What are primary and secondary body cavities?

The primary body cavity is a persistent blastocoel in which the internal organs remain free and not bound by peritoneum or mesoderm. It is a space bounded by ectoderm on the outside and endoderm on the inside, and not lined with mesoderm on either side. This pseudo-whole is found in many worms such as animals, including rotifers, roundworms, etc.

More highly developed bilateria have a secondary body cavity. The blastocel is gradually destroyed by the archenteron. inside the embryonic mesoderm, a new cavity or true body cavity develops, which is lined on the outside with somatic mesoderm, and on the inside with splenic mesoderm (peritoneum).