The body cavity, or coelom, enabled the development of larger, more complex animals. Animals with one are coelomates, while those without are acoelomates. Some animals have a pseudocoele. The cavity allows for organ movement, cushioning, and chemical exchange. Coelomates evolved from cnidarians, and acoelomates evolved from coelomate ancestors. Acoelomates tend to be flat and lack efficient nutrient transfer. Coelomates often have dedicated circulatory organs.
The evolution of the body cavity (also known as the coelom, pronounced “seel-um”) was instrumental in enabling the development of larger, more complex animals. Animals with one – which are the vast majority – are known as coelomates, while animals without one are called acoelomates. There are also animals without a true coelom, which instead have a structure called pseudocoele. These include nematodes, rotifers, kinorhynchans (mud dragons), nematomorphs, gastrotrichans, loricifera (brush heads), priapulidans (penis worms), acanthocephalans (spiny-headed worms), and entoproctans (calyx worms). Most of these are microscopic.
Body cavities are always filled with fluid. The purpose of the space is for the organs to slide independently of the body wall, cushion the organs, and allow for easy chemical exchange between the organs and the rest of the body. They are thought to have evolved into triploblastic bilaterians (bilaterally symmetrical animals with three layers of tissue) about 600 million years ago, around the dawn of generally known multicellular organisms. Animals with a body cavity evolved from basic cnidarians, which might have resembled something modern like the hydra, which doesn’t have one. One theory holds that the cavity evolved from the gastric pouches (stomachs) of early cnidarians.
At one point, acoelomate animals were thought to be all related to each other, forming an independent clade. This is now known to be false, as most scientists agree that acelomatous triploblasts (any animal besides cnidarians or sponges) evolved from coelomate ancestors. This coelom reduction occurs due to a process of evolutionary rationalization, and is particularly common among parasites. Because they lack an efficient means of transferring nutrients and other chemicals between organs and the rest of the body, acoeloma animals tend to be flat, as this makes diffusion possible. In addition to a simple body cavity, coelomate animals often have dedicated circulatory organs for moving in nutrient- and oxygen-rich body fluids.
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