The axis of the backbone of all Vertebrates is formed by an elastic rod known as the "notochord" (Figs. 72, 115), which lasts throughout life in some of the lowest forms, but in the higher forms appears only in the embryo. The universal occurrence of this structure has been regarded as the most important characteristic of the Vertebrata and their allies, which are accordingly grouped together in the Phylum CHORDATA. The members of this Phylum are further distinguished from other animals by several important features. Of these one of the most important appears to be the existence of lateral {4}outgrowths of the pharynx, which unite with the skin of the neck and form a series of perforations leading to the exterior. These structures are the gill-slits, and in the Fishes their walls give rise to vascular folds or gills. With the assumption of a terrestrial life, the higher Vertebrates lost their gills as functional organs, respiration being then performed by entirely different organs, the lungs. But even in these cases, the gill-slits appear in the embryo; and remains of one pair can usually be recognised in the adult state of even the highest Vertebrates. Another fundamental characteristic of the Chordata is given by the central nervous system, which lies entirely above the alimentary canal, just dorsal to the notochord. Not only does this position of the nerve-centres distinguish the Chordata from Invertebrates, but a further point of difference is found in the development. While in Invertebrates the ventral nerve-cord is formed as a thickening of the ectoderm or outermost layer of the embryo, in the Chordata the nervous system is usually formed as a longitudinal groove running medianly along the back of the embryo. This groove closes to form a tube of nervous matter, the cavity of which always persists throughout life as the "central canal" of the spinal chord and its anterior prolongation which constitutes the "ventricles" of the brain.