Tissues
Dinosaurs are prehistoric and extinct, but they are not the most primitive animals. That honor belongs to sponges. We are not talking about the type of sponge at the kitchen sink. We're talking about the SpongeBob SquarePants type of sponge. They're bright pink, purple, and orange sponges that live in the ocean and make up the phylum Porifera. Sponges are joined by another phylum, Placozoa, in a clade called the Parazoa. The animals in both phyla are asymmetrical. They have specialized cells that perform different functions within the organism but their cells do not join together to form tissues. Tissues are groups of cells, like skin cells, working together on the same job.