Precisely the same arrangement of veno-pericardial muscles and of longitudinal venous collecting sinuses occurs in the scorpions. It is one of the fundamental characters of the group, and we may fairly assume that a similar arrangement existed in the extinct forms from which I imagine the vertebrate to have arisen. The further consideration of this group of muscles will be given in Chapter IX. Passing now to the condition of the branchial blood-vessels of Ammocoetes, we see that the blood passes into the gill-lamellæ from a blood-space in the appendage, which can hardly be dignified by the name of a blood-vessel. This blood-space is supplied by the branchial artery which arises segmentally from the ventral aorta (_V.A._), as seen in Fig. 71 (taken from Miss Alcock's paper). From the gill-lamellæ the blood is collected into an efferent or branchial vein (_v. br._), which {178}runs, as seen in Fig. 65, along the free edge of the diaphragm, and terminates in the dorsal aorta. The ventral aorta is a single vessel near the heart, but at the commencement of the thyroid it divides into two, and so forms two ventral longitudinal vessels, from which the branchial arteries arise segmentally. [Illustration: FIG. 71.--DIAGRAM CONSTRUCTED FROM A SERIES OF TRANSVERSE SECTIONS THROUGH A BRANCHIAL SEGMENT, SHOWING THE ARRANGEMENT AND RELATIVE POSITIONS OF THE CARTILAGE, MUSCLES, NERVES, AND BLOOD-VESSELS. Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system. _Br. cart._, branchial cartilage; _M. con. str._, striated constrictor muscles; _M. con. tub._, tubular constrictor muscles; _M. add._, adductor muscle; _D.A._, dorsal aorta; _V.A._, ventral aorta; _S._, sense-organs on diaphragm; _n. Lat._, lateral line nerve; _X._, epibranchial ganglia of vagus; _R. br. prof. VII._, _ramus branchialis profundus_ of facial; _J.v._, jugular vein; _Ep. pit._, epithelial pit.] From this description it is clear that the vascular supply of the branchial segment of Ammocoetes would resemble most closely the vascular supply of the Limulus branchial appendage, if the ventral aorta of the former was derived from two longitudinal veins, homologous with the paired longitudinal venous sinuses of the latter. {179}_A priori_, such a derivation seems highly improbable; and yet it is precisely the manner in which embryology teaches us that the heart and ventral aorta of the vertebrate have arisen. THE ORIGIN OF THE INVERTEBRATE HEART AND THE ORIGIN OF THE VERTEBRATE HEART.