(3) In most Teleostomi the air-bladder is supplied with blood by branches of the coeliac artery, with the addition of small branches arising directly from the dorsal aorta. Polypterus and Amia[[388]] are, however, exceptional, inasmuch as the arteries for the air-bladder are derived from the last or fourth pair of efferent branchial vessels, and in this respect, but not in the destination of the corresponding veins, the two genera exhibit a significant resemblance to the Dipnoi.
In the Dipnoi the ventral aorta is so short that the afferent branchial arteries arise almost directly from the conus arteriosus with their roots in close contiguity to one another (Fig. 200).
Fig. 200.—Branchial arterial system of Neoceratodus. Lateral view. The conus arteriosus and the afferent branchial vessels are represented in solid black, the efferent vessels and their derivatives with double contours. a, Auricle; a.c.a, anterior carotid; a.cb.a, anterior cerebral artery; af.b.a′, first afferent branchial artery; br.a, brachial artery; c.a, coronary artery; c.ar, conus arteriosus; c.m.a, coeliaco-mesenteric; ep.a, epibranchial artery; hb.a, hypobranchial artery; hy.a, hyoid artery; hy.ar, hyoidean arch; l.a, lingual; l.d.a, r.d.a, left and right dorsal aortae; oc.a, occipital artery; oes.a, oesophageal artery; p, pericardium; p.a, pulmonary artery; p.c.a, posterior carotid; p.cb.a, posterior cerebral artery; s.v, sinus venosus; v, ventricle; 1, hyobranchial cleft; 2-5, branchial clefts. (After Baldwin Spencer, diagrammatic.)
In Neoceratodus (Fig. 200),[[389]] there are two efferent vessels to each gill-bearing branchial arch, which unite above to form an epibranchial artery, and by the successive union of the four epibranchial arteries a short common trunk is formed on each side. Posteriorly, the two trunks unite to form a median dorsal aorta. Immediately above the gill-clefts each efferent vessel gives off a branch which, passing either forwards or backwards, unites with the corresponding branch of the efferent vessel in front or behind as the case may be. A hyoidean artery arises from the ventral extremity of the anterior efferent artery of the first branchial arch, and, after giving off a lingual artery, ascends the hyoid arch and supplies the hyoidean pseudobranch. The efferent vessel of the pseudobranch (a.c.a) or anterior carotid artery, eventually enters the cranial cavity and subdivides into anterior and posterior cerebral arteries for the brain, also giving off a branch which unites with its fellow of the opposite side directly behind the infundibulum. A posterior carotid springs from the epibranchial of the first branchial arch and divides into palatine, orbital, and ocular branches; and from the ventral end of the anterior efferent vessel of the second branchial arch is derived a hypobranchial artery for the heart and pericardium. The pulmonary arteries for the lung-like air-bladder have their origin from the fourth pair of epibranchial arteries.
As in so many other details of its anatomy, Neoceratodus exhibits in its arterial system abundant evidence of the wide-spreading affinities of the group to which it belongs. In its branchial arterial system Neoceratodus presents a singular combination of features which, individually, are characteristic of Amphibia and Elasmobranchs. Special Amphibian features may be noted in the origin of the afferent branchial arteries almost simultaneously from the anterior end of the conus arteriosus; in the mode of union of the epibranchial arteries to form the dorsal aortae; in the origin of a lingual artery from the efferent vessel of the first branchial arch; and in the derivation on either side of a pulmonary artery from the fourth epibranchial artery. Agreement with Elasmobranchs is to be found in the presence of two efferent branchial vessels in each branchial arch, although the relations of these arteries are more primitive than in most adult Elasmobranchs, inasmuch as the two efferent vessels of the same arch unite to form an epibranchial artery; and also in the origin and distribution of the anterior and posterior carotids. Lastly may be mentioned the fact that Neoceratodus agrees not only with the Amphibia but also with those generalised Teleostomi, Polypterus and Amia, in the mode of origin of the great arteries for the air-bladder.
Of the two remaining Dipnoi, the arterial system of Protopterus[[390]] is better known than that of Lepidosiren, but in both cases further research is needed before a satisfactory comparison can be made with Neoceratodus and other Vertebrates. It is evident, nevertheless, that both genera differ from Neoceratodus in approximating more closely to the Amphibia than to the lower Fishes, in so far as the branchial part of the arterial system is concerned.
Fig. 201.—Branchial arterial system of Protopterus (diagrammatic). a, Auricle; a.c.a, carotid artery; af.b.a1-4, afferent branchial arteries; af, ef, afferent and efferent vessels of the hyoidean pseudobranch; b.a2, second branchial arch, the vestigial first arch being omitted; c.a, conus arteriosus; e.g, external or cutaneous gill; ep.a, epibranchial artery; hy.ar, hyoid arch; hy.ps, hyoidean pseudobranch; l.a, lingual artery; l.d.a and r.d.a, right and left dorsal aortae; l.p.a, left pulmonary artery; s.v, sinosus venosus; v, ventricle; 2-6, the second branchial and succeeding clefts, the hyobranchial cleft being closed. The vestigial first branchial arch is not shown. The epibranchial arteries unite to form the right or left dorsal aorta at the same point and not in succession as in the figure. (Altered from Newton Parker.)
In their origin from the conus the four afferent branchial arteries of Protopterus resemble those of Neoceratodus, but their relations to the branchial clefts are somewhat different (Fig. 201). The first or hyoidean cleft is closed, and the first afferent vessel lies between the second cleft and the third, and is therefore in relation with the second branchial arch. The remaining afferent arteries are disposed between the succeeding clefts and are related to the corresponding arches. As the second and third arches, like the vestigial first arch, bear no gill-lamellae, their afferent arteries are directly continuous with the corresponding efferent vessels, as in those Teleosts in which certain arches are gill-less, as well as in the Tadpole-stage of the tailless Amphibia when the internal gills begin to degenerate; and they apparently transmit arterial blood directly to the dorsal aorta.[[391]] The third and fourth afferent arteries, on the contrary, supply venous blood to the two hemibranchs which are borne by each of the two corresponding arches, viz.: the fourth and fifth, and from each pair of hemibranchs the blood is collected into two efferent vessels which unite dorsally to form an epibranchial artery. From the dorsal end of the fourth afferent artery there arises a recurrent branch which curves round the upper margin of the sixth cleft and supplies the gill-lamellae on the posterior margin of that cleft, a fact which lends support to the view that these lamellae are "emigrants" from the anterior margin of the cleft; the efferent vessel from the "emigrant" lamellae joins the fourth epibranchial artery. The blood-supply of the external or cutaneous gills is derived from the dorsal extremities of the second, third, and fourth afferent arteries, while the efferent vessels from these organs join the corresponding epibranchial arteries; in this respect there is a close resemblance between Protopterus and those larval Amphibians which possess similar cutaneous gills. All four epibranchial arteries unite together at about the same point to form a short common trunk, the right or left dorsal aorta, which subsequently unites with its fellow to form the median dorsal aorta.