It does actually become a body cavity, it becomes the digestive tract; the tube that passes through the body from mouth to anus.
peritoneal cavity.
abdominal cavity
Protists do not have a body cavity.
The coelom is a fluid-filled body cavity between the intestines and the body wall of some higher metazoans. It is lined with a mesodermal epithelium. The coelom is formed within the mesoderm of the embryo.
Pisces or fish have true internal body cavity.
The body cavity of a frog is much larger in ratio to its body than a man's body cavity is. A frog's body cavity is also much more condensed than a man's.
thoracic cavity
thoracic and abdominopelvic cavity
The dorsal cavity.
abdominal cavity
abdominal cavity
cranial cavity