When we think of animals, we think of movement. Surprisingly, the diverse and graceful ballet of animal movement may have started with cnidarians (pronounced "ny-DAIR-ee-ans), a group that includes corals, sea anemones, sea pens and jellyfish. All of these animals, with few exceptions, have nerves and muscles. Because cnidarians are the simplest animals to possess this complexity, their direct ancestors were very likely the first animals to bundle the power of nerves and muscles together, enabling them to move and exhibit discernible behavior.
Cnidarians are also the first animals with an actual body of definite form and shape. Most feature tentacles with stinging cells used to capture prey. The cnidarian's sting comes from tiny, often toxic harpoons called nematocysts. Triggered by touch or by certain chemicals, nematocysts fire out of the stinging cells at lightning speed.