Locomotion of the cercaria ofParorchis acanthus,Nicoll and the ultrastructure of the tail

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Abstract
The cercaria ofParorchis acanthusswims by a series of spasmodic ventral flexions of the body moving with the dorsal surface of the body foremost. The wave of contraction continues along the tail which therefore moves dorso-ventrally. Creeping movements precede cyst formation. The structure of the tail is admirably suited to swimming movements and to rapid shortening and lengthening. The wall is provided with circular and longitudinal muscles and abundant mitochondria. The centre of the tail contains fluid in which is a network of large myoblasts the processes of which overlap and probably slide over one another during the tail movements. Glands opening into the terminal tail invagination secrete an adhesive substance consisting of mucoprotein and glycoprotein which together with the vesicular secretion around the aperture enables the tail tip to attach itself to the substratum.I would like to express my sincere thanks to Dr I. ap Gwynn of this department for invaluable help in the preparation of the electron-micrographs and Mr R. A. Moore and Mr M. C. Bibby for technical assistance.