The migration ofCercaria XBaylis (Strigeida) within the molluscan intermediate hostLymnaea stagnalis

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Abstract
The route of migration ofCercaria Xwithin the host snail,Lymnaea stagnalis, has been demonstrated by means of serial sections and a method of numerical assessment of the distribution of cercariae in the various tissues and organs of the body.The main pathway along which migrating cercariae travel is the blood system, which differs from the route in the fish second intermediate host. The cercariae enter the main visceral haemocoel by rupture of the wall of the digestive gland and from there enter the main venous vessels deploying through the rectal sinus, sub-renal sinus, lung and heart. The cercariae travel from the heart via the mantle arteries to localize in the mantle sinuses prior to emergence.The inner surface of the mantle, and in particular its leading edge, is the main site of emergence, and escape of cercariae is, in the final stages at least, an active process. It is suggested that passage of parasites in the blood stream is partly active.Migration and subsequent emergence is probably facilitated by the activity of the oral cap of spines, the general activity of the cercaria and the activity of the oral and ventral suckers.A tissue response to the presence of cercariae in the blood vessels and tissues of the mantle is evident, the cells responsible for this being, possibly, actively phagocytizing amoebocytes of the mollusc's blood. The swellings in the epidermis of the mantle in heavy infections may be the result of an accumulation of these cells.Grateful acknowledgement is expressed to Professor J. Brough for the facilities and encouragement provided during the course of this study. This work was carried out during the tenure by A. J. Probert of a D.S.I.R. Research studentship.