The life-cycle ofProsorhynchus crucibulum(Rudolphi, 1819) Odhner, 1905, and a comparison of its cercaria with that ofProsorhynchus squamatusOdhner, 1905

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Abstract
The adult ofP. crucibulumis redescribed to form a basis for comparison with the cercaria and metacercaria. Particular emphasis is given to the structure and function of the rhynchus, digestive system and reproductive system. Some features, notably prepharyngeal glands and spematophores, have not previously been recorded in this species.The sporocyst and cercaria which were found inMytilus edulisare described for the first time. Out of 6907 mussels examined from Borth Rocks, Cardigan Bay, 0·26% were infected withP. crucibulumand 0·06 % withP. squamatus. The cercaria of the latter is redescribed on a comparative basis with that ofP. crucibulum, to avoid confusion. The two cercariae are clearly distinguished by three features – flame-cell formula, shape of the excretory vesicle, and structure of the tail stem (this being bilobed inP. crucibulumand trilobed inP. squamatus). The significance of the two types of tail in securing attachment to the second intermediate host is discussed.Metacercariae ofP. crucibulumwere obtained experimentally in ‘O’ group specimens ofScophthalmus maximus. Three other species of fish, namelyLimanda limanda, Onus mustelusandGobius minutuswere also successfully infected. The metacercariae did not develop in the following fish:S. rhombus, Pleuronectes platessa, Cottus bubalisandBlennius pholis. Possible mechanisms which might account for host specificity within Pleuronectidae andScophthalmusare suggested.The development of the metacercaria in the connective tissues and musculature ofS. maximuswas followed over a period of 10 months. Within 1 month of infection the hyaline cyst of parasitic origin breaks down, enabling the parasite to feed directly on the host tissues. This feature is thought to account for the advanced development of the metacercaria, which 1 month after infection could clearly be identified with the adult on the basis of comparative morphology, and at 2 months resembles the adult in every respect except the development of the vitellaria. Egg production only occurs within the intestine ofConger conger, the definitive host.