180 THE ORIGIN OF VERTEBRATES 
line of two longitudinal venous channels, which were originally situ- 
ated one on each side of the notochord, in what was then the ventral 
surface of this part of the embryo. 
This history is especially in- 
structive in showing how the 
pharyngeal region is formed by 
the growing round of the lateral 
mesoblast, 7.¢e. the muscular and 
other mesoblastic tissues of the 
branchial segments, and how the 
two longitudinal veins take part 
in this process. The phyloge- 
netic interpretation of this em- 
bryological fact seems to be, 
that the new ventral surface of 
the vertebrate in this region is 
formed, not only by the branchial 
appendages, but also by the 
Fic. 72.—Diacram (UPPER Hair oF 
Ficurr) OF THE Oricivat Position growth ventrally of that part 
or Veins (H) wHicu coms tocrtueR of the original ventral surface 
M : . * P ‘ 
TO FORM THE HEASTOF A VEERTEGEAT: schich covered éach longitudinal 
C.N.S., central nervous system; 7C¢., 
venous sinus. 
notochord; m., myotome. : 
The lower half of figure shows compara- The following out of the 
tive position of the longitudinal venous consecutive clues, which one 
sinus (L.V.S.) in Limulus. C.N.S., : : 
central nervous system ; Al., alimentary after the other arise in har- 
canal; H., heart; m., body-muscles. monious succession as the neces- 
sary sequence of the original 
working hypothesis, brings even now into view the manner in which 
the respiratory portion of the alimentary canal arose, and gives 
strong hints as to the position of that part of the arthropod which 
gave origin to the notochord. Here I will say no more at present, 
for the origin of the new alimentary canal of the vertebrate and of 
the notochord will be more fittingly discussed as a whole, after all 
the other organs of the vertebrate have been compared with the 
corresponding organs of the arthropod. 
The strong evidence that the vertebrate heart was formed from a 
pair of longitudinal venous sinuses on the ventral side of the central 
canal, carries with it the conclusion that the original single median 
dorsal heart of the arthropod is not represented in the vertebrate, 
° 
