Encystment of Cercariae

Observations were made to determine the effect of successive dilutions of sea-water on cercariae. There appeared to be no visible effects until the sea-water was diluted 50% by distilled water. At this dilution, the body of the cercaria expanded slightly, and within five minutes, individual cystogenous granules were page 23

Text-fig. 7.—Bucephalus longicornutus. Attachment and method of entry of the cercaria, into the fish intermediate host (semi-diagrammatic): Fig. A, adherence of furcae; Fig. B, attachment of tail-stem; Fig. G, U-shape assumed by body as the enterior end is brought into, contact with the skin of host; Fig. D, penetration of the anterior end of the cercaria and dislodgement of the tail stem and furcae from the body; Fig. E, complete penetration and encystment. For abbreviations see p. 9.

page 24 broken down into a large number of minute, vibrating particles. These particles were compacted together in the shapes of the individual cystogenous granules from which they were derived. Approximately one minute later, the particles were exuded through the cuticle and enveloped the body of the cercaria as a mucus secretion. Simultaneously, the tail stem and furcae become detached from the body.

With still further dilutions, the same effect was observed but it occurred more rapidly. After 100cc of distilled water had been added a fluid concentration corresponding approximately to that of fish body fluids was reached. The formation of the enveloping secretion around the body of the cercaria then occurred after approximately two minutes.

Precisely the same sequence as that already described was observed in cercariae treated with the body fluids from Tripterygion sp., when encystment took place in approximately three minutes.

These experiments strongly suggest that encystment of the cercaria is a phenomenon controlled osmotically. It is assumed that the body fluids of the cercaria are more or less isotonic with sea-water whereas those of the fish are hypotonic. Hence, when a cercaria penetrates the host, osmosis occurs which results in water uptake by the body of the cercaria. This appears to initiate the breakdown of the cystogenous granules into small particles which are eventually exuded through the cuticle to restore osmotic equilibrium and form the initial cyst wall.