It's hard to imagine an organism much simpler than the hydra. It doesn't have brains, hearts, or eyes - it's basically just tentacles with a mouth attached. And yet these simple creatures pull of biological feats no other animal can.
Hydra are one of a few species - flatworms are another recently discussed example - that can regenerate their body parts, seemingly indefinitely. They also possess a surprisingly complex weapon system known as cnidocytes, which enable their tentacles to shoot powerful neurotoxins into their enemies. What's interesting is that these attacks seem to be connected to light levels, and yet hydra don't just lack eyes - they don't even have a visual system at all. And yet they hunt on a clear night-day cycle and have been observed moving in response to sudden changes in light.