• 1
    The circulation of blood in the intestinal wall has been studied in living lamprey embryos and unpigmented larvae.
  • 2
    A simple circulation is established just before the time of hatching.
  • 3
    The anterior mesenteric artery supplies the intestinal wall and the sub-intestinal vein drains it.
  • 4
    Shortly after hatching, posterior mesenteric vessels appear.
  • 5
    When pigmentation sets in the posterior mesenteric artery develops a connection with the posterior cardinal veins by way of recto-cloacal vessels.
  • 6
    Torsion of the intestine and its blood vessels extends gradually towards the posterior end but ceases at the end of the typhlosole.
  • 7
    A median dorsal branch of the posterior mesenteric artery comes to lie in or on the wall of the intestine on its left side and enters the typhlosole to become confluent with the anterior mesenteric artery as the typhlosolar artery.
  • 8
    Additional posterior mesenteric arteries supply the peripheral part of the intestinal wall and persist throughout the ammocoete stage.
  • 9
    In the ammocoete a sub-cardinal vein takes blood from the kidneys to the post-cardinal veins.
  • 10
    At or just before metamorphosis outgrowths from the sub-cardinal vein grow down within the connective tissue coat of the posterior mesenteric arteries to reach the intestinal wall, thus forming the “vascular couples” characteristic of the adult.
  • 11
    During metamorphosis secondary venous connections are developed between the kidney veins and the post-cardinal veins.
  • 12
    Questions of terminology are discussed and the term “supra-renal sinus”abandoned in favour of“sub-cardinal sinus”.
  • 13
    The term “vascular couple” is retained. “Veins”, “arteries” and “ligaments” of previous authors are interpreted according to the mode of development of the vascular couple.
  • 14
    The “valves” of Keibel (1924) are shown to be sphincters in the wall of the dorsal aorta at the origin of the segmental parietal arteries.
  • 15
    The general plan of the intestinal blood system of the adult lamprey shows the persistence of several features characteristic of the earlier developmental stages.
  • 16
    The functional significance of the vascular couples is still not fully understood.