Larval Trematodes 
The Life History of Parorchis acantlius Nicoll. 
Parorchis acanthus (Nicoll 1907 a) is common in the cloaca of the 
Herring Gull at Millport and also at St Andrews where Nicoll got his 
type specimens. Among the adults in the Millport Gulls were several 
young forms which correspond so closely with the cercariae Avhich 
I have named Cercaria purpurae from Purpura lapillus that there can 
be no doubt that it is the same species. 
This cercaria is one of our commonest larval Trematodes and occurs 
in extraordinary abundance completely filling the spine of the mollusc 
when present and giving a curious swollen pale yellowish appearance to 
the digestive gland, so that it can at once be recognised. Some years 
ago Dr Nicoll told me he thought that this was the larva of Paroixhis 
acanthus but it has not been possible to prove this until now. The 
intermediate host is still unknown in spite of constant searchings on my 
part in the animals living side by side with Purpura. I have 
found these cercariae in Purpura in the following localities : Fenham 
Flats, Budle Bay and Cullercoats in Northumberland (Fenham 
Flats being muddy ground with few rocks, Budle Bay rocky 
surrounded by sand, Cullercoats very rocky), Robin Hood’s Bay in 
Yorkshire (rocky), the shores of Loch Ryan on the west coast of 
Scotland (muddy and sandy ground with stones) and on the coast round 
about Millport in Cumbrae (rocks surrounded by. muddy sand). It 
occurs therefore in various habitats. The Herring Gull {Larus argentatus) 
is a constant feeder in all these places. 
The animals almost always to be found with Purpura are Sea 
Anemones {Actinia inesemhryantheniuni). Green Crabs {Carcinus moenas), 
Rock Barnacles (Balanus balanoicles), Limpets {Patella vulgata), Peri¬ 
winkles {Littorina littorea, also frequent!}^ L. rudis and L. ohtwrata) and 
very often Mussels {Mytiius edulis and Modiola modiola). All these 
I have examined for the encysted stages but with no success. 1 have 
also examined various worms from the mud or sand below the rocks and 
MyOj arenaria from Millport and tSaxicava rugosa from Robin Hood’s 
Bay, all without success. The cercaria has a p^owerful tail which points 
to its leaving its first host and swimming to its second so that it seems 
unlikely that it ensysts in the Purpura or in an animal that feeds on 
Purpura. 1 should expect to find it in some animal living near and most 
probably in a mollusc as the cercaria closely resembles an Echinosto- 
mum cercaria, and those members of this genus whose complete life 
histories are known encyst in moWnscs {E, secunduui in Mytiius edulis, 
