Parorchis acanthus is a Trematode described by Nicoll (1907) from the bursa Fabricii and rectum of the Herring Gull, Larus argentatus; in 1906 he first described it as Zeugorchis acanthus. Originally found at St. Andrews, it is now known to be common at Millport and also occurs in gulls from the Northumberland coast, and probably is abundant elsewhere. Nicoll found it once in the Common Gull, Larus canus. In 1907 (Lebour, 1907) one of the present writers described a cercaria occurring in rediæ from Purpura lapillus. This was named Cercaria purpurœ sp. inq., and afterwards identified with young stages of Parorchis acanthus in the Herring Gull (Lebour, 1914). It was thus shown that the first host of this Trematode is Purpura lapillus and the final host the Herring Gull, but the intermediate host was unknown. Localities for the cercaria were Loch Ryan, Wigtownshire; Budle Bay, Fenham Flats and Cullercoats in Northumberland; Robin Hood's Bay in Yorkshire; and Millport. Adults occur at St. Andrews, Northumberland coast and Millport, as already stated.

The Cercaria appeared to be closely related to Echinostomum cercariæ, and it was suggested (Lebour, 1914) that the intermediate host would probably be some marine bivalve, as it is often so with Echinostomum (e.g. E. secundum in Mytilus edulis and Cardium edule, E. leptosomum in Scrobicularia tenuis). This suggestion now proves to be correct, and we are able to show that the intermediate host of Parorchis acanthus is Mytilus edulis or Cardium edule.

On August 13th, 1921, at the Millport Station some larval cercariæ were noticed by Elmhirst swimming in a small glass aquarium, which contained a collection of Purpura lapillus, Cardium edule and Mytilus edulis.