Fig. 67.—Heart of Ceratodus.
a, Atrium; b, Conus arteriosus; d, Papillary valve within the conus; e-g, Transverse rows of Ganoid valves; h, i, Anterior arcus aortæ; k, l, Posterior arcus aortæ; v, Ventricle.
The heart is, relatively to the size of the body, very small, and consists of three divisions: the atrium, with a large sinus venosus into which the veins enter; the ventricle; and a conical hollow swelling at the beginning of the arterial system, the structure of which forms one of the most important characters used in the classification of fishes. In all Palæichthyes (Figs. [66] and [67]) this swelling is still a division of the pulsating heart, being provided with a thick muscular stratum; it is not separated from the ventricle by two valves opposite to each other, but its interior is fitted with a plurality of valves, arranged in transverse series more or less numerous in the various groups of Palæichthyes. Lepidosiren and Protopterus offer an example of a modification of this valvular arrangement, their valves being longitudinal, each valve in fact being formed by the confluence of several smaller ones situated behind one another. This Palæichthyan type is called conus arteriosus.
In Cyclostomes and Teleosteans (Fig. [68]) the enlargement is a swelling of the artery, without muscular stratum and without contractility; with the exception of the Myxinoids its walls are thick, fibrous, with many trabeculæ and pouches, but it has no valves in its interior, and is separated from the ventricle by two valves opposite to each other. This Teleostean type is called bulbus aortæ.
Fig. 68.—Bulbus aortæ of Xiphias gladius, opened.
a, Section through part of the wall of ventricle; b, Section through the bulbus; c, Teleosteous valves of the ostium arteriosum; d, Accessory valves, of rudimentary nature and inconstant; e, Trabeculæ carneæ of the bulbus.
The sinus venosus sends the whole of the venous blood by a single orifice of its anterior convexity into the atrium; two thin membranous valvules turned towards the atrium, prevent the blood from re-entering the sinus. A pair of other valves between atrium and ventricle have the same function. The walls of the ventricle are robust, and, internally, it is furnished with powerful fleshy trabeculæ.
The bulbus or conus arteriosus is prolonged into the branchial artery which soon divides, sending off a branch to each branchial arch. On returning from the respiratory organ the branchial veins assume the structure and functions of arteries. Several branches are sent off to different portions of the head and to the heart, but the main trunks unite to form the great artery which carries the blood to the viscera and all the parts of the trunk and tail, and which, therefore, represents the aorta of higher animals.