The ultrastructure of the capsule of the larval stages of Moniliformis dubius (Acanthocephala) in the cockroach Periplaneta americana

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Abstract
The capsule, which surrounds the acanthocephalan parasite Moniliformis dubius in its intermediate host, the cockroach, Periplaneta americana, has been studied in the electron microscope. Some observations were also made with the light microscope.The capsule surrounding the developing parasite (acanthella) taken from the haemocoel of its host consists largely of closely packed vesicles, which are empty of electron-dense contents.The vesicles arise from the surface membrane of the insect's haemocytes, which surround the parasite. Long tubular protrusions arise from the haemocyte's surface and, on separating, swell to form rounded vesicles.The capsule, which surrounds the later resting stage (cystacanth), has apparently been stretched by the growth of the parasite after haemocytic activity has declined. An amorphous, electron–dense component is also evident at this stage.We are grateful to Mrs A. Baker and Miss R. Jantunen for their technical assistance and to Dr A. C. S. Crossley for his comments on the work.