i 7 6 



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 lamella? from sinuses or 

 blood-spaces (b.s., Fig. 66) at the base of each of the lamelke, which 

 sinuses are filled by a vessel which may be called the branchial 



Fig. 70. — Longitudinal Diagrammatic Section through the Mesosomatic 

 Region of Limulus, to show the origin of the Branchial Arteries. 

 (After Benham.) 



L.Y.S., 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. h\). The blood of the branchial artery 

 flows into the blood-spaces between the anterior and posterior 

 lamina? of the appendage and thence into the gill-lamella?, 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 



