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_. [Illustration: 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.