Evolutionary trends. Circulation in agnathans
In the lamprey heart the atrium and ventricle are side
by side, with the sinus venosus enteringthe atrium laterally.
Nonmuscular valves prevent backflow of blood, and the
conus arteriosus contains no cardiac muscle. There is
no separate coronary blood supply, and the heart must
obtain its oxygen from the blood as it goes through.
The arterial system in agnathans is most obviously
modified because there are more than six sets of gills.
Eight branches emerge from the ventral aorta, which
splits into two, unlike the single vessel in most vertebrates
with gill slits. Oxygenated blood from the gills is
then collected into eight efferent vessels, which join
to form a dorsal aorta, single for most of its length.
Internal carotid arteries arise from the dorsal aorta,
but the ventral part of the head is supplied from anterior
efferent branchial (gill) vessels, not from the anterior
part of the ventral aorta.
The venous system does not include a renal portal section,
and there is asymmetry of the common cardinal veins,
which take blood from the dorsal anterior and posterior
cardinal veins down to the ventral heart. In embryos
there are two of these, one on each side of the body;
in lampreys, the left one disappears during development,
while in hagfishes the right one disappears. Hagfishes
also have accessory hearts in the venous system at several
points. No other vertebrate has these structures.
It is not clear why the circulatory system of agnathans
differs in these ways from the basic vertebrate plan.
It is important to remember, however, that lampreys
and hagfishes are specialized descendants of what was
once a more diverse and widespread group of animals.
Their circulatory systems may be less similar to the
basic vertebrate plan than those of their ancestors
because of their present mode of life.
