These arrangements in the conus of the Dipnoans are of the highest morphological interest, pointing in an unmistakable way towards the condition found in the higher lung-breathing vertebrates. Of the two cavities into which the conus is partially divided in the Dipneusti the one which begins posteriorly on the right receives the (venous) blood from the right side of the heart, and ending up anteriorly dorsal to the other cavity communicates only with aortic arches V. and VI. In the higher vertebrates this cavity has become completely split off to form the root of the pulmonary arteries, and a result of aortic arch V. receiving its blood along with the functionally much more important VI. (the pulmonary arch) from this special part of the conus has been the almost complete disappearance of this arch (V.) in all the higher vertebrates. _Arterial System._--There are normally six aortic arches laid down corresponding with the visceral arches, the first (mandibular) and second (hyoidean) undergoing atrophy to a less or greater extent in post-embryonic life. Where an external gill is present the aortic arch loops out into this, a kind of short-circuiting of the blood-stream taking place as the external gill atrophies. As the walls of the clefts assume their respiratory function the aortic arch becomes broken into a network of capillaries in its respiratory portion, and there is now distinguished a ventral afferent and a dorsal efferent portion of each arch. Complicated developmental changes, into which it is unnecessary to enter,[38] may lead to each efferent vessel draining the two sides of a single cleft instead of the adjacent walls of two clefts as it does primitively. In the Crossopterygians and Dipnoans as in the higher vertebrates the sixth aortic arch gives off the pulmonary artery to the lung. Among the Actinopterygians this, probably primitive, blood-supply to the lung (swimbladder) persists only in _Amia_. [Illustration: FIG. 28.--Venous System of Polypterus 30 mm. larva (dorsal view). a.c.v, Anterior cardinal vein. d.C, Ductus Cuvieri. h.v, Hepatic vein. i.j.v, Inferior jugular vein. ir.v, Inter-renal vein. l.v, Lateral cutaneous vein. p.c.v, Posterior cardinal vein. p.n, Pronephros. p.v, Pulmonary vein. s, Subclavian vein. s.v, Sinus venosus. th, Thyroid. v, Vein from pharyngeal wall. * Anterior portion of left posterior cardinal vein.]