A coelom is a fluid-filled body cavity found in most triploblastic animals, allowing internal organs to move and develop independently. Animals with a coelom are called coelomates, while those without are acoelomates. Pseudocoelomates possess a “false coelom.” The earliest known animal with a coelom is Vernanimalcula guizhouena. The protection coelomous animals obtain from their body cavities comes from the fact that fluids are incompressible, while organs are not. Many smaller animals are pseudocoelomates.
A coelom (pronounced “seal-um”) is a fluid-filled body cavity found in most animals. It is found in the mesoderm, the middle germ layer found only in triploblastic (three-layered) organisms. Simpler animals such as cnidarians (jellyfish, corals, etc.) and sponges are respectively diploblastic and monoblastic, with no coelom. Although the cavity has developed in triploblastic animals, some of these animals have lost it.
Animals with a coelom, including most animal phyla, are called coelomates. Animals without it, such as flatworms, are called acoelomates. In between are some animals called pseudocoelomates, which possess a “false coelom,” which is an unlined or partially lined body cavity between the intestine and the body wall.
A coelom is defined as a cavity that separates the intestine from the body wall. It allows internal organs to move and develop independently of the body wall, creating greater physiological and evolutionary flexibility. Although the term usually refers to the largest body cavity, coelomous animals (such as humans) can have 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 allowed almost all large and complex animals to exist. The only acoelomate animals are sponges, flatworms (flatworms, tapeworms, etc.), nemerthei (ribbon worms), and cnidarians (jellyfish, etc.), none of which are particularly complex.
The biggest downside to not having a coelom is that internal organs are much more susceptible to compression and damage. The protection coelomous animals obtain from their body cavities comes 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.
Many of the smaller, simpler, and more numerous animals are pseudocoelomates, including nematodes, rotifers, kinorhynchans (mud dragons), nematomorphs (horsehair worms), gastrotrichans, loriciferans, priapulid worms, spiny worms, and entoprocts. Many of these organisms are less than 0.04 inches (1 millimeter) in size, but some, such as priapulid worms, grow up to 6 inches (15 cm).
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