Annelida
While some small segmented worms of the phylum Annelida
have no separate circulatory system, most have a well-developed
closed system. The typical arrangement is for the main
contractile dorsal vessel to carry blood anteriorly
while a number of vertical segmental vessels, often
called hearts, carry it to the ventral vessel, in which
it passes posteriorly. Segmental branches supply and
collect blood from the respiratory surfaces, the gut,
and the excretory organs.
There is, however, great scope for variation on the
basic circulatory pattern. Many species have a large
intestinal sinus rather than a series of vessels supplying
the gut, and there may be differences along the length
of a single individual. The posterior blood may flow
through an intestinal sinus, the medial flow through
a dense capillary plexus, and the anterior flow through
typical segmental capillaries. Much modification and
complication may occur in species in which the body
is divided into more or less distinct regions with specific
functions.
Many polychaete worms (class Polychaeta), especially
the fanworms but also representatives of other families,
have many blind-ending contractile vessels. Continual
reversals of flow within these vessels virtually replace
the normal continuous-flow capillary system.
In most leeches (class Hirudinea), much of the coelomic
space is filled with mesodermal connective tissue, leaving
a series of interconnecting coelomic channels. A vascular
system comparable to other annelids is present in a
few species, but in most the coelomic channels containing
blood (strictly coelomic fluid) have taken over the
function of internal transport, with movement induced
by contraction of longitudinal lateral channels.
The blood of many annelids contains a respiratory pigment
dissolved in the plasma, and the coelomic fluid of others
may contain coelomic blood cells containing hemoglobin.
The most common blood pigments are hemoglobin and chlorocruorin,
but their occurrence does not fit any simple evolutionary
pattern. Closely related species may have dissimilar
pigments, whiledistant relatives may have similar ones.
In many species the pigments function in oxygen transport,
but in others they are probably more important as oxygen
stores for use during periods of hypoxia.
In addition to internal circulation, many polychaete
worms also set up circulatory currents for feeding and
respiration. Tube-dwelling worms may use muscular activity
to pass a current of oxygenated water containing food
through their burrows, while filter-feeding fanworms
useciliary activity to establish complicated patterns
of water flow through their filtering fans.
