygen. The turbellarians cause no obvious ill-effects apart from a single instance of parasitic castration, the hosts’ capacities for regeneration allowing tissue repairs to keep pace with the damage caused by the parasites’ feeding activities.
