Cercaria vaullegeardi Pelseneer, 1906 (Digenea: Hemiuridae); the infection mechanism

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Abstract
SUMMARY: The inoculative mechanism whereby cystophorous cercariae infect the copepod 2nd intermediate host is described for the first time in Cercaria vaullegeardi. Experimental infections of the harpacticoid copepod Tigriopus brevicornis are recorded and the infection process is related to the ultrastructure of the cercaria and to the feeding mechanics of harpacticoids. The cystophorous tail of C. vaullegeardi is shown to be a device whose shape and construction ensure that the cercarial body is neither damaged by copepod mouthparts nor swallowed, but reaches the host haemocoel during the initial stages of feeding. The cystophorous tail consists essentially of a caudal cyst, into which the cercarial body retracts at encystment, and various appendages including a delivery tube. The infection mechanism is triggered when the copepod mandibles bite the narrow posterior caudal cyst beak, when the delivery tube everts into the mouth and penetrates the dorsal mid-gut wall. The cercarial body, lubricated by tegumental secretions, passes simultaneously through the delivery tube and is inoculated into the host haemocoel.