Scanning electron microscopy of rediae, cercariae, metacercariae and adults of Mesorchis denticulatus (Rudolphi, 1802) (Trematoda, Echinostomatidae)

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Abstract
A detailed study of the surface topography of the echinostome digenean Mesorchis denticulatus (Rudolphi 1802) Dietz 1909 was carried out using the scanning electron microscope. The surface area of the redia is increased by microvilli, and ciliate sensory structures are common around the mouth. The cannibalistic behaviour of the rediae is demonstrated by rediae which have the posterior end and tail of extraredial cercariae protruding from the mouth. The partly devoured cercariae had occasionally — apparently as a defence reaction — covered their bodies with a thick layer of cystogenous material, which, however, does not protect them from being eaten. The surface area of the tegument of the youngest extraredial cercariae is increased by small microvilli or knobs with a presumed function in nutrient absorption. The knobs disappear when the cercariae are fully developed. Flattened spines occur peripherally in the suckers. Different types of ciliate structures are especially common anteriorly. The infective metacercariae, which occur in the gill filaments of euryhaline fishes, have conspicuous collar spines and smaller pointed spines occur on most of the body surface. The spines continue to grow in the experimental final host, chickens. The flattened spines of the oral sucker remain, whereas those of the ventral sucker disappear. The ventral area between the suckers is spineless.