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

Abstract

Although Bonellia viridis is the best studied echiurid species, many aspects of its anatomy and ultrastructure are still unclear. In this study, detailed description of proboscis epithelium of females of B. viridis is done and illustrated by accurate schemes. Accordingly 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 supplies certain functions: protective for dorsal, transport of food particles for ventral, and agglutination of food particles for lateral. Ciliated cells have been described in epithelium of the dorsal side of the proboscis stem for the first time. These cells, probably, supply slow ciliary movements of proboscis. For the first time, the organization of epithelial cells is described and the ability of epithelial cells to consume nutrients is detected. The absence of continuous basal lamina under the ventral epithelium is ascertained. This unusual state may be explained by activity of bonellin-producing cells, which constantly penetrate between epithelial cells. In bonellin-producing cells, voluminous perinuclear space and contribution of rough endoplasmic reticulum into formation of bonellin-containing granules are detected for the first time. Innervation of epithelial and gland cells occurs via nerve tracts originating from proboscis nerve cords. In general, echiurid proboscis epithelium exhibits many unusual features, which are unknown for any other food collecting apparatuses of annelids. The specificity of organization of proboscis correlates with unique evolutionary transformation, which echiurids underwent from initially segmented ancestor to recent non-segmented state.

Ultrastructure of proboscis blood vessels in females of Bonellia viridis (Annelida: Bonellinae): New reconstructions

“…Little is known about the ultrastructure of echiurid organ systems and recent studies have revealed new unusual facts (Kuznetsov & Temereva, 2021; Kuznetsov, Ereskovsky, et al, 2021). The ultrastructure of blood vessels of echiurids is poorly studied.…”