A pseudocoelomate animal possesses no internal space between its gut and its external musculature. A coelomate animal possesses a true body cavity between the external surface of the gut and the internal surface of the body wall, in which many of the organs are suspended by connective tissues. Coelomates have as an advantage over pseudocoelomates and acoelomates the potential space for digestive tracts that are longer than their own body lengths, permitting a wider range of feeding strategy, as well as internal spaces for the development and storage of gametes. Cells are organized into functional units called tissues, which aggregate to form organs. The simplest group in which organ level of development is seen is the Platyhelminthes.
Protostome animals exhibit spiral determinate cleavage, with the blastopore becoming the mouth.
Deuterostomes exhibit radial indeterminate cleavage, with the blastopore becoming the anus. The major coelomate protostome phyla include the annelids, molluscs, and arthropods. Echinoderms and chordates are deuterostomes. In protostomes, the coelom arises as an enlarging space within the mesoderm. In deuterostomes, the coelom is formed from an evagination of the archenteron.