What defines the distinction between Parazoa and Eumetazoa?

The distinction between Parazoa and Eumetazoa primarily lies in the structural complexity of their cellular organization. Parazoa, which includes organisms like sponges, lack true tissues. This means that their cells are not organized into specialized tissues that work together to perform specific functions. Instead, Parazoa have a more loosely organized cellular structure, where different types of cells carry out various functions somewhat independently.

In contrast, Eumetazoa includes all animals with true tissues, which are organized into distinct layers and perform coordinated functions. This group encompasses a wide range of organisms, from cnidarians like jellyfish to more complex animals like mammals. The presence of these true tissues is a key characteristic that differentiates Eumetazoa from Parazoa.

The other factors listed, such as body cavities, bilateral symmetry, and coelom, pertain to classifications within the Eumetazoa specifically and do not apply to the fundamental distinction between these two groups. For example, while many Eumetazoans exhibit bilateral symmetry and have body cavities, these traits do not define the core difference from Parazoa, which is their lack of true tissue organization.