A new species of marine triclad, Ectoplana undata n. sp., ectoparasitic on the horseshoe crab Tachypleus gigas, is described. Cocoons of triclads were found on T. gigas as well as on the horseshoe crab Carcinoscorpius rotundicauda; since from the latter species no triclads were collected, it remains unknown whether these cocoons are those of E. undata or of another species. The genus Ectoplana, though poorly defined, is here retained, for stated reasons, pending a revision of the Procerodidae.