Anatomy: Animals Inside

Circulatory System

Do all animals have blood?

Some invertebrates, such as flatworms and cnidarians, lack a circulatory system that contains blood. These animals possess a clear, watery tissue that contains some phagocytic cells, a little protein, and a mixture of salts similar to seawater. Invertebrates with an open circulatory system have a fluid that is more complex and is usually referred to as hemolymph (from the Greek term haimo, meaning “blood,” and the Latin term lympha, meaning “water”); their hemoglobin is not concentrated in cells within the hemolymph, but rather is found floating in the hemolymph. Invertebrates with a closed circulatory system have blood that is contained within blood vessels. Other examples of circulatory systems are found in squids, octopi, and crustaceans—animals that also have oxygen-carrying molecules in their plasma, but their bodies use a copper-based molecule hemocyanin to carry oxygen instead.


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