Formation of the capsule around Filicollis anatis (Acanthocephala) in its intermediate host.

Morphogenesis of the capsule around larvae of Filicollis anatis was studied by electron microscopy. The capsule is formed at early, middle, and late acanthella development. The inner layer is formed at the late acanthella stage apparently from material secreted via a system of hypodermal canals. The central layer of the capsule is formed mainly of detached microvilli of the acanthella. The source and mechanism of formation of the outer layer, which appears during the early acanthella stage, are unclear.