Above the gut lies a straight, cylindrical rod of cartilage, pointed at either end. This is the highly important structure known as the _Notochord_, which is present in all the vertebrates, although in the higher forms it is replaced during development by the vertebræ, the bony segments of the backbone. Above the notochord again lies the main nerve cord, a position which it retains throughout the whole vertebrate group. The nerve cord is simple in structure, with only a very slight swelling at the front end, representing the brain. There are two main blood vessels, an upper and a lower, which expand and contract alternately throughout their whole length, and thus maintain the circulation. The blood passes forward in the ventral vein, is pumped through the fine vessels of the gills, and collected into the upper artery. From this it is distributed throughout the body by branch vessels, to be re-collected into the ventral vein. If the reader will refer to the illustrations in Figs. 61, 62, and 63, the relationships of these parts will be more easily understood. There is a single small eye-spot, a single organ of smell, but no hearing organs. It seems probable that this extremely ill-developed condition of the sensory system is due in some measure to degeneration, and is not a primitive characteristic. There are numerous pairs of simple nephridia, which open into the peribranchial chamber, and bear a close resemblance to those of the worms. The lancelet forms a most important link between the lower and the higher animals. It is in all probability derived from some form similar to Balanoglossus, and it certainly leads up to the round-mouths, which form the next step in the ladder. [Illustration: FIG. 61.--The Lancelet--Amphioxus.] [Illustration: FIG. 62.--Diagrammatic cross-section of Amphioxus. _n_, Nerve cord; _ch_, notochord; _mus_, muscle tissue; _ec_, skin; _bl_, blood vessel; _cöl_, body cavity; _kd_, pharynx; _ld_, liver sac; _g_, reproductive gland; _p_, peribranchial chamber.] [Illustration: FIG. 63.--Diagrams of Tunicate (on the left), Amphioxus (centre), and young Lamprey (right). _o_, Mouth; _au_, eye; _c_, peribranchial chamber; _ch_, notochord; _d_, alimentary canal; _g_, ear; _hz_, heart; _k_, gills; _lb_, liver; _m_, nerve cord; _m_, brain; _mg_, stomach; _mt_, mantle; _z_, tunicate embryos.]