Sponges often don’t even look alive, but an ancient sponge was actually the first animal. The apparent simplicity of sponges is actually quite perplexing. Find out about the many responsibilities of sponge cells, and how they retain the freedom to continuously reinvent themselves.
Cnidarians were the first animals to have muscles and nerves to produce behavior. They were also the first to have a mouth and stomach to digest food. We learn about nematocysts when we watch an anemone catching a goby and two anemones fighting. Cnidarians come in various body shapes and have different ways of living. Corals are cnidarians that build reefs. One anemone, Stomphia, can swim away from predators by contracting its entire body. The jellyfish body plan is like an anemone that has been turned upside down and a diverse group of cnidarians thrives at all depths of the ocean.
Learn about the anatomy of the flatworm in this video segment. Flatworms look simple, but they were the first animals to evolve a bilateral body plan with a head, a brain, and stereo senses enabling them to actively hunt. Flatworms are hermaphroditic with both male and female reproductive organs, and have internal fertilization. With no circulatory system, their gut distributes food to all parts of their bodies. Learn more about the structure and evolution of this fascinating creature with The Shape of Life.