The functional significance of scolex retraction and subsequent cyst formation in the cysticercoid larva of Hymenolepis microstoma

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Abstract
The cysticercoid type of larval development, involving the retraction of the scolex and the associated development of a cyst, is of widespread occurrence within cyclophyllidean cestodes, but previously its possible adaptive value has been unexplained. Scolex retraction in Hymenolepis microstoma is an active, muscular process which takes only about a minute to complete. Following its retraction the scolex develops adult structures. The cyst enclosing the scolex also differentiates further, involving the secretion of three layers of collagen fibres and the development of a myelin-like layer adjacent to the retracted scolex. The cells within the cyst, including the muscle, become condensed or are autolysed causing a decrease in thickness of the cyst and an increase in the proportion of fibrous tissue. The collagenous part of the cyst provides mechanical protection for the scolex during grinding by the teeth on entry into the mouse host. The myelin-like layer insulates the scolex from the low pH conditions of the stomach. Values of lower than pH 3·0 are lethal to newly excysted worms. The myelin-like layer loses its insulatory capacity following exposure to bile salts.