Cercariae are motile larvae with a tail which are produced asexually by the redia (see lifecycle).
The cercariae are a type called cystophorous. The tail is used in at least three ways to assist the cercaria’s passage:
- it is enlarged and when swimming freely, the body of the cercaria is withdrawn within it
- the narrow posterior part of the tail is forked, and beats vigorously to keep the cercaria afloat
- when ingested by copepods or larvae of barnacles and crabs, the cercariae turn part of their tail inside out to produce a delivery tube which
- passes up into the predator’s gut and perforates the gut wall and,
- provides a tunnel for the body of the cercaria to pass along into the invertebrate’s body-cavity
- in the body-cavity the larva develops into a metacercaria -a stage where the larva grows, but does not usually become mature
The tail is in two parts:
- a spherical part which contains a coiled delivery tube and into which the body can be withdrawn
- an elongate forked swimming or flotation organ
Metacercariae, in relatively large crustaceans, can grow large enough to infect fish when the crustacean is swallowed.
In smaller crustaceans the metacercariae may first need to get into a larger host - such as an arrow worm - to grow large enough to infect fish. In some cases these metacercariae produce eggs.