BLOOD-VESSELS OF MUSTELUS ANTARCTICUS. 
719 
vessel of some Annelids. Thus the study of the vascular system would seem to lend 
more support to the Annelid than to the Nemertean theory, a strong point against the 
latter hypothesis being the absence of a ventral vessel in any known Nemertean. 
The most recent speculation of this sort is that of Bateson (3), who proposes to 
include Balanoglossus in the Chordate Phylum. This theory clearly receives no support 
from a comparison of the blood-vessels of the Enteropneusta with those of the 
Vertebrata. Balanoglossus certainly possesses dorsal, ventral, and lateral trunks, but 
the blood in the dorsal vessel—which, by the hypothesis, answers to the dorsal vessel 
of vertebrates—flows from behind forwards, and both dorsal vessel and heart he dorsad 
of the supposed notochord. 
Fig. A. 
Diagrammatic transverse section of the trunk region of a Selachian, showing the principal longitudinal 
blood-vessels. The arteries are distinguished by dotting, the veins by vertical shading. (For 
explanation of reference letters see p. 726.) 
The diagram, fig. A (woodcut), will probably be useful as showing the position of 
the chief longitudinal vessels—arteries and veins—in the trunk region of a typical 
Selachian. Of these the dorsal aorta and the intra-intestinal (sub-intestinal) vein are 
of primary importance in any phylogenetic speculations which avoid favouritism towards 
particular organs or sets of organs ; the lateral veins are probably, and the cardinal 
veins certainly, sub-primary, while the remaining vessels, with the possible exception 
of the myelonal vein, are, in all probability, of altogether secondary significance. 
