Development of the blood vascular system in Sabellaria cementarium (Annelida, Polychaeta)

The development of the blood vascular system (BVS) in larvae of the polychaete (Sabellaria cementarium was studied by light and electron microscopy. BVS formation begins in the metatrochophore, concomitant with onset of segmentation, and all major vessels and sinuses of the BVS have formed by the nectochaeta stage. Blood vessels form de novo by a separation of apposing basal extracellular matrices (ECM) of adjacent myoepithelial peritoneal cell layers, and blood sinuses also form de novo by a separation of the basal ECM of peritoneal cells from the basal ECM of the gut epithelium. Blood vessels and sinuses are lined only by the ECM of overlying cell layers. Podocytes are present overlying lateral esophageal and ventro-lateral trunk blood vessels. The results support the blastocoel theory of Lang (1904) and the “segmentation hypothesis” and structural model of Ruppert and Carle (1983) which presents the BVS of triploblastic Metazoa as a developmental and evolutionary modification of the basal ECM of overlying cell layers and argues that the adaptive significance of the BVS is to bypass septal partitions with a fluid transport system.