Coelomic Cavities May Function as a Vascular System in Amphioxus Larvae.

At the level of light microscopy, Branchiostoma lanceolatum may be the best studied species of the Cephalochordata-the probable sister taxon of the craniates (1, 2, 3, 4). The blood vascular system of adult lancelets was studied by Rahr (5) and the detailed anatomical similarities of the major vessels prompted him to propose their homology with the vascular anatomy of craniates. He also showed that, in contrast to those of the craniates, most of the vessels in B. lanceolatum are not lined by an endothelium (6). In addition, there are rarely any hemocytes in the blood of cephalochordates. Nevertheless, certain parts of the circulatory system-endostylar artery, bulbilli, glomus, hepatic portal vein, hepatic vein, sinus venosus, subintestinal vein, and others-are reported to be contractile. Ruppert (7) proposed that the myoepithelial lining of adjacent coelomic cavities may provide the driving force for such contractions. On the basis of an extensive study by transmission electron microscopy as well as light microscopical observations of living larvae, I propose that Ruppert's hypothesis also holds true for larval stages. Moreover, the observations reported here suggest that the coelomic canal system is the functional circulatory system at a stage when the blood vessels are rudimentary.