Which of the following defines a coelomate organism?

A coelomate organism is defined by the presence of a true coelom, which is a body cavity that is fully lined by tissue derived from the mesoderm. This structure provides a space for internal organs to develop and function independently of the outer body wall, allowing for more complex movement and organization of bodily systems. The unique lining of this cavity enables the body cavity to support organs, facilitate the transport of nutrients, and house circulatory systems.

The correct choice states that a coelomate has a body cavity lined by muscle tissue and blood vessels both on the outer and inner surfaces. This accurately describes the mesoderm-derived tissue that lines the coelom, allowing for efficient physiological processes.

In contrast, organisms that do not have a true coelom, such as acoelomates, lack any true body cavity, while pseudocoelomates possess a body cavity that is partially lined with mesodermal tissue but remains not fully encased in muscular tissue like true coelomates. The other options reflect these conditions that do not meet the complete structural definition of a coelomate organism.