Sexually mature individuals, which live on the egg masses of crabs, are 20-70 mm long. Juvenile worms, found on the host gills, reach about 15 mm in length. The body is slender and yellowish, orange, pale reddish, rose pink or bright brick red in colour. Two small eyes near the anterior tip, occasionally these may have become fragmented (C. carcinophila-head).
The anal blood vessel commissure passes ventrally below the posterior end of the intestine. Ovaries are arranged in a single row on each side of the intestine.
The entire life cycle is spent on the crab hosts, although artificially removed specimens are capable of surviving for several weeks in clean sea water. In the British Isles recorded hosts are Carcinus maenas and Liocarcinus depurator . 40-50 or more nemerteans may be found upon a single host. Juvenile worms live among their host gill filaments, adhering to them by means of sticky mucous secretions. In a heavy infestation some of the gill plates may become damaged, blackened and degenerated. The nemerteans remain on the gills until the host produces eggs, then migrating to the egg masses where they attain sexual maturity and lay their own eggs in mucous tubes. The newly hatched nemerteans tend to remain among the host eggs until the end of the breeding period, after which they move to the gill chamber.
Found on galatheid, portunid and xanthid crabs from Europe to the Atlantic coast of North America. The related variety, Carcinonemertes carcinophila imminuta , which differs from the European form in length and in the size of the proboscis armature, is found in the Gulf of Mexico, the West Indies, Central America and Brazil.