Where does true coelom present in the body?

The correct option is between mesoderm and endoderm (body wall).

To understand why this is the correct answer, let us break down the concept of a true coelom and its embryonic origin step-by-step:
1. Definition of Coelom: A coelom is the principal body cavity in most animals, located between the intestinal canal (gut wall) and the outer body wall.
2. True Coelom (Eucoelomate): A true coelom is defined as a fluid-filled body cavity that is completely lined by mesoderm. This lining (known as peritoneum) covers both the inner side of the body wall and the outer side of the visceral organs (the gut).
3. Embryonic Layer Relations: During embryonic development, three primary germ layers are formed: ectoderm (outermost layer), mesoderm (middle layer), and endoderm (innermost layer). The ectoderm forms the outer skin/nervous system, the endoderm forms the lining of the gut (alimentary canal), and the mesoderm forms the muscles, bones, and connective tissues including the lining of the body cavity.
4. Position of the Cavity: Structurally, the coelomic cavity is positioned between the mesodermal lining associated with the body wall (somatic mesoderm) and the mesodermal lining associated with the endoderm-derived gut wall (splanchnic mesoderm). In basic anatomical reference, this body cavity sits anatomically between the mesoderm (which lines the body wall) and the endoderm (which forms the gut wall structure).
5. Why other options are incorrect:
- between body wall and ectoderm: The body wall itself contains ectoderm on the outside, and there is no primary body cavity formed between them.
- mesoderm and ectoderm: A cavity between ectoderm and mesoderm does not constitute a coelom.
- ectoderm and endoderm: A cavity between ectoderm and endoderm that is not lined by mesoderm is called a pseudocoelom (found in roundworms/Aschelminthes).

Therefore, the true coelom is located between the mesoderm lining the body wall and the endoderm lining the gut.