Abstract
The cystophorous cercariae of Halipegus occidualis develop in rediae in the pulmonate snail, Helisoma anceps. Fully developed cercariae are shed from the snail and remain motionless on pond substrata. Manipulation of the caudal appendage by feeding microcrustaceans (copepods and ostracods) results in instantaneous eversion of the delivery tube within the cyst, through which the cercarial body is expelled into the crustacean hemocoel, wherein mesocercariae develop. Morphological and field evidence indicates that mesocercariae become metacercariae in the intestine of several species of larval odonate nymphs when they prey upon microcrustacea. The life cycle is completed when green frogs, Rana clamitans, ingest infected odonates. Phase contrast microscopy, scanning and transmission electron microscopy reveal that the tegument of the cercarial body is covered by microvilli, but internally lacks cellular and tissue differentiation. The apical portion of the delivery tube is covered by glandular papillae and the tip possesses a spined structure. Both represent modifications facilitating penetration and anchorage of the delivery tube through the crustacean intestine. While within the cyst, the body of the cercaria appears to be in a state of anhydrobiosis, an adaptation which presumably prolongs the life of this infective stage and enhances the temporal opportunity for transmission to microcrustacean hosts.
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