178 THE ORIGIN OF VERTEBRATES 
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 com- 
mencement of the thyroid it divides into two, and so forms two ventral 
longitudinal vessels, from which the branchial arteries arise segmentally. 
"Madd. 
ay -—- 
NED CS 2. con. tub. 
Fic. 71.—D1aGRAM CONSTRUCTED FROM A SERIES OF TRANSVERSE SECTIONS THROUGH 
A BRANCHIAL SEGMENT, SHOWING THE ARRANGEMENT AND RELATIVE POSITIONS 
oF THE CaRTILAGE, Muscies, 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; MM. 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., epibran- 
chial ganglia of vagus; R. br. prof. VII., ramus branchialis ¢rofundus 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 Ammoccetes 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, homo- 
logous with the paired longitudinal venous sinuses of the latter. 
