Schizocoelic development
The coelom is a fluid-filled body cavity, where the internal organs are suspended in. It is the cavity between the wall of the body and the digestive tract.
In the schizocoelic development of the embryo, the coelom, called the schizocoel, develops as a split in the mesoderm. The outer layer of the mesoderm attaches with the ectoderm to form a body's musculature, while the inner layer attaches with the endoderm to form the wall of the digestive tract.
This type of development is commonly found in annelids, arthropods, and mollusks.