Paravortex karlingi sp. nov. from Cerastoderma edule L., in Britain

Paravortex karlingi sp. nov. collected from the intestine of the bivalve mollusc Cerastorderma edule from the Ythan Estuary, N. E. Scotland, and elsewhere, is distinguished from a closely related species, P. cardii, also occurring in this host, on the basis of differences in habitat occupied by the two species as well as behavioural and morphological differences. P. karlingi is smaller, has fewer embryos in the gravid adult and shows a different behaviour pattern when released from the host intestine. It is also negatively phototactic whereas P. cardii is initially positively phototactic, only later becoming negatively phototactic. The occurrence of both species in Britain is briefly described.Abbreviations bc brood capsules - c core of indigestible particles - ca common atrium - ch chamber - e eye - fa female atrium - fol fibre optic light - gc gut caecum - gp genital pore - j juvenile worm - m mouth - o ovary - oe oesophagus - p pharynx - pe penis - sg shell gland - sv seminal vesicle - t testis - u uterus - vg vitelline gland - w window