Larval development with transitory epidermis in Paranemertes peregrina and other hoplonemerteans

We describe development of the hoplonemertean Paranemertes peregrina from fertilization to juvenile, using light, confocal, and electron microscopy. We discovered that the uniformly ciliated lecithotrophic larva of this species has a transitory epidermis, which is gradually replaced by the definitive epidermis during the course of planktonic development. The approximately 90 large multiciliated cleavage-arrested cells of the transitory larval epidermis become separated from each other by intercalating cells of the definitive epidermis, then gradually diminish in size and disappear more or less simultaneously. Rudiments of all major adult structures-the gut, proboscis, cerebral ganglia, lateral nerve cords, and cerebral organs-are already present in 4-day-old larvae. Replacement of the epidermis is the only overt metamorphic transformation of larval tissue; larval structures otherwise prefigure the juvenile body, which is complete in about 10 days at 7-10 degrees C. Our findings on development of digestive system, nervous system, and proboscis differ in several ways from previous descriptions of hoplonemertean development. We report development with transitory epidermis in two other species, review evidence from the literature, and suggest that this developmental type is the rule for hoplonemerteans. The hoplonemertean planuliform larva is fundamentally different both from the pilidium larva of the sister group to the Hoplonemertea, the Pilidiophora, and from the hidden trochophore of palaeonemerteans. We discuss the possible function and homology of the larval epidermis in development of other nemerteans and spiralians in general.