Ultrastructural Observations on the Dwarf Male of Bonellia viridis (Echiura)

Abstract

AbstractThe organization of the dwarf male of Bonellia viridis was studied by electron microscopy. The epidermis is formed by two types of epithelial cells: the majority are multiciliated cells; highly vacuolated, non‐ciliated cells are less abundant. The body wall musculature consists of an outer circular, a diagonal, and a longitudinal layer. As a unique feature in coelomate spiralians it was found that the perikarya of all muscle cells are located internal to the entire contractile muscular layer. The muscles are solitary myocytes embedded in extracellular matrix. Masses of secretory and indifferent cells occur inside the muscles. Two types of secretory cells were distinguished. Both of them apparently undergo holocrine secretion. A complete lining of thin peritoneal cells delimits the body cavity. Also, the gut and sperm sac have a complete peritoneal lining. The coelomic lining of the gut is a single‐layered myoepithelium, that of the sperm sac a pseudo‐stratified myoepithelium. The vas deferens was seen to be ciliated. The entrance of the sperm sac is formed by a ciliated funnel that leads into the reservoir by means of a thin, ciliated canal. The existence of repeated transverse nerves and of four longitudinal nerve cords is described for the first time.