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 organs directly.

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.

flatworm
nematode
jellyfish