Modifications among the vertebrate classes. Fishes

The hearts of fishes show little modification from the basic plan, except that lungfish hearts tend to become subdivided. In them, the oxygenated blood carried by the pulmonary vein does not enter the sinus venosus along with the deoxygenated blood from the body. Instead, the oxygenated blood remains separate and enters the left side of the atrium. The atrium is partially divided into two auricles, and the ventricle also has a partial septum. Lungfishes show further signs of circulatory developments in their venous system. As in land vertebrates, there is a median posterior vena cava, and the posterior cardinal veins are reduced.

The arterial system of fishes is also altered from the basic plan. First there are the afferent (leading to) and efferent (leading from) parts of the gill (branchial) blood vessels. Each pair ofblood vessels looping up between a pair of gills is called an arterial arch. During the development of embryos, the arterial arches become interrupted by capillaries in the gills. Thus, each arch consists of a ventral afferent section that brings blood to the gills from the heart and a dorsal efferent section that collects blood from the gill capillaries and carries it tothe dorsal aorta. The whole circulatory system is a one-way arrangement, with the heart pumping only deoxygenated blood from the body forward to the gills to be oxygenated and redistributed to the body.

Although six gill slits appear in embryos, few adult fishes retain all six. The first and most anterior gill slit in the series becomes the spiracle, and the first branchial arch is much modified; parts of it disappear altogether. The second branchial arch is variable in its presence in different fishes. In general, therefore, adult fishes often have only four of the six original arterial arches found in embryos. The external carotid arteries also show modifications. Instead of arising from the anterior part of the ventral aorta, they become connected with the efferent portion of the second branchial arch. This change ensures that, despite modifications to the most anterior of the arterial arches, blood just oxygenated in the gills will reach the head.

It may be that the prevalence of poorly oxygenated water in certain habitats explains the evolution of lungs and, hence, of land vertebrates. Fishes also have evolved accessory structures for obtaining oxygen from the air. These are often modified gill chambers, with dense capillary networks. Even the intestine may be involved, as in the loach Haplosternum.

Except for sharks and their relatives (elasmobranchs), most fishes have a swim bladder, the structure from which lungs may have evolved. Although its prime function in fishes is to control buoyancy, the swim bladder may also act as an oxygen reserve, for the gas in it often contains a high concentration of oxygen derived from the blood's own supply. Blood to the swim bladder usually comes from the dorsal aorta. One African fish, Polypterus , uses its swimbladder for respiration, and the veins from it join the posterior cardinal veins close to the heart. These swim bladder veins are almost where pulmonary veins would be expected to be, if they were bringing oxygenated blood from lungs straight to the heart.

The lungfishes have gone further in adapting their circulatory systems to the presence of lungs, although the different species do not breathe air to the same extent. Some of their modifications foreshadow the changes that have taken place in amphibians. The divided atrium of the lungfish heart receives blood from the body on the right side and from the lungson the left. The conus is large and is divided by a complex system of valves arranged in a spiral pattern and called the spiral valve. The ventral aorta is also subdivided internally. The result is that oxygenated blood from the left side of the ventricle is directed into the ventral division of the ventral aorta and passes to the anterior of the arterial arches, while deoxygenated blood from the right side of the ventricle is directed into the two most posterior arterial arches and passes mainly to the lungs.

Four arterial arches are present even in the lungfish species most dependent on breathing air (Lepidosiren), where gills still exist. These are arches three to six of the original series of six present in fish embryos. Their arrangement is largely unaffected by the presence of lungs, except that the gills may be reduced and the arteries may pass straight through without intervening capillaries. Arches five and six, however, join together before entering the dorsal aorta and give rise to a large pulmonary artery to the lungs. Thus, in lungfishes, lungs and gills can be seen working side by side.

The circulatory systems of lungfishes are strikingly similar to those of amphibians, and although lungfishes do not seem to have been amphibian ancestors, they are related to fishes that were. It is likely that several groups of ancient fishes had lungs, partially divided hearts, and ventral aortas, and from one of these groups arose the land vertebrates.