Coelomic cavity

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

The coelom is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In developed animals, it is lined with a mesodermal epithelium. In other animals, such as molluscs, it remains undifferentiated. The word coelom comes from the similar Greek word meaning a cavity.

Coelom formation begins in the gastrula stage. The developing digestive tube of an embryo forms as a blind pouch called the archenteron.

In Protostomes, the coelom forms by a process known as schizocoely. The archenteron initially forms, and the mesoderm splits into two layers: the first attaches to the body wall or ectoderm, forming the parietal layer and the second surrounds the endoderm or alimentary canal forming the visceral layer. The space between the parietal layer and the visceral layer is known as the coelom or body cavity. Examples of protostome coelomates include earthworms, snails, clams, slugs and octopuses.

In Deuterostomes, the coelom forms by enterocoely: mesoderm buds from the walls of the archenteron and hollows to become the coelomic cavities. Some examples of deuterostome coelomates are sea urchins, fish, sea stars and humans.

The evolutionary origin of the coelom is uncertain. The oldest known animal to have had a body cavity was the Vernanimalcula. Current hypothesis include:

A coelom can absorb shock or provide a hydrostatic skeleton. It can also support an immune system in the form of coelomocytes that may either be attached to the wall of the coelom or may float about in it freely. The coelom allows muscles to grow independently of the body wall — this feature can be seen in the digestive tract of tardigrades (also known as water bears) which is suspended within the body in the mesentery derived from a mesoderm-lined coelom.