Final answer:
A true coelom is completely lined with tissue derived from mesoderm and is characteristic of eucoelomates, providing space for internal organs and allowing for efficient organ function.
Step-by-step explanation:
In biology, during embryonic development, body cavities in multicellular animals can be classified as acoelomates, pseudocoelomates, or eucoelomates. Acoelomates have no body cavity, pseudocoelomates have a body cavity that is not completely lined by mesodermal tissue, and eucoelomates have a body cavity entirely lined by mesodermal tissue, called a coelom. Eucoelomates, such as earthworms, snails, insects, starfish, and vertebrates, possess this true coelom. This coelom can form in one of two ways: schizocoely, where the body cavity forms when a mass of mesodermal cells splits, generally seen in protostomes; and enterocoely, where the mesoderm develops as pouches off the endoderm during gastrulation, which is typical in deuterostomes like chordates and echinoderms.
Understanding the structure of a true coelom is crucial as it allows for more complex organ development and efficient functioning of the organism's body systems. It provides space for the internal organs, allows for their movement and growth, and facilitates the distribution of nutrients and removal of waste.