Occurrence of the symbiotic Rhabdocoele flatworm Paravortex gemellipara in Chesapeake Bay and Gulf of Mexico molluscs, with notes on its biology and geographic range.

Paravortex gemellipara is reported from its type host Geukensia (=Modiolus) demisa and from two new hosts Ischadium (=Brachidontes) recurvum and Mytilopsis (=Congeria) leucopheata. Its range on the Atlantic seaboard is extended from Woods Hole, Massachusetts south to the lower Chesapeake Bay. The worm is reported from the Gulf of Mexico at Galveston Bay, Texas. The presence of dermal protruberences and posterior adhesive areas bearing mucous strands is recorded. Mature worms were found living in the digestive tracts of their host, contrary to the findings of previous investigators who reported them from the gill and kidney.