The fluid-filled cavity in an embryo, in the early embryonic stage, is called a blastocoele, or blastocyst cavity; this is perhaps the cavity you're referring to.
See the link below for more information on the human embryo.
Fistula of what? A fistual is an abnormal tubelike passage from a normal cavity or tube to a free surface or to another cavity. So depending on where the fistual is depends on the severity of the case. Fistula near rectum
The ventral cavity that develops in the embryo is called the coelom. It is a fluid-filled cavity that forms during early embryonic development and gives rise to the body cavities in higher organisms. The coelom helps provide space for organ development and movement.
Pisces or fish have true internal body cavity.
An internal body cavity is a space in the body that does not contain something, much like a cavity in a tooth is where the tooth has rotted away. There are several examples of this: your sinuses are internal body cavities - they are an open space in your body. There is a body cavity in your chest as well surrounding your lungs and heart.
Arthropods have an internal body cavity called a hemocoel which bathes the internal organs in nutrients, and is a component of their open circulatory system.
Probably because you can visualize the same from out side. You can list the same in internal body cavity and you wont be wrong. Mostly because the mouth and the anus both lead to the external environment.
It does actually become a body cavity, it becomes the digestive tract; the tube that passes through the body from mouth to anus.
The function of the chest cavity is to support some internal organs. The lungs and the heart are located in the chest cavity which protects and supports them.
An internal cavity is a hollow space or void within a structure or organism, such as the empty space within a tooth or a body cavity within an animal. These cavities can serve various functions, including housing organs, providing protection, or aiding in movement or structural stability.
pneumothorax
internal OS
internal iliac artery