New data on the ultrastructure of the proboscis in females of Bonellia viridis (Annelida: Bonellinae)

Although Bonellia viridis is the best studied echiurid species, many features of its anatomy and ultrastructure are still unclear. Detailed data on the organisation of the proboscis may help us to understand the echiurid biology and some aspects of their feeding behaviour. In this study, a detailed description of the proboscis epithelium of females of B. viridis is provided. According to histological specificities, four zones of proboscis stem can be distinguished: dorsal, ventral, and two lateral. These zones differ from each other in ciliation and abundance of bonellin-producing and gland cells. Each zone provides certain functions: protective for dorsal, transport of food particles for ventral, and agglutination of food particles for lateral. Ciliated cells have been described in the epithelium of the dorsal side of the proboscis stem for the first time. These cells probably supply the slow ciliary movements of the proboscis. For the first time, the organisation of epithelial cells is described: these cells contain many pinocytotic vesicles, multivesicular bodies, and phagosomes, whose presence may provide evidence of the ability of epithelial cells to consume nutrients. The absence of continuous basal lamina under the ventral epithelium is ascertained and may be explained by the activity of bonellin-producing cells, which constantly penetrate between epithelial cells. In bonellin-producing cells, voluminous perinuclear space and the contribution of rough endoplasmic reticulum into the formation of bonellin-containing granules are detected for the first time. For the first time, the neurite bundles extending from lateral nerve cords to gland cells and bonellin-producing cells are discovered.