ULTRASTRUCTURE OF THE DIGESTIVE AND PROTONEPHRIDIAL SYSTEMS OF THE METACERCARIA OF EUCLINOSTOMUM-MULTICAECUM

The structure of the digestive tract of Euclinostomum multicaecum (Dig enea: Euclinostomatidae) is unusual, comprising several main lateral c aeca which extend posteriorly and further divide, giving rise to numer ous smaller branches which are widely distributed throughout the fluke . These multicaeca presumably promote nutrient absorption during rapid and prolonged feeding directly following excystment. The caecal wall consists of a syncytial gastrodermal epithelium, bearing loop-like lam ellae which extend into the lumen and enclose spherical inclusion bodi es and presumably also, increase the absorptive surface area. There wa s no evidence of endo- or exocytosis, nor were lysosomes, phagosomes o r residual bodies observed. The gut caeca are supported by a fibrous b asal lamina and an underlying layer of muscle fibres, while parenchyma l cells occupy much of the extracellular space. The protonephridial sy stem resembles that observed in other digeneans consisting of flame ce lls and collecting ducts which join to form a bladder that opens exter nally through a single excretory pore. The syncytial epithelium of the collecting ducts is elevated by numerous lamellae while the basal lam ina is highly infolded. The luminal contents of these ducts comprise b ar-shaped crystalline structures, lipid droplets and electron-dense in clusion bodies. The excretory system is supported by a network of musc le fibres and parenchymal cells.