Here's a breakdown of what makes a coelom "true":
* Origin: It develops from the mesoderm, the germ layer that also gives rise to muscles, bones, and circulatory systems.
* Lining: The coelom is completely lined by mesoderm-derived tissue called peritoneum. This lining is distinct from the outer layer of the body (ectoderm) and the inner lining of the digestive tract (endoderm).
* Fluid-filled: The coelom is filled with fluid, which acts as a hydrostatic skeleton, providing support and allowing for movement.
Why is a true coelom important?
A true coelom offers several evolutionary advantages:
* Internal organ support: It provides space for organs to grow and develop, preventing them from being crushed by muscles or other tissues.
* Internal transport: The coelomic fluid facilitates the circulation of nutrients and waste products throughout the body.
* Movement: It acts as a hydrostatic skeleton, allowing for more complex and efficient movement.
* Organ specialization: The coelom allows for greater specialization of organs, as they are no longer confined to a single cavity.
Examples of animals with true coeloms:
* Annelids (earthworms, leeches)
* Molluscs (snails, clams, squids)
* Arthropods (insects, spiders, crustaceans)
* Echinoderms (starfish, sea urchins)
* Chordates (vertebrates, including humans)
Contrast with a pseudocoelom:
A pseudocoelom is a body cavity that is not fully lined by mesoderm. It is only partially lined by mesoderm, with the outer lining being ectoderm. Examples include roundworms (nematodes).
In summary, a true coelom is a significant evolutionary development that contributes to the complexity and success of many animal groups. It provides crucial support for internal organs, facilitates internal transport, and allows for more efficient movement and organ specialization.