In its basic plan, the circulatory system of the cyclostome conforms to the characteristic craniate pattern. In the pharyngeal region, the ventral aorta gives rise to a series of aortic arches supplying the gills, of which there are eight in the lamprey; the first — the hyoid — serving the anterior hemibranch of the most anterior gill pouch. The remaining afferent arteries divide to supply the anterior and posterior hemibranchs of adjacent gill pouches, and a similar pattern is seen in the efferent arteries, joining the dorsal aorta below the notochord (Fig. 6.1). In myxinoids, the number of arterial arches varies with the number of gill pouches and unlike those of lampreys or other craniates, each arch supplies the hemibranchs of the same pouch. This also applies to the efferent vessels, which join two lateral aortae above the pharynx. These unite behind the branchial region to form the median dorsal aorta, which is continued forwards between the lateral vessels. The homologies of these vessels are by no means certain, but Holmgren (1946) considers that in its embryonic development, the dorsal aorta of Myxine is closer to the gnathostome pattern than that of the lamprey.