The formation of the circulatory system in this animal is not less interesting. All the other vertebrates have a compressed, thick, pouch-shaped heart, which develops from the wall of the gut at the throat, and from which the blood-vessels proceed; in the Amphioxus there is no special centralised heart, driving the blood by its pulsations. This movement is effected, as in the annelids, by the thin blood-vessels themselves, which discharge the function of the heart, contracting and pulsating in their whole length, and thus driving the colourless blood through the entire body. On the under-side of the gill-crate, in the middle line, there is the trunk of a large vessel that corresponds to the heart of the other vertebrates and the trunk of the branchial artery that proceeds from it; this drives the blood into the gills (Figure 2.210 l). A number of small vascular arches arise on each side from this branchial artery, and form little heart-shaped swellings or bulbilla (m) at their points of departure; they advance along the branchial arches, between the gill-clefts and the fore-gut, and unite, as branchial veins, above the gill-crate in a large trunk blood-vessel that runs under the chorda dorsalis. This is the principal artery or primitive aorta (Figure 2.214 D). The branches which it gives off to all parts of the body unite again in a larger venous vessel at the underside of the gut, called the subintestinal vein (Figures 1.210 o and 2.212 E). This single main vessel of the Amphioxus goes like a closed circular water-conduit along the alimentary canal through the whole body, and pulsates in its whole length above and below. When the upper tube contracts the lower one is filled with blood, and vice versa. In the upper tube the blood flows from front to rear, then back from rear to front in the lower vessel. The whole of the long tube that runs along the ventral side of the alimentary canal and contains venous blood may be called the "principal vein," and may be compared to the ventral vessel in the worms. On the other hand, the long straight vessel that runs along the dorsal line of the gut above, between it and the chorda, and contains arterial blood, is clearly identical with the aorta or principal artery of the other vertebrates; and on the other side it may be compared to the dorsal vessel in the worms. (FIGURE 2.214. Transverse section of a young Amphioxus, immediately after metamorphosis, through the hindermost third (between the atrium-cavity and the anus). FIGURE 2.215. Diagram of preceding. (From Hatschek.) A epidermis, B medullary tube, C chorda, D aorta, E visceral epithelium, F subintestinal vein. 1 corium-plate, 2 muscle-plate, 3 fascie-plate, 4 outer chorda-sheath, 5 myoseptum, 6 skin-fibre plate, 7 gut-fibre plate, I myocoel, II splanchnocoel, I1 dorsal fin, I2 anus-fin.)