Not less interesting and important is an organ that develops from the ventral wall in all vertebrates—the gill-groove or hypobranchial groove. In the acrania and the ascidiæ it consists throughout life of a glandular ciliated groove, which runs down from the mouth in the ventral middle line of the gill-gut, and takes small particles of food to the stomach (Fig. 101 _z_). But in the craniota the thyroid gland (_thyreoidea_) is developed from it, the gland that lies in front of the larynx, and which, when pathologically enlarged, forms goitre (_struma_).
From the head-gut we get not only the gills, the organs of water-breathing in the lower vertebrates, but also the lungs, the organs of atmospheric breathing in the five higher classes. In these cases a vesicular fold appears in the gullet of the embryo at an early stage, and gradually takes the shape of two spacious sacs, which are afterwards filled with air. These sacs are the two air-breathing lungs, which take the place of the water-breathing gills. But the vesicular invagination, from which the lungs arise, is merely the familiar air-filled vesicle, which we call the floating-bladder of the fish, and which alters its specific weight, acting as hydrostatic organ or floating apparatus. This structure is not found in the lowest vertebrate classes—the acrania and cyclostoma. We shall see more of it in Volume II.
The second chief section of the vertebrate-gut, the trunk or liver-gut, which accomplishes digestion, is of very simple construction in the acrania. It consists of two different chambers. The first chamber, immediately behind the gill-gut, is the expanded stomach (_ma_); the second, narrower and longer chamber, is the straight small intestine (_d_): it issues behind on the ventral side by the anus (_af_). Near the limit of the two chambers in the visceral cavity we find the liver, in the shape of a simple tube or blind sac (_l_); in the amphioxus it is single; in the prospondylus it was probably double (Figs. 98, 100 _l_).
Closely related morphologically and physiologically to the alimentary canal is the vascular system of the vertebrate, the chief sections of which develop from the fibrous gut-layer. It consists of two different but directly connected parts, the system of blood-vessels and that of lymph-vessels. In the passages of the one we find red blood, and in the other colourless lymph. To the lymphatic system belong, first of all, the lymphatic canals proper or absorbent veins, which are distributed among all the organs, and absorb the used-up juices from the tissues, and conduct them into the venous blood; but besides these there are the chyle-vessels, which absorb the white chyle, the milky fluid prepared by the alimentary canal from the food, and conduct this also to the blood.