Lepidosteus[[384]] offers a singularly interesting transition from the Elasmobranch to the Teleost. As indicated in the preceding chapter, this Fish possesses both a hyoidean gill and a spiracular pseudobranch (Figs. 197 and 198). The hyoidean gill is supplied by an afferent artery direct from the ventral aorta, but the proper efferent vessel of the gill, which primitively joined the dorsal aorta, is suppressed, and the blood is collected into a vessel, which, like the hyoidean artery in Elasmobranchs, becomes the afferent artery of the spiracular pseudobranch. The latter artery unites, however, with a second hyoidean artery derived from the efferent branchial vessel of the first branchial arch, and represents the artery termed "hyoidean" in Teleosts. The efferent vessel from the spiracular pseudobranch joins an internal branch from the carotid artery, and then distributes its blood both to the eye and the brain.
Fig. 197.—Blood-vessels of the spiracular pseudobranch and the hyoidean gill in Lepidosteus. af.a, ef.a, Afferent and efferent vessels of the hyoidean gill; af.ps.a, ef.ps.a, afferent and efferent vessels of the spiracular pseudobranch; ca, carotid (posterior); d.a, dorsal aorta; ef.b.a1-4, efferent branchial vessels; hy.a, hyoidean artery; hy.h, hyoidean gill; hy.ps, spiracular pseudobranch; v.a, ventral aorta. (From F. W. Müller, after Johannes Müller.)
In Teleosts, as already mentioned in a preceding chapter, it is probable that the hyoidean hemibranch is suppressed, the so-called hyoidean pseudobranch being a spiracular pseudobranch. The latter is now supplied by a "hyoidean" artery, which has its origin from the ventral end of the efferent branchial artery of the first branchial arch, the corresponding efferent trunk forming an ophthalmic artery, and passing to the choroid gland of the eye (Fig. 199). Both the proper afferent and efferent arteries of the hyoidean hemibranch either disappear or, as in the Cod (Gadus morrhua), the efferent artery may be represented on each side by an anastomosis between the hyoidean artery and the cephalic circle. Hence, the "hyoidean" artery of Teleosts corresponds to the one which has a similar origin in Lepidosteus.
A brief description of the remaining efferent branchial arteries and their derivatives in the Cod (Gadus morrhua) will illustrate the condition of these structures in a well-known Teleost.
Fig. 198.—The branchial circulation in Lepidosteus (diagrammatic). a, a, Afferent branchial arteries; c, carotid; d.a, dorsal aorta; e, e, efferent branchial arteries; ef.a, efferent vessel from the hyoidean gill which, after its union with the hyoidean artery, becomes the afferent vessel of the spiracular pseudobranch; ef.a′, efferent vessel of the spiracular pseudobranch; hy.a, hyoidean artery; hy.g, hyoidean gill; sp.ps, spiracular pseudobranch; v.a, ventral aorta; 1-5, the hyo-branchial and succeeding gill-clefts. (After F. W. Müller and Ramsay Wright.)
In this Fish the efferent branchial vessels open dorsally into right and left suprabranchial arteries,[[385]] which unite behind to form a median dorsal aorta (Fig. 199). Anteriorly, the paired suprabranchial arteries extend towards the base of the skull as the so-called "carotid" arteries. The two carotids enter the cranial cavity, and there unite in the median line, as in the Cyclostomes. By the union of these arteries in front, and of the right and left suprabranchial arteries behind, the characteristic "circulus cephalicus" of Teleosts is completed.[[386]] From the anterior part of the cephalic circle are derived two internal carotid arteries[[387]] for the brain, and also a pair of orbito-nasal arteries for the eye-muscles and the nasal sacs, while more posteriorly an external carotid has its origin from each suprabranchial artery.
Fig. 199.—Branchial arterial system of the Cod (Gadus morrhua). Lateral view. af.b.a, First afferent branchial artery; cl.a, coeliac artery; d.a, median dorsal aorta; ef.b.a, first efferent branchial artery; ex.c, external carotid; H, heart; hy.a, hyoidean artery; Hy.b.a, hypobranchial artery for the heart and pelvic fins; hy.ps, spiracular pseudobranch; in.c, internal carotid;[[387]] l.d.a, left suprabranchial artery; m.a, mesenteric artery; on, orbito-nasal artery; oph.a, ophthalmic artery; r.d.a, right suprabranchial artery; sb.a, subclavian artery; v.a, ventral aorta; 1-5, hyobranchial and succeeding gill-clefts. (Altered from T. Jeffery Parker.)