542-488 million years ago
THE CIRCULATORY SYSTEM
The contractile ventral blood vessel of cephalochordates is homologous to the vertebrate heart. Ancestral cephalochordates evolved a pattern of blood circulation which is retained in all vertebrates (at least as embryos) which included a dorsal aorta and a hepatic portal system. Blood vessels possess a simple lining.
The vertebrate heart is a modified blood vessel. This raises the obvious question: at what point should a contractile ventral blood vessel be called a heart? While hemichordates, urochordates, and pognophorans possess definitive, albeit primitive hearts, the existence of a true heart in lancelets is less clear. While some classify the rhythmically contracting ventral blood vessel in lancets as the branchial artery, others consider it as a 1 chambered heart which may be homologous to the truncus arteriosus or to the sinus venosus (with a conus arteriosus) of vertebrate hearts (Prosser, 1973; Weichert, 1970; Willey, p. 47). The growing body of genetic evidence suggests that this structure is homologous to the vertebrate heart. Although the Amphioxus heart has no separate chambers, valves, endocardium, or epicardium, it does express amphiNk2-tin, a homolog of vertebrate NK2 and Drosophila tinman genes which are expressed in the developing heart. Vertebrates and Amphioxus also express members of the BMP family, TGF , GATA, MEF, and FGF in the developing heart (Holland, 2003).
Venous valves are known from Amphioxus and valves may also exist in arteries in Amphioxus and sharks (Dutta, 367; Webster, 1974, p. 67; Hoar, 1983).
In Amphioxus, blood vessels develop which all craniates will retain,
at least as embryos. In all vertebrate embryos, a primordial circulatory
system develops in which blood travels from the heart cranially through
a ventral aorta and, after passing through an aortic arch, proceeds caudally
through the dorsal aorta to the yolk sac and then back to the heart. (Torrey).In
Amphioxus and all higher chordates, a pair of anterior cardinal veins
develops to drain blood from the head and all vertebrate embryos retain
these primitive vessels. (Weichert, 1970, p.560). In Amphioxus, there
is a large subintestinal vein which transports nutrients from the gut
cranially. In Amphioxus, the hepatic portal vein forms a plexus of vessels
in the hepatic cecum which rejoin to form the hepatic vein and subsequently
the portion of the endostylar artery called the sinus venosus. (Ruppert,
from Harrison, 1997, p. 445-52; Willey 49,54). The hepatic portal system,
which brings blood directly from the GI tract to the liver, has been little
modified since the early chordates. In both lancelets and hagfish, the
hepatic portal vein is contractile (Guenther, 154). In Amphioxus, blood
vessels reach the myosepta but not the muscle myotomes themselves and
skeletal muscle receives little vascular supply (Ruppert, from Harrison,
1997, p. 458; Willey ).