Cercaria

Cercaria, the technical name applied to an embryonic form of many flukes. In all the two-suckered flat parasitic worms (the Distomea division of Trematodes) the development is indirect or circumscribed. The eggs develop into embryos, which, instead of growing into adults, produce asexually one or more sets of intermediate forms. The final form, produced more or less directly from the embryo, is called a cercaria, and grows up into the adult fluke. It differs from the adult in having only rudiments of reproductive organs, in possessing eye-spots, and in being (except in one genus) equipped with a very movable tail. It is (1) born within some host, such as a water-snail; (2) leaves this and swims freely in the water; (3) becomes sluggish, and enters a second host, or fixes itself on some foreign body. In this state it loses its tail and encapsulates itself, and does not experience any further change till (4) it or its host is eaten by a vertebrate, within which the cercaria becomes an adult and sexual fluke. From the latter the embryos which eventually enter the first-mentioned host arise. Sometimes the life-history is simpler, but in all cases the cercaria is the form produced (generally indirectly) by the original embryo, and developing into the adult. See FLUKE.