Provisional Structures of the Larva of <i>Nipponomicrura</i> sp. (Nemertea, Pilidiophora)

Nemertea is a phylum of predominantly marine worms that exhibit various larval forms, including the iconic pilidium. Pelagic lecithotrophic pilidia are considered more derived than pelagic planktotrophic pilidia, but data on the structure of lecithotrophic larvae are limited to the light-optical level. Here, we study the lecithotrophic reversed Iwata's larvae of an undescribed heteronemertean, Nipponomicrura sp. Using transmission electron microscopy and confocal laser scanning microscopy with F-actin, acetylated α-tubulin, and serotonin (5-hydroxytryptamine) labeling, the provisional structures of the larva are described. The larval envelope of Nipponomicrura sp. consists of three layers: the epidermis, the circular musculature, and the epithelium of the amnion. The larval epidermis contains a considerable amount of yolk, only half of which is consumed by the end of metamorphosis. The apical plate consists of 5-hydroxytryptamine-negative cells, each bearing a cilium surrounded by a collar of eight to nine microvilli. Four monociliated 5-hydroxytryptamine-like-immunoreactivity sensory apical neurons are associated with the apical plate. For the first time, a pair of longitudinal muscles running along the body of the juvenile and joining the anterior and posterior parts of the provisional epithelium has been identified in nemertean larvae. These muscles serve as retractors of the apical plate and fix the position of the juvenile within the larva. The obtained data indicate a similar morphology of the apical organ in Pilidiophora larvae; however, in the Nipponomicrura sp. larva, there are more layers under the apical plate, and the muscle-retractor is derived from two longitudinal muscle cords that pass through the juvenile's body, and in posterior pole, attach at the base of the larval envelope.