Broad, flattened and leech-like species with a typically elongate oval shape. Sexually mature individuals reach lengths of 20-40 mm and are 8-15 mm wide. Immature worms are translucent white or pale grey in colour, with no evidence of gonads. In ripe males, however, the testes are a rich cream colour, often with a pink or rosy tint, and gravid females contain dark olive-green to deep yellowish-green ovaries. Occasional specimens may be dark brown, chocolate-brown or almost black due to an infection with the sporozoan parasite Haplosporidium malacobdellae . A heavy infection leads to parasitic castration of the nemerteans. Rhynchocoel for the full body length. Nephridiopores of the excretory system open ventrolaterally. Lateral nerve cords unite above the intestine just in front of the anus. Gonads are numerous and closely packed (M. grossa-detail). The anus is terminal, but above the sucker.
The only North Sea representative of this unusual nemertean genus. It possesses a low degree of host specificity and has been recorded from 23 species of bivalve molluscs. British hosts reported are Acanthocardia echinata , Arctica islandica , Cerastoderma edule , Hiatella arctica , Mya truncata , Pecten sp.,Venus casina and Zirfaea crispata . A solitary record exists of the nemertean being found in an unspecified fish. The maximum number of nemerteans obtained from a single host is five and such multiple infections are invariably composed of young individuals. Double infections are not uncommon, but more usually only a single worm occurs in any one host.
Widely distributed on the coasts of Europe and on the Atlantic and Pacific coasts of North America.