We have a further important advance in the Tunicates, which we have recognised as the nearest blood-relatives of our early vertebrate ancestors. Here we find for the first time a real heart--i.e. a central organ of circulation, driving the blood into the vessels by the regular contractions of its muscular wall, it is of a very rudimentary character, a spindle-shaped tube, passing at both ends into a principal vessel (Figure 2.221). By its original position behind the gill-crate, on ventral side of the Tunicates (sometimes more, sometimes less, forward), the head shows clearly that it has been formed by the local enlargement of a section of the ventral vessel. We have already noticed the remarkable alternation of the direction of the blood stream, the heart driving it first from one end, then from the other (Chapter 2.16). This is very instructive, because in most of the worms (even the Enteropneust) the blood in the dorsal vessel travels from back to front, but in the Vertebrates in the opposite direction. As the Ascidia-heart alternates steadily from one direction to the other, it shows us permanently, in a sense, the phylogenetic transition from the earlier forward direction of the dorsal current (in the worms) to the new backward direction (in the Vertebrates). (FIGURE 2.363. Head of a fish-embryo, with rudimentary vascular system, from the left. dc Cuvier's duct (juncture of the anterior and posterior principal veins), sv venous sinus (enlarged end of Cuvier's duct), a auricle, v ventricle, abr trunk of branchial artery, s gill-clefts (arterial arches between), ad aorta, c carotid artery, n nasal pit. (From Gegenbaur.) FIGURE 2.364. The five arterial arches of the Craniotes (1 to 5) in their original disposition, a arterial cone or bulb, a double apostrophe aorta-trunk, c carotid artery (foremost continuation of the roots of the aorta). (From Rathke.) FIGURE 2.365. The five arterial arches of the birds; the lighter parts of the structure disappear; only the shaded parts remain. Letters as in Figure 2.364. s subclavian arteries, p pulmonary artery, p apostrophe branches of same, c apostrophe outer carotid, c double apostrophe inner carotid. (From Rathke.) FIGURE 2.366. The five arterial arches of mammals; letters as in Figure 2.365. v vertebral artery, b Botall's duct (open in the embryo, closed afterwards). (From Rathke.))