Let’s first start by examining what blood is
and what it does. Blood is a fluid within most
animals that delivers oxygen and nutrients to
different parts of the body. It can be several
different colors depending on the organism.
Vertebrates (mammals, amphibians, reptiles, and
birds) have red-blood cells that travel through a
closed circulatory system (a series of arteries
and veins). The circulatory system is
considered closed because the blood is always
contained within blood vessels. Vertebrate
blood is red because it has an iron-containing
substance called hemoglobin.
The blood of most mollusks, which include
squid, octopus, snails, slugs, and horseshoe
crabs, is blue! It is blue because it has a
copper-containing substance called
hemocyanin. Mollusks have an open
circulatory system where blood is contained in a
cavity where it surrounds and bathes the internal
Flatworms, nematodes, and cnidarians
(jellyfish, sea anemones, and corals) do not
have a circulatory system and thus do not have
blood. Their body cavity has no lining or
fluid within it. They obtain nutrients and oxygen
directly from the water that they live in. When
the cells on the outside of their body come into
contact with water, oxygen and nutrients can
diffuse into their body to provide them with
everything they need.
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