ИСТИНА

Vestimentifera – a group of gutless tubeworms Siboglinidae – inhabits reduced environments and relies their nutrition on bacterial endosymbionts. Here we arise a question if organizations of their excretory and circulatory systems share common features with sedentary annelids, or is a result of adaptations to peculiar mode of life. The organ systems were studied based on Ridgeia piscesae, Oasisia alvinae, Riftia pachyptila with histology, 3D modeling, and electron microscopy. Tubeworms’ vascular system comprises two main vessels (dorsal and ventral) and resembles the one of typical annelids. Unlike other annelids, vestimentiferans have sinus valvatus and abundant vascular net of blood supply of trophosome inhabited by endosymbionts. Non-obvious organization of coelomes arises questions on segmentation pattern to compare with annelids. In vestimentum there are separate coelomic cavities with undefined connection with each other: obturacular, tentacular, enteral coeloms and one around the dorsal vessel. Spacious coelomic cavities are visible in extended trunk and in each segment of opisthosome. Excretory and genital systems serve as coelomic ducts in vestimentum and trunk, correspondingly. Excretory system is unusual for annelids. This is an excretory tree comprising numerous blind tubules formed by ciliated terminal cells. Cells and tubules are coated by ECM and considered as a site of ultrafiltration. Tubules enter excretory ducts which proximal parts lined by absorptive epithelium. The distal part of the ducts opens by two pores on the dorsal side. Our results represent an important basis to unveil the evolution of the Vestimentifera and could place them into the annelid radiation.