Larval Development With Transitory Epidermis inParanemertes peregrina and Other Hoplonemerteans

摘要

We describe development of the hoplone-mertean Paranemertes peregrina from fertilization to juve-nile, using light, confocal, and electron microscopy. Wediscovered that the uniformly ciliated lecithotrophic larva ofthis species has a transitory epidermis, which is graduallyreplaced by the definitive epidermis during the course ofplanktonic development. The approximately 90 large mul-ticiliated cleavage-arrested cells of the transitory larval epi-dermis become separated from each other by intercalatingcells of the definitive epidermis, then gradually diminish insize and disappear more or less simultaneously. Rudimentsof all major adult structures—the gut, proboscis, cerebralganglia, lateral nerve cords, and cerebral organs—are al-ready present in 4-day-old larvae. Replacement of the epi-dermis is the only overt metamorphic transformation oflarval tissue; larval structures otherwise prefigure the juve-nile body, which is complete in about 10 days at 7-10 °C.Our findings on development of digestive system, nervoussystem, and proboscis differ in several ways from previousdescriptions of hoplonemertean development. We reportdevelopment with transitory epidermis in two other species,review evidence from the literature, and suggest that thisdevelopmental type is the rule for hoplonemerteans. Thehoplonemertean planuliform larva is fundamentally differ-ent both from the pilidium larva of the sister group to theHoplonemertea, the Pilidiophora, and from the hidden tro-chophore of palaeonemerteans. We discuss the possiblefunction and homology of the larval epidermis in develop-ment of other nemerteans and spiralians in general.