The body of the preserved specimens may reach a length of 7 mm and width of 4 mm. The outline of the body is broad and leaf-like; the head is rounded. The animal lacks both eyes and pigment. The posterior end with a flat and well-delimited attachment zone. The internal organs are usually visible through the body wall; the small pharynx and well-developed anterior ramus of the intestine are particularly noteworthy.
An unarmed penis forms a broad cone that fills the male atrium. The vasa deferentia unite anterior to the penis and form a common vas deferens that enters the penis bulb and opens, via an intrapenial papilla, into a wide ejaculatory duct. The bursa copulatrix, usually greatly reduced, is behind the penis and opens via the bursal canal into the roof of the common atrium, which is sharply delimited from the male atrium. The oviducts open separately into the terminal part of the bursal canal, or into the bursa itself; extensive shell-glands empty into the bursal canal beneath them (M. parasitica-detail).
Known only as an ectoparasite on three species of skates; never found free-living.
Reported from northern Europe and from the North Atlantic off Newfoundland, Canada.