What body cavity arises from the archenteron of an embryo?
It does actually become a body cavity, it becomes the digestive tract; the tube that passes through the body from mouth to anus.
The term "pseudocoelom" refers to how the body cavity appears in the embryo of an organism. As you probably know, the bilaterians (all organisms with left-right symmetry, and the echinoderms for phylogenetic reasons) have three layers of tissue in the embryo: the ectoderm, endoderm, and mesoderm. In the "true" coelomates, the body cavity ("coelom") is completely surrounded by mesodermal tissue. In the pseudocoelomates, though, the body cavity is partly surrounded by mesoderm but partly surrounded…
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.
yes, I think it's called coelom. An organism with a true body cavity is called a eucoelomate. This means that the body cavity exists between the inner layer of mesoderm and the outer layer of mesoderm. A pseudocelomate kind of has a body cavity but it is the space between the endoderm and mesoderm. This is not a true body cavity because it is not mesoderm on mesoderm.