What was the first group of animals to show true tissues?

Cnidarians represent the first group of animals to exhibit true tissues, which are organized collections of cells that perform specific functions. This is an important evolutionary development, as it signifies a move away from the simpler, more primitive body structures seen in organisms like sponges and placozoans, which lack true tissues.

In cnidarians, the cells are organized into layers that form two distinct tissue layers: the outer epidermis and the inner gastrodermis, with a jelly-like mesoglea in between. These tissue layers allow for the development of specialized functions such as digestion, reproduction, and response to stimuli, enhancing their capability for survival and interaction with the environment.

This level of organization is more advanced than that found in sponges and placozoans, which possess simple aggregates of cells without specialized tissues, and is foundational for the complexity seen in later animal phyla. Cnidarians include familiar organisms such as jellyfish, corals, and anemones, all showcasing this tissue organization and enabling more complex behaviors and interactions.