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What is a Coelom?

Michael Anissimov
Updated May 21, 2024
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A coelom (pronounced "seal-um") is a fluid-filled body cavity found in most animals. It is located in the mesoderm, the middle germ layer only found in triploblastic (three-layered) organisms. Simpler animals like cnidarians (jellyfish, coral, etc.) and sponges are diploblastic and monoblastic respectively, lacking a coelom. Though the cavity developed in triploblastic animals, some of these animals have lost it.

Animals with a coelom, including the majority of animal phyla, are called coelomates. Animals without it, such as flatworms, are called acoelomates. In between there are some animals called pseudocoelomates, which possess a "false coelom," which is an unlined or partially lined body cavity between the gut and body wall.

A coelom is defined as a cavity that separates the gut from the body wall. It allows the internal organs to shift around and develop independently of the body wall, creating more physiological and evolutionary flexibility. Though the term usually refers to the largest body cavity, coelomate animals (such as humans) may feature several strategically placed body cavities.

The earliest known animal with a coelom is Vernanimalcula guizhouena, which lived 600 million years ago, during the Ediacaran period. The cavity is a crucial evolutionary innovation that enabled the existence of nearly all large, complex animals. The only acoelomate animals are sponges, platyhelminthes (flatworms, tapeworms etc.), nemerteans (ribbon worms), and cnidarians (jellyfish, etc.), none of which are especially complex.

The biggest disadvantage of lacking a coelom is that the internal organs are much more susceptible to compression and damage. The protection that coelomate animals get from their body cavity stems from the fact that fluids are incompressible, while organs are not. Since the organs are surrounded by an incompressible fluid, they are well-protected in coelomates.

Lots of the smallest, simplest, and most numerous animals are pseudocoelomate, including nematodes, rotifers, kinorhynchans (mud dragons), nematomorphs (horsehair worms), gastrotrichs, loriciferans, priapulid worms, spiny-headed worms, and entoprocts. Many of these organisms are less than 0.04 inches (1 millimeter) in size, but some, like priapulid worms, grow as large as 6 inches (15 cm).

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Michael Anissimov
By Michael Anissimov
Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology, astronomy, chemistry, and futurism to his articles. An avid blogger, Michael is deeply passionate about stem cell research, regenerative medicine, and life extension therapies. His professional experience includes work with the Methuselah Foundation, Singularity Institute for Artificial Intelligence, and Lifeboat Foundation, further showcasing his commitment to scientific advancement.
Discussion Comments
By Nawaf — On Jun 07, 2011

@SouthernMuse - The difference between pseudo coeloms and true coeloms is that a true coelom has a mesodermal lining on all sides, allowing muscles to develop that surround the organs and support them within the body wall. This support system allows more efficient digestion of foods and the passing of other substances through the internal organs.

By SouthernMuse — On Jun 04, 2011

So can anybody tell me what the difference is between pseudo coeloms and true coeloms? I know that it's something to do with a lining, but I'm really not sure, and I can't find the answer in my textbook. I really need to finish this homework, somebody help me out!

By panda2006 — On Jan 30, 2011

While it might seem easy to say that all animals with coeloms are also vertebrates, and all animals without them are invertebrates, it is not quite that simple. Some simple vertebrates do have a pseudo coelom, meaning that they are somewhat in between, at least having enough of one to make it hard to say they do not.

By anon141587 — On Jan 10, 2011

Thanks for the info mate! needed it for biology, now to find out how it evolved.

Michael Anissimov
Michael Anissimov
Michael Anissimov is a dedicated All The Science contributor and brings his expertise in paleontology, physics, biology...
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