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 cr aniates (1, 2, 3, 4). The blood vascular system of adult lancelets was stud ied by Rahr (5), and the detailed anatomical similarities of the major vess els prompted him to propose their homology with the vascular anatomy of cra niates. 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 addi tion, there ave rarely any hemocytes in the blood of cephalochordates. Neve rtheless, certain parts of the circulatory system-endostylar artery, bulbil li, 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 dri ving force for such contractions. On the basis of an extensive study by tra nsmission electron microscopy as well as light microscopical observations o f living larvae, I propose that Ruppert's hypothesis also holds true for la rval stages. Moreover, the observations reported here suggest that the coel omic canal system is the functional circulatory system at a stage when the blood vessels are rudimentary.