1 7 8 



THE ORIGIN OF VERTEBRATES 



rims, as seeu 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 segmen tally. 



''it.lrfrof-W 



'TfT-KT.aai- 

 ^^'M-Con- Cut. 



-Jyf. Con- Str. 



Fig. 71. — Diageam 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., epibran- 

 chial ganglia of vagus ; B. 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 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. 



