Blood
The primary body cavity (coelom) of triploblastic multicellular
organisms arises from the central mesoderm, which emerges
from between the endoderm and ectoderm during embryonic
development. The fluid of the coelom containing free
mesodermal cells constitutes the blood and lymph. The
composition of blood varies between different organisms
and within one organism at different stages during its
circulation. Essentially, however, the blood consists
of an aqueous plasma containing sodium, potassium, calcium,
magnesium, chloride, and sulfate ions; some trace elements;
a number of amino acids; and possibly a protein known
as a respiratory pigment. If present in invertebrates,
the respiratory pigments are normally dissolved in the
plasma and are not enclosed in blood cells. The constancy
of the ionic constituents of blood and their similarity
to seawater have been used by some scientists as evidence
of a common origin for life in the sea.
An animal's ability to control its gross blood concentration
(i.e., the overall ionic concentration of the blood)
largely governs its ability to tolerate environmental
changes. In many marine invertebrates, such as echinoderms
and some mollusks, the osmotic and ionic characteristics
of the blood closely resemble those of seawater. Other
aquatic, and all terrestrial, organisms, however, maintain
blood concentrations that differ to some extent from
their environments and thus have a greater potential
range of habitats. In addition to maintaining the overall
stability of the internal environment, blood has a range
of other functions. It is the major means of transport
of nutrients, metabolites, excretory products, hormones,
and gases, and it may provide the mechanical force for
such diverse processes as hatching and molting in arthropods
and burrowing in bivalve mollusks.
Invertebrate blood may contain a number of cells (hemocytes)
arising from the embryonic mesoderm. Many different
types of hemocytes have been described in different
species, but they have been studied most extensively
in insects, in which four major types and functions
have been suggested: (1) phagocytic cells that ingest
foreign particles and parasites and in this way may
confer some nonspecific immunity to the insect; (2)
flattened hemocytes that adhere to the surface of the
invader and remove its supply of oxygen, resulting in
its death; metazoan parasites that are too large to
be engulfed by the phagocytic cells may be encapsulated
by these cells instead; (3) hemocytes that assist in
the formation of connectivetissue and the secretion
of mucopolysaccharides during the formation of basement
membranes; they may be involved in other aspects of
intermediate metabolism as well; and (4) hemocytes that
are concerned with wound healing; the plasma of many
insects does not coagulate, and either pseudopodia or
secreted particles from hemocytes (cystocytes) trap
other such cells to close the lesion until the surface
of the skin regenerates.
While the solubility of oxygen in blood plasma is adequate
to supply the tissues of some relatively sedentary invertebrates,
more active animals with increased oxygen demands require
an additional oxygen carrier. The oxygen carriers in
blood take the form of metal-containing protein molecules
that frequently are coloured and thus commonly known
as respiratory pigments. The most widely distributed
respiratory pigments are the red hemoglobins, which
have been reported in all classes of vertebrates, in
most invertebrate phyla, and even in some plants. Hemoglobins
consist of a variable number of subunits, each containing
an iron–porphyrin group attached to a protein. The distribution
of hemoglobins in just a few members of a phylum and
in many different phyla argues that the hemoglobin type
of molecule must have evolved many times with similar
iron–porphyrin groups and different proteins.
The green chlorocruorins are also iron–porphyrin pigments
and are found in the blood of a number of families of
marine polychaete worms. There is a close resemblance
between chlorocruorin and hemoglobin molecules, and
a number of species of a genus, such as those of Serpula,
contain both, while some closely related species exhibit
an almost arbitrary distribution. For example, Spirorbis
borealis has chlorocruorin, S. corrugatus has hemoglobin,
and S. militaris has neither.
The third iron-containing pigments, the hemerythrins,
are violet. They differ structurally from both hemoglobin
and chlorocruorin in having no porphyrin groups and
containing three times as much iron, which is attached
directly to the protein. Hemerythrins are restricted
to a small number of animals, including some polychaete
and sipunculid worms, the brachiopod Lingula,and some
priapulids.
Hemocyanins are copper-containing respiratory pigments
found in many mollusks (some bivalves, many gastropods,
and cephalopods) and arthropods (many crustaceans, some
arachnids, and the horseshoe crab, Limulus). They are
colourless when deoxygenated but turn blue on oxygenation.
The copper is bound directly to the protein, and oxygen
combines reversibly in the proportion of one oxygen
molecule to two copper atoms.
The presence of a respiratory pigment greatly increases
the oxygen-carrying capacity of blood; invertebrate
blood may contain up to 10 percent oxygen with the pigment,
compared with about 0.3 percent in the absence of the
pigment. All respiratory pigments become almostcompletely
saturated with oxygen even at oxygen levels, or pressures,
below those normally found in air or water. The oxygen
pressures at which the various pigments become saturated
depend on their individual chemical characteristics
and on such conditions as temperature, pH, and the presence
of carbon dioxide.
In addition to their direct transport role, respiratory
pigments may temporarily store oxygen for use during
periods of respiratory suspension or decreased oxygen
availability (hypoxia). They may also act as buffers
to prevent large blood pH fluctuations, and they may
have an osmotic function that helps to reduce fluid
loss from aquatic organisms whose internal hydrostatic
pressure tends to force water out of the body.
