In amniotes and some invertebrates the peritoneum lines their largest body cavity called the coelom.

Cavitation in the early embryo is the process of forming the blastocoel, the fluid-filled cavity defining the blastula stage in non-mammals, or the blastocyst in mammals.

The abdominal cavity occupies the entire lower half of the trunk, anterior to the spine, and houses the organs of digestion.

[2] At the end of the third week of gestation, the neural tube, which is a fold of one of the layers of the trilaminar germ disc, called the ectoderm, appears.

The space between the visceral and parietal layers of lateral plate mesoderm is the primitive body cavity.

Ventral mesentery, derived from the septum transversum, exists only in the region of the terminal part of the esophagus, the stomach, and the upper portion of the duodenum.

The body maintains its internal organization by means of membranes, sheaths, and other structures that separate compartments.

The lungs, heart, stomach, and intestines, for example, can expand and contract without distorting other tissues or disrupting the activity of nearby organs.

[5] In amniotes and some invertebrates, the coelom is the large cavity lined by mesothelium, an epithelium derived from mesoderm.

Organs formed inside the coelom can freely move, grow, and develop independently of the body wall while fluid in the peritoneum cushions and protects them from shocks.

Arthropods and most molluscs have a reduced (but still true) coelom, the hemocoel (of an open circulatory system) and the smaller gonocoel (a cavity that contains the gonads).

This Wikipedia entry incorporates text from the freely licensed Connexions [1] edition of Anatomy & Physiology [2] text-book by OpenStax College