Juvenile Formation in Lecithotrophic Pilidium of Nipponomicrura sp. (Pilidiophora, Lineidae)

Lecithotrophic larvae of Pilidiophora represent a modified pilidium without its hat-like appearance and reduced larval lobes and lappets but sharing its pattern of juvenile development from imaginal discs and juvenile rudiments. Until the 1960s, only three types of lecithotrophic larvae of Pilidiophora were described: incapsulate Desor’s larva, Schmidt’s larva, and swimming Iwata’s larva. To date, about 18 forms of swimming lecithotrophic larvae have been identified, of which only pilidium nielseni has been studied by confocal laser scanning microscopy (CLSM). Here, using CLSM with phalloidin labeling, we have described the juvenile development of the swimming lecithotrophic larva of the heteronemertean Nipponomicrura sp. In the gastrula stage, six juvenile rudiments synchronously appear; in 2-day-old larvae, two more rudiments (cerebral organ discs) are formed. All eight juvenile rudiments are derived from the ectoderm and, in 5-day-old larvae, they gradually fuse together to form the complete juvenile body. Larvae of Nipponomicrura sp. are more similar to the Iwata’s larva: almost all rudiments appear at about the same time in the embryo in the gastrula stage, while the cerebral organ discs are formed in free-swimming larvae. Issues concerning the development of the cerebral organ and proboscis apparatus in nemerteans are discussed. A unique feature of lecithotrophic larvae of Pilidiophora is that the rudiments of the cerebral organs are initially associated with the foregut. We assume that the proboscis epithelium in Pilidiophora and Hoplonemertea has an ectodermal origin, regardless of the way it forms.