Not all blood is red. We are so accustomed to the idea that blood is red
that some of us are surprised to learn that it comes in other colors. It is
true that all animals with backbones have red blood -- mammals,
birds, turtles, snakes, frogs, salamanders and fish. However, among
lower animals we find blood that is colorless, blue or green -- as well
as a few with red blood.

If an animal is large enough to be seen with the naked eye, it is almost
certain to have some sort of a circulatory system. Usually, this is a
plumbing system with pipes, valves and pumps which brings
nourishment and oxygen to the living cells of the animal's body and
carries away waste products. The fluid flowing in this plumbing
system is blood. In man and the higher animals this blood stream is
completely enclosed in the heart, arteries, veins and their smaller
branches. That is called a closed circulatory system.

Many lower animals, in contrast, have an open circulatory system in
which blood fills the body spaces and bathes the living tissues directly.
A few lower forms of animal life, although quite large, have no blood
at all. The sponge, for example, instead of blood, circulates water
through a network of small canals. The living parts of a jellyfish are in
a thin outer layer in close contact with the water while its inner jelly is
practically lifeless.

An insect has a simple tubular heart which pumps the blood forward
into the head. From there it flows around the internal organs, through
the legs and other appendages, then back into a central pool, or blood
sinus, in which the heart lies. In many naked caterpillars with a
transparent streak down the back, this pulsating heart can be seen just
beneath the surface, and, under a microscope, the blood corpuscles as
they rush into the heart each time the valves along its sides open. In
insects which squirm or wriggle, the blood and the internal organs
surge back and forth with every movement.

Insects are exceptional. Their blood furnishes little or no oxygen to the
living tissues. Instead, they have many-branched ducts opening to the
outside, called a tracheal system, which carry air directly to every part
of the body.

The blood of all higher animals, from man down to fish, is made up
largely of red blood cells whose principal work is to carry oxygen from
the lungs or gills throughout the body. These red blood cells contain
hemoglobin -- a protein with an atom of iron in each of its large
molecules -- which is able to pick up or release large amounts of
oxygen readily. Such blood is bright red when it leaves the lungs or
gills and is dark purplish red when it returns after giving up its
oxygen. Some lower animals such as earthworms, pond snails, land
snails, water fleas and midge larvae also have red blood. In these the
hemoglobin is not in blood corpuscles but is dissolved in the blood

Many other lower animals carry oxygen in their blood stream by
means of a dissolved substance called hemocyanin which is blue. At
least it is blue when it is oxygenated, but colorless after the oxygen is
released. Hemocyanin is much like hemoglobin except that the iron
atom in the protein molecule is replaced by one of copper. The lobster,
crab, crayfish, scorpion, octopus, squid, clam and mussel all have this
blue blood. A few marine worms have green blood.

The blood stream of all kinds of animals has a variety of free-floating
or creeping cells much like the white blood cells in our own blood.
Among other tasks, these devour bacteria, foreign substances and bits
of dead tissue.