Vascular System of Bdellostoma dombeyt. 43 
to occur in some Testudinata and Amphibia, however. The 
portal system of Bdellostoma is also remarkable in being 
developed chiefly from the supra-intestinal vein, and in 
having no connection whatever with the sub-intestinal vein. 
A well developed portal heart, contractile and supplied 
with a complete system of valves, so far as I know, occurs 
nowhere else among Vertebrates (excepting the closely 
related Myxine?). Contractile veins, however, are not 
uncommon. 
I may sum up my conclusions in regard to the Comparative 
Anatomy as follows: 
PRIMITIVE CHARACTERS IN THE BLOOD- VASCULAR 
SYSTEM OF BDELLOSTOMA. 
1. Persistent pericardio-peritoneal foramen. 
2. The simple tubular heart. 
3. The large number (up to 14) of functional branchial 
vessels. 
4. The origin of the carotid arteries from a lateral com- 
missural vessel on each side connecting all the efferent 
branchial arteries. 
5. The complete sub-chordal aorta (dorsal aorta) extend- 
ing forward into the head region. 
6. Segmental arrangement of the somatic and renal 
arteries and veins. 
7. Frequent anastomosis between the posterior cardinal 
veins. 
8. Persistent sub-intestinal vein which does not join the 
portal system. 
g. The presence of an inferior jugular vein. 
10. The contractility of the portal heart. 
CHARACTERS SECONDARILY ACQUIRED. 
1. The asymmetry of the venous system. 
2. Distribution of branchial vessels to gill slits instead of 
gill arches. 
3. The extension of the portal system into the territory of 
the right anterior cardinal vein. 
31 
