Coelomates. Echinodermata
The circulatory systems of echinoderms (sea urchins,
starfishes, and sea cucumbers) are complicated as they
have three largely independent fluid systems. The large
fluid-filled coelom that surrounds the internal organs
constitutes the major medium for internal transport.
Circulatory currents set up by the ciliated cells of
the coelomic lining distribute nutrients from the gut
to the body wall. Phagocytic coelomocytes are present,
and in some species these contain hemoglobin. The coelomic
fluid has the same osmotic pressure as seawater, and
the inability to regulate that pressure has confined
the echinoderms to wholly marine habitats.
The blood-vascular (hemal) system is reduced and consists
of small, fluid-filled sinuses that lack a distinct
lining. The system is most highly developed in the holothurians
(sea cucumbers), in which it consists of an anterior
hemal ring and radial hemal sinuses. The most prominent
features are the dorsal and ventral sinuses, which accompany
the intestine and supply it through numerous smaller
channels. The dorsal sinus is contractile, and fluid
is pumped through the intestinal sinuses into the ventral
sinus and thence to the hemal ring. Most members of
the class Holothuroidea have a pair of respiratory trees,
located in the coelom on either side of the intestine,
which act as the major sites for respiratory exchange.
Each tree consists of a main tubular trunk with numerous
side branches, each ending in a small vesicle. Water
is passed through the tubules by the pumping action
of the cloaca. The branches of the left tree are intermingled
with the intestinal hemal sinuses and provide a means
of oxygenating the blood via the coelomic fluid. The
right tree is free in the coelomic fluid and has no
close association with the hemal system. Respiratory
exchange in other echinoderms is through thin areas
of the body wall, and the hemal system tends to be reduced.
The water vascular system of echinoderms is best developed
in the starfishes and functions as a means of locomotion
and respiratory exchange. The entire system consists
of a series of fluid-filled canals lined with ciliated
epithelium and derived from the coelom. The canals connect
to the outside through a porous, button-shaped plate,
called the madreporite, which is united via a duct (the
stone canal) with a circular canal (ring canal) that
circumvents the mouth. Long canals radiate from the
water ring into each arm. Lateral canals branch alternately
from the radial canals, each terminating in a muscular
sac (or ampulla) and a tube foot (podium), which commonly
has a flattened tip that can act as a sucker. Contraction
of thesac results in a valve in the lateral canal closing
as the contained fluid is forced into the podium, which
elongates. On contact with the substratum, the centre
of the distal end of the podium is withdrawn, resulting
in a partial vacuum and adhesion that is aided by the
production of a copious adhesive secretion. Withdrawal
results from contraction of the longitudinal muscles
of the podia.
