Acoelomate

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

The coelom (/ˈsiːləm/ SEE-ləm, plural coeloms or coelomata /siːˈloʊmətə/ see-LOH-mə-tə) (from Greek: κοῖλοςkoîlos, hollow, cavity) 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.

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.