An Adult Digenetic Trematode From an Invertebrate Host:Proctoeces Subtenuis (Linton) From the Lamellibranch Scrobicularia Plana(Da Costa)

The digenetic trematodeProctoeces subtenuis(Linton, 1907) is recorded from the kidney of the lamellibranchScrobicularia plana(da Costa). At Chalkwell in Essex all of nearly a thousandS. planaexamined were found to be infected, as were three of approximately 150S. planaat Whitstable in Kent. The average number of parasites per host was between four and five. NoProctoeces subtenuiswere found inScrobicularia planaexamined from other localities in Essex, Devon, Wales and Suffolk. The environmental conditions of a parasite in the lamellibranch kidney are discussed. It was found that the kidney fluid surrounding the parasites varies in osmotic pressure. The anatomy of the parasite is described. The excretory bladder has a cellular wall, thus disagreeing with the inclusion ofProctoeces subtenuisin La Rue's superorder Anepitheliocystidia. The red colour of the parasite is due to a native haem pigment. The egg capsules are hardened by a quinone-tanning mechanism, but incubation with catechol, protocatechuic acid, and dopa failed to reveal the presence of a phenol-oxidase. The species ofProctoecesare reviewed, and some revisions suggested.P. magnorusManter, 1940 is considered a synonym ofP. subtenuis, and the synonymy ofP. erythraeusOdhner, 1911 withP. subtenuisis confirmed. The differences betweenP. insolitus(Nicoll, 1915) andP. subtenuis, and betweenP. maculatus(Looss, 1901) andP. subtenuis, require re-examination. It is considered thatP. macrovitellusWinter, 1954 should be excluded from the genusProctoeces. A definitive host list ofProctoecesspp. is given. The adult nature ofP. subtenuisfromScrobicularia planais discussed, and it is suggested thatProctoeces subtenuisin the Thames estuary shows an abbreviated life cycle restricted to invertebrate hosts.