ИСТИНА

Echiurida is a small group of marine annelid lost their segmentation. They lead a hidden lifestyle and are extremely difficult to access for morphological studies, especially for investigation of ultrastructure. Despite the fact that Bonellia viridis is the main object for morphological research in the echiurids, many details of the anatomical structure still remain unclear. New data on the anatomy and ultrastructure of B. viridis will help to find the main ways of evolution of the organ systems within Echiura. Epithelium of proboscis lacks the basal lamina and therefore can not be regarded as true epithelium. The same concerns the coelothelium, which lines the coelomic canals. This is probably caused because of bonellin producing cells, which are able to penetrate into epithelium and into coelomic lumen. Cells of dorsal side of proboscis are able to absorb small particles from the environment. The nerve tracts innervating large epidermal glands of the proboscis are described for the first time. According to our data, the wall of the axial blood vessel is formed by myoepithelial cerlls, which form numerous thin projections directing into the lumen of vessel and into the connective tissue. The study is supported by Russian Foundation for Basic Research (#20-04-00096).