True coelom is the cavity between alimentary canal and body wall which is lined by mesoderm (NCERT 11th . pg 48)

**The Coelom: A Brief Introduction**

The coelom is a fluid-filled cavity that is found in many animals, including humans. It is located between the alimentary canal (digestive tract) and the body wall. The coelom plays a crucial role in the body's development, organization, and functioning.

**The Coelom Development: Mesoderm**

The coelom is derived from the mesoderm, one of the three primary germ layers in early embryonic development. The mesoderm gives rise to various tissues and structures, including muscles, bones, and organs. It also gives rise to the coelom.

**The Coelom Enclosure: Mesoderm on Both Sides**

The correct answer to the question is option D, which states that the coelom is enclosed by mesoderm on both sides. This means that the mesoderm forms the inner lining of the coelom, as well as the outer lining. The mesoderm on both sides of the coelom forms a continuous layer, encasing the cavity.

**Embryonic Development and Coelom Formation**

During embryonic development, a process called schizocoely occurs. In schizocoely, the mesoderm splits into two layers: the somatic mesoderm and the splanchnic mesoderm. The somatic mesoderm lines the outer body wall, while the splanchnic mesoderm lines the inner surface of the alimentary canal.

As development progresses, the space between these two layers expands and forms the coelom. The coelom thus becomes enclosed by the mesoderm on both sides, with the somatic mesoderm forming the outer lining and the splanchnic mesoderm forming the inner lining.

**Importance of the Coelom**

The coelom has several important functions in the body. It provides a protective cushion for the internal organs, allowing them to move and function without being damaged by external forces. It also serves as a space for organ development and movement, facilitating the proper functioning of systems such as the circulatory, respiratory, and reproductive systems.

Additionally, the coelom plays a crucial role in the development and evolution of animals. It provides a framework for the organization of the body, allowing for the differentiation and specialization of tissues and organs. The presence of a coelom is considered a defining characteristic of certain groups of animals, such as the coelomates.

In conclusion, the coelom is the cavity located between the alimentary canal and body wall. It is enclosed by mesoderm on both sides, as the mesoderm forms the inner and outer linings of the coelom during embryonic development. The coelom is essential for the proper functioning, development, and organization of the body.