Protostomes form a coelom by the use of a 'blind pouch' that appears when the organism's digestive tract is beginning to develop. It occurs when the mesoderm, or middle layer of the three basic layers of tissue, begins to split, which develops into the coelom.

What is coelom and how is it formed?

Coelom is the mesodermally lined cavity between the gut and the outer body wall. During the development of the embryo, coelom formation begins in the gastrulation stage. The developing digestive tube of an embryo forms as a blind pouch called the archenteron.

Where does the coelom come from?

Derived from the mesoderm, the coelom is found between the intestinal canal and the body wall, lined with mesodermal epithelium. The mesodermal tissue also goes on to form the blood, bones, digestive tract, gonads, kidneys, and other organs. Organisms that possess a true coelom are called (true) coelomates.