First Detection of Podocytes in the Circulatory System of Enigmatic Echiurids (Annelida: Thalassematidae).

The fine structure of echiurid blood vessels in the proboscis is known in detail, but the circulatory system of the trunk is still understood mainly at the level of general anatomy. The trunk circulatory system was studied in Bonellia viridis females, and specialized podocytes were found to form the walls of the ring vessel and the anterior part of the ventral vessel. Podocytes were for the first time described in the echiurid circulatory system. Podocytes of B. viridis displayed a typical cell architecture, which is known for other bilaterians. A podocyte consists of a cell body; primary processes; and pedicels, which extend from the primary processes and are interconnected via specialized slit diaphragms. The presence of podocytes indicates that the ventral and ring vessels act as ultrafiltration sites, where the plasma is filtered through the basal lamina into the body cavity.