176 THE ORIGIN OF VERTEBRATES 
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. 
In Limulus the blood flows into the lamelle from sinuses or 
blood-spaces (0.s., Fig. 66) at the base of each of the lamelle, which 
sinuses are filled by a vessel which may be called the branchial 
Fie. 70.—LoneitupinaL DisGRAMMATIC SECTION THROUGH THE MEsSOsOMATIC 
ReGIon oF LIMULUS, TO SHOW THE ORIGIN OF THE BRANCHIAL ARTERIES. 
(After Bennam.) 
L.V.S8., longitudinal venous sinus, or collecting sinus; a. br., branchial arteries; 
V.p., veno-pericardial muscles; P., pericardium. 
artery, since it is the afferent branchial vessel. On each side of the 
middle line of the ventral surface of the body a large longitudinal 
venous sinus exists, called by Milne-Edwards the venous collecting 
sinus, L.V.S., (Fig. 70 and Fig. 58), which gives off to each of the 
branchial appendages on that side a well-defined afferent branchial 
vessel—the branchial artery (a. br.). The blood of the branchial artery 
flows into the blood-spaces between the anterior and posterior 
lamine of the appendage and thence into the gill-lamelle, from 
which it is collected into an efferent vessel or branchial vein, termed 
by Milne-Edwards the branchio-cardiac canal, which carries it back 
to the dorsal heart. The position of the branchial artery and vein 
is shown in Fig. 66, which represents a section through the branchial 
appendage of Limulus at right angles to the cartilaginous branchial 
bar (br. cart.), just as Fig. 65 represents a section through the 
