On the histochemistry and ultrastructure of the tegument and associated structures of the cercaria ofZoogonoides viviparusin the first intermediate host

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Abstract
The tegument of the cercaria of Zoogonoides viviparus has been studied in detail using the scanning and transmission electron microscopes. The greatest part of the cercarial body is provided with long, recurved spines. The anterior end of the cercaria is extremely specialized. A circle of microvilli, about 5 µ long, is found around the mouth. Numerous sensory structures composed of a central cilium and nervous tissue penetrating the tegument are found throughout the body, but in large numbers on the anterior end near the mouth. Apical to the mouth the invaginated stylet and the openings of the penetration glands are found. At least three distinct types of cells connect with the syncytial tegument. One type is similar to tegumentary cells previously described from most adult trematodes. In addition, what appear to be two types of cystogenous cells are found. The secretory products of these cells are released into the tegument. The tegument covers the external surface as well as the pharynx and oesophagus. The two caeca are lined with cells with long, lamellate projections. The tegument and associated cells reacted positively for acid and alkaline phosphatases, non-specific esterase and aminopeptidase, whereas none of these enzymes were found in the penetration glands. The tegument and spines are covered with a 1 µ-thick filamentous coat containing acid mucopolysaccharides. This mucous coat probably protects the cercaria during the migration in the snail tissue and during the free-living aquatic phase. Only the host cells in close contact with the migrating cercariae are injured by the parasite, as revealed by observations in the transmission electron microscope.