AbstractThe morphology of the circulatory system of Amphioxus (Branchiostoma lanceolatum(Pallas)) has been investigated using a new intravascular injection technique. A survey of the vessels of Amphioxus using this technique is given. The dorsal arteries and their ramifications are described in detail. The new injection technique brought to light myoseptal plexi, supplied from the dorsal arteries, between every two myomeres. Also the ventral parietal arteries have a much more complicated course than hitherto accepted. They are connected with an atrial plexus which is a continuous net of small vessels in the whole length of the dorso‐lateral wall of the atrial cavity. It is postulated that this plexus has a supplementary function in respiration. Plexi of minute vessels in the gonads and a real blood circulation with afferent and efferent gonadal vessels have been demonstrated. Two vessels connecting the liver plexus with the cardinal vein (or the atrial plexi) have been noticed, the v. communicans accessoria anterior mentioned in 1900 by Burchardt, and a so‐called oblique vessel never described before. The vessels of the caudal region are analyzed completely and also here a real blood circulation appears possi