In this representative of the earliest vertebrates the method of manufacturing an efficient gill out of a lacunar blood-space is precisely the same as that which existed in Limulus and the scorpion, and, therefore, as that which existed in the dominant invertebrate group at the time when vertebrates first appeared. This similarity indicates a close resemblance between the circulatory systems of the two groups of animals, and therefore, to the superficial inquirer, would indicate an homology between the heart of the vertebrate and the heart of the higher invertebrate; but the former is situated ventrally to the gut and the nervous system, while the latter is composed of a long vessel which lies in the mid-dorsal line immediately under the external dorsal covering. Indeed, this ventral position of the heart in the one group of animals and its dorsal position in the other, combined with the corresponding positions of the central nervous system, is one of the principal reasons why all the advocates of the origin of vertebrates from the Appendiculata, with the single exception of myself, feel compelled to reverse the dorsal and ventral surfaces in deriving the vertebrate from the invertebrate. But there is one most important fact which ought to make us hesitate before accepting the homology of the dorsal heart of the arthropod with the ventral heart of the vertebrate--The heart in all invertebrates is a systemic heart, _i.e._ drives the arterial blood to the different organs of the body, and then the veins carry it back to the respiratory organ, from whence it passes to the heart. [Illustration: FIG. 69.--COMPARISON OF BRANCHIAL LAMELLÆ OF LIMULUS AND SCORPIO WITH BRANCHIAL LAMELLÆ OF AMMOCOETES. A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of Ammocoetes (after Nestler).] The only exception to this scheme is found in the vertebrate where the heart is essentially a branchial heart, the blood being {176}driven from the heart to the ventral aorta, from which by the branchial arteries it is carried to the gills, and then, after aeration, is collected into the dorsal aorta, whence it is distributed over the body. The distributing systemic vessel is the dorsal aorta, not the heart which belongs essentially to the ventral venous system. This constitutes a very strong reason for believing that the systemic heart of the invertebrate is not homologous with the heart of the vertebrate. How, then, did the vertebrate heart arise? Let us first see how the blood is supplied to the gills in Limulus. [Illustration: FIG. 70.--LONGITUDINAL DIAGRAMMATIC SECTION THROUGH THE MESOSOMATIC REGION OF LIMULUS, TO SHOW THE ORIGIN OF THE BRANCHIAL ARTERIES. (After BENHAM.) _L.V.S._, longitudinal venous sinus, or collecting sinus; _a. br._, branchial arteries; _V.p._, veno-pericardial muscles; _P._, pericardium.]