Structure of the Proboscis Endothelium in Nemertea.

We studied the ultrastructure of the proboscis endothelium of 14 nemertean species. In all nemerteans examined, the endothelium is organized as a pseudostratified myoepithelium consisting of two types of cells resting on the basal extracellular matrix: apically situated supportive cells and subapical myocytes covered by cytoplasmic sheets of the supportive cells. Myocytes form the inner circular musculature of the proboscis; the endothelium in the bulb of monostiliferous nemerteans lacks myocytes. The endothelium of the studied species differs in the number of rows of muscle fibres (one vs. several rows), the number of myofibrils in myocytes (one vs. two to five), the number of processes of myocytes covered by one supportive cell (one vs. two to 23), and in the number of processes in supportive cells (one vs. two to five). In some of the species, rudimentary cilia of supportive cells were revealed by using cLSM and an antibody against tubulin. The data obtained indicate that the proboscis endothelium in nemerteans is in fact a coelothelium, but recognition of the ancestral state of the coelomic lining in Nemertea is problematic, as the rhynchocoel peritoneum lacks myocytes.