Ultrastructure of the stomochord and the heart?glomerulus complex in Rhabdopleura compacta (Pterobranchia): phylogenetic implications

Abstract

Pterobranchia and Enteropneusta traditionally form the highest ranking sister taxa within the Hemichordata. The most prominent feature of all representatives of the Hemichordata is the stomochord, which is associated with an anterior excretory complex. The stomochord has been homologized with the notochord of the Chordata species, though this hypothesis is still under debate. Recently, even the validity and position of the Pterobranchia and the Hemichordata has been doubted. These uncertainties are along with a lack of information on the Rhabdopleurida, one of the highest ranking taxa within the Pterobranchia. This study analyzes the stomochord and the heart–glomerulus complex of Rhabdopleura compacta on the ultrastructural level. The data confirm that a glomerulus exists in representatives of the Rhabdopleurida, which previously was only known from the Cephalodiscida, the second highest ranking Pterobranchia taxon. The heart–glomerulus complex is, as expected, associated with the stomochord. Consisting of monociliated, non-vacuolated cells that form a monolayered epithelium of presumed ectodermal origin in R. compacta, the stomochord belongs to the ground pattern of the Pterobranchia and Enteropneusta. Information that would support a homology hypothesis of the chordate notochord and the hemichordate stomochord was not found. Furthermore, there is some evidence that the stomochord might represent a glandular organ. Thus, differences concerning the possible ontogenetic origin, morphology, and function are assumed to result from an independent evolution of the stomochord and the notochord.