Within bilaterian animals several taxa have spacious body cavities. The structural basis of such cavities can be divided into primary and secondary body cavities, the latter being the coelom. Body cavities are absent in compact (‘acoelomate’) animals. Primary and secondary body cavities can also fuse to form a mixocoel. More than one type of body cavity can be present in one animal at the same time or during its life cycle. Some phylogenetic hypotheses include the evolution of body cavities as a central part. Nevertheless, it appears that both types of body cavities evolved in parallel in several cases.
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