Endosymbiosis

Endosymbiosis sounds very fancy, but is just means that a relationship between two organisms evolved where one lived inside ("endo") another. Again the Amoeba Sisters give is a good overview of the Theory of Endosymbiosis.

Cells can work in amazing ways, whether their very small or very large. The Hydra is not some shadow organization or monster, but a very successful little microbe. Enjoy.