On the morphology and life-history ofLepidapedon elongatum(Lebour, 1908) Nicoll, 1910(Trematoda, Lepocread iidae)

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Abstract
The cercaria of Lepidapedon elongatum develops in rediae in Onoba aculeus (Gould) (Prosobranchia, Rissoidae). The cercaria differs from all known lepocreadiid cercariae in being non-ocellate and in having a small, nearly spherical tail. The cercaria has penetration glands but no stylet. Encysted meta-cercariae of L. elongatum were found in polychaetes and molluscs from the northern Øresund, Denmark. Under experimental conditions cercariae encysted in brittle stars too. Metacercariae develop to infectivity during four weeks in polychaetes, whereas they never or rarely become infective in molluscs and echinoderms. The Atlantic cod Gadus morhua L. is the final host in Danish waters, in Greenland waters it also occurs in G. ogac Richardson. Rediae, cercariae, metacercariae and adults of L. elongatum were examined by means of light microscopy and stereoscan electron microscopy. The tegument of undeveloped extraredial cercariae is covered by short microvillus-like structures. The long microvilli which encircle the oral and ventral suckers of fully developed cercariae are lost a few days after encystment. The metacercariae do not grow, but the shape of the spines changes gradually from being pointed to being flattened and multi-pointed. The larval stages and the life-cycle of L. elongatum are compared with those of other lepocreadiids. L. microcotyleum Odhner is regarded as a synonym of L. elongatum. L. elongatum has been recorded from several families of littoral and deep-water fishes from throughout the world, but more than one species are obviously involved.