Fun animal you didn't know about

Bet you hadn't heard about Hydra! Here's one:

They're little carnivorous fresh water cnidarians (cousins with jellyfish) that live their lives attached to rocks. Though they can reproduce sexually, they choose to do so asexually, for efficiency presumably.

Hydra bodies are made up for not very many cells (50-100,000) and grow up to 10mm, you can't really do much with so few cells. Their most fun quirk is the ones that gives them their name which, as you may be able to guess, is regrowing severed appendages. Oh, they're also immortal.

First came across cnidarians in the book Metazoa, where they're one of the animals used to illustrate the strange evolution of sensing and, eventually, consciousness. Fun little guys.