Coelomata

Body cavity
Cross-section of an oligochaete worm. The worm's body cavity surrounds the central typhlosole.
Anatomical terminology []

A body cavity is any fluid-filled space in a multicellular organism other than those of vessels (such as blood vessels and lymph vessels). The term usually refers to the space located between an animal’s outer covering (epidermis), and the outer lining of the gut cavity, where internal organs develop.

The human body cavity normally refers to the ventral body cavity, because it is by far the largest.

A coelom is a cavity lined by an epithelium derived from mesoderm. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.

Arthropods and most mollusks have a reduced (but still true) coelom, usually the pericardial cavity and the gonocoel. Their principal body cavity is the hemocoel or haeomocoel of an open circulatory system, often derived from the blastocoel.

Mammalian embryos develop two cavities: the intraembryonic coelom and the extraembryonic coelom (or chorionic cavity). The intraembryonic coelom is lined by somatic and splanchnic lateral plate mesoderm, while the extraembryonic coelom is lined by extraembryonic mesoderm. The intraembryonic coelom is the only cavity that persists in the mammal at term, which is why its name is often contracted to simply coelomic cavity. Subdividing the coelomic cavity into compartments, for example, the pericardial cavity / pericardium, where the heart develops, simplifies discussion of the anatomies of complex animals.