RECENTLY, the opinion has been expressed that the endoparasitic monogenetic trematode Dictyocotyle coeliaca is simply a cœlomic form of the common cloacal parasite Calicotyle which has entered, through the abdominal pores, the body cavity of certain species of Raia 1. The normal posterior adhesive organ (‘opisthaptor’) was thought to be lost, and its central boss to regenerate a ‘pseudohaptor’. The evidence given in support of this supposed ontogenetic migration was: (1) the finding, in an abdominal canal, of a trematode which had lost its opisthaptor; (2) the recovery, from the cloaca, of six isolated opisthaptors each with a central perforation which matched the boss of the mutilated specimen.