Developmental changes in the tegument of four microphallid metacercariae in their second (crustacean) intermediate hosts

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Abstract
The morphology of the tegument of four microphallid metacercariae from the stage of invasive cercariae to their maturation as encysted metacercariae inside their crustacean second intermediate hosts is described. The tegument of the metacercariae developed surface lamellae and projections which, along with coated vesicles in the surface syncytium, indicated that the tegument had an absorptive function. The disappearance of secretory granules from the tegument at the same time as the appearance of the first cyst wall suggested that the tegument had a role in primary cyst production. Following this, the metacercariae continued to grow and seemingly retained their absorptive ability. The tegument was also involved in the transport of material into the perimetacercarial lumen prior to its eventual inclusion in the developing inner cyst layers. It appeared that this material originated in tegumental cells located amongst the parenchymal cells beneath the tegumental syncytial layer. On completion of the secondary cyst layers there was a gradual degeneration of structures associated with absorption and a progressive accumulation of dense discoid granules traceable to underlying tegumental cells. All four microphallid species (Maritrema arenaria,M. subdolum,Levinseniella brachysomaandMicrophallus claviformis) demonstrated the same developmental pattern but the period spent in each stage differed depending on the time spent migrating to encystment sites. The pattern of tegumental development described is thought to be applicable to all microphallid metacercariae and possibly to other metacercariae which undergo growth and development in their second intermediate hosts.

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