The anatomy of the blood vascular system of the giant vestimentiferan tubeworm Riftia pachyptila (Siboglinidae, Annelida).

The giant dimensions of vestimentiferan Riftia pachyptila (Jones, ) are achieved thanks to the well-developed vascular system. In the vestimentum, there is a complicated net of lacunae, including the brain blood supply and the ventral lacuna underlying the ciliary field. The trunk region has an extensive network of blood vessels feeding the gonads («rete mirabile»). The thick muscular lining of the mesenterial vessels in the trunk and the dorsal vessel in the opisthosome serves as an additional pump, pushing blood into numerous vessels in the segments. It was hypothesized that the blood envelope of the ventral blood vessel in the trunk provides the blood supply to the trophosome. The 3D reconstruction has revealed that there are two vascular systems of the tentacular crown of R. pachyptila. Blood runs into the tentacles via axial afferent vessels, as described earlier only for Riftia, and also via basal ones, as described for other vestimentiferans except Riftia. The basal ones are poorly developed, and the number of lamellar blood vessels is small, indicating a lack of demand for these within huge R. pachyptila. It appears that the presence of these vessels is the preserved ancestral state of Vestimentifera. In different portions of the dorsal vessel, the morphology of the intravasal body varies, depending on function.