The flatworms, , soft-bodied invertabrate animals. Unlike other bilaterians, they have no body cavity, and no specialized circulatory and respiratory organs, which restricts them to flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion.
The first flatworms, or platyhelminthes, probably evolved from something like a hydra about 550 million years ago. Like all other living things at this time, these early flatworms all lived in the ocean. A flatworm has a body shaped roughly like a bag, like the earlier hydras.
Flatworm Flatworms were the first animals that had bilateral symmetry, instead of radical symmetrylike a hydra, or no symmetry at all like a sponge. Bilateral symmetry must have been a big advantage, because nearly all later animals had bilateral symmetry. Inside, flatworms had a nervous system with most of the senses concentrated in the head end, so that when the flatworm moved forward, the head, which was in front, could detect danger in time to escape it. Besides this early brain, there were just two long neurons that ran the length of the flatworm's body. On top of the head, there were simple eyes that could sense light, like the eyespots on eukaryotic cells.
Flatworm Flatworms were the first animals that had bilateral symmetry, instead of radical symmetrylike a hydra, or no symmetry at all like a sponge. Bilateral symmetry must have been a big advantage, because nearly all later animals had bilateral symmetry. Inside, flatworms had a nervous system with most of the senses concentrated in the head end, so that when the flatworm moved forward, the head, which was in front, could detect danger in time to escape it. Besides this early brain, there were just two long neurons that ran the length of the flatworm's body. On top of the head, there were simple eyes that could sense light, like the eyespots on eukaryotic cells.