[Illustration: FIG. 41.--_A_, The Notochord of Vertebrates. Section, considerably magnified, through the middle of an embryo one inch long, of Acanthias, one of the Spiny Dog-fishes allied to the sharks. 1, Section through Spinal Cord; 2, Section through Notochord; below it lies a bean-shaped space, which is a section through a large blood-vessel; _sk_, epiblast or skin; _me_, mesoblast or middle layer of the body; the dots represent the nuclei of its transparent cells. The intestine, _i_, lined with hypoblast, is traversed by a spiral valve, and surrounded by the horse-shoe shaped body-cavity. _B_, Diagram indicating the position of the Notochord in the vertebra of an adult Common Dog-fish (_Scyllium canicula_). 1, "Neural arch" of the vertebra, consisting of processes of bone enclosing the central nervous system, or spinal cord; 2, bony centrum of the vertebra, hollowed out into a cup, in which lies a soft pad, the remains of the notochord.] The lowest member of the vertebrate group, separated in fact from the true vertebrates and placed in a lower division all by itself, is the little animal called the Lancelet or Amphioxus. It is often spoken of as a "fish"; but it is only by a stretch of our courtesy that it can receive that name, being an animal of a much lower form than the fishes. It was discovered in 1834, in the Mediterranean, and described as a fish; but it had previously been discovered in 1778, by a German naturalist who described it as a slug. The latter was misled by its external shape. He had not the advantage of the modern methods of preparing animals for examination under the microscope; in these days, Amphioxus is cut into successive slices along its whole length, and each of these carefully magnified, so that no detail of structure is lost. The Amphioxus burrows in the sea-sand; it lies buried in it, with its mouth just uncovered. Its food consists of microscopic vegetable organisms. Its distribution is very wide; it is found in both the Atlantic and Pacific waters. It occurs most abundantly in the salt-water lakes of Sicily, and in the Gulf of Naples. The specimen first seen, in 1778, came from the coast of Cornwall. There are eight species; the one which is found in the English Channel is the _Amphioxus lanceolatum_, also found in the Mediterranean and on the shores of North America. The classes of the Vertebrata are Fishes, Amphibia, Reptiles, Birds and Mammals. We used to learn that of these, fishes had gills, and Amphibia gills for a time; but, to be strictly accurate, we must say that fishes have gills, and _all_ the rest of the Vertebrata have gills for a time. There is no exception to this rule, not even among the highest vertebrates of all. But in those vertebrates which stand higher in the scale of life than Amphibia, viz., Reptiles, Birds, and Mammals, these gills are never brought into use. They only exist in the early embryo, and afterwards disappear, giving rise by their modification to other structures.