Coelomates. Chordata
The phylum Chordata contains all animals that possess,
at some time in their life cycles, a stiffening rod
(the notochord), as well as other common features. The
subphylum Vertebrata is a member of this phylum and
will be discussed later (see below The vertebrate circulatory
system). All other chordates are called protochordates
and are classified into two groups: Tunicata and Cephalochordata.
The blood-vascular system of the tunicates, or sea
squirts, is open, the heart consisting of no more than
a muscular fold in the pericardium. There is no true
heart wall or lining and the whole structure is curved
or U-shaped, with one end directed dorsally and the
other ventrally. Each end opens into large vessels that
lack true walls and are merely sinus channels. The ventral
vessel runs along the ventral side of the pharynx and
branches to form a lattice around the slits in the pharyngeal
wall through which the respiratory water currents pass.
Blood circulating through this pharyngeal grid is provided
with a large surface area for gaseous exchange. The
respiratory water currents are set up by the action
of cilia lining the pharyngeal slits and, in some species,
by regular muscular contractions of the body wall. Dorsally,
the network of pharyngeal blood vessels drains into
a longitudinal channel that runsinto the abdomen and
breaks up into smaller channels supplying the digestive
loop of the intestine and the other visceral organs.
The blood passes into a dorsal abdominal sinus that
leads back to the dorsal side of the heart. The circulatory
system of the sea squirt is marked by periodic reversals
of blood flow caused by changes in the direction of
peristaltic contraction of the heart.
Sea squirt blood has a slightly higher osmotic pressure
than seawater and contains a number of different types
of amoebocytes, some of which are phagocytic and actively
migrate between the blood and the tissues. The blood
of some sea squirts also contains green cells, which
have a unique vanadium-containing pigment of unknown
function.
Amphioxus (Branchiostoma lanceolatum) is a cephalochordate
that possesses many typical vertebrate features but
lacks the cranial cavity and vertebral column of the
true vertebrate. Its circulatory pattern differs from
that of most invertebrates as the blood passes forward
in the ventral and backward in the dorsal vessels. A
large sac, the sinus venosus, is situated below the
posterior of the pharynx and collects blood from all
parts of the body. The blood passes forward through
the subpharyngeal ventral aorta, from which branches
carry it to small, accessory, branchial hearts that
pump it upward through the gill arches. The oxygenated
blood is collected into two dorsal aortas that continue
forward into the snout and backward to unite behind
the pharynx. The single median vessel thus formed branches
to vascular spaces and the intestinal capillaries. Blood
from the gut collects in a median subintestinal vein
and flows forward to the liver, where it passes through
a second capillary bed before being collected in the
hepatic vein and passing to the sinus venosus. Paired
anterior and posterior cardinal veins collect blood
from the muscles and body wall. These veins lead, through
a pair of common cardinal veins (duct of Cuvier), to
the sinus venosus.
There is no single heart in the amphioxus, and blood
is transported by contractions that arise independently
in the sinus venosus, branchial hearts, subintestinal
vein, and other vessels. The blood is nonpigmented and
does not contain cells; oxygen transport is by simple
solutionin the blood.
