coelomista

Coelomista, more commonly referred to as coelomates or coelomata, denotes animals that possess a true coelom, a fluid-filled body cavity completely lined by mesoderm-derived tissue. The term derives from Greek koilos, meaning cavity, and soma, meaning body. In coelomates the coelom serves as an internal space for organs and as a flexible framework for body plan.

The coelom forms during embryogenesis. In many protostomes, development proceeds by schizocoely, in which a solid mass of mesoderm splits to create the coelomic cavity. In deuterostomes, coelom formation typically occurs by enterocoely, where mesodermal pouches bud from the gut to generate coelomic spaces. This distinction reflects broad developmental patterns across major animal groups.

Coelomates include most animal phyla: annelids, mollusks, arthropods, echinoderms, and chordates. Historically, animals were categorized as coelomates, pseudocoelomates, or acoelomates, depending on the presence and completeness of a body cavity. Modern phylogenetic views emphasize developmental mechanisms and genetic relationships rather than a single three-way division, but the concept of a true coelom remains a useful anatomical and evolutionary criterion.

Functions and variation: the coelom provides a cushioned, movable space for organs, supports a hydrostatic skeleton in many invertebrates, and allows greater organ differentiation and larger body sizes. Some lineages exhibit reduced or modified coelomic spaces, especially in parasitic or highly derived taxa.

In vertebrates, the coelomic spaces give rise to major body cavities such as the pericardial cavity around the heart, the pleural cavities around the lungs, and the peritoneal cavity within the abdomen.