The hydra is a favorite subject of middle school science. It is a fearsome-looking tentacled predator, but it is tiny — less than a half inch — and lives in ponds. You can collect or buy hydras and the tiny crustaceans they eat, then watch the capture under a dissecting microscope. Hydras, like jellyfish, have stinging cells in their tentacles.

They usually reproduce by budding. If you cut them up in pieces, the odds are good that a piece will become a new hydra, sometimes a hydra with two heads. But if you want to get children interested in science, that is hardly a drawback.

Adult, professional scientists are interested in them too, particularly the way a hydra snippet grows up. For example, one of the great discoveries of modern science, is the way that genes control how a few cells develop into a highly complex organism. Fruit flies, zebrafish and humans share some of the same genes that direct this process.

Hydras are much simpler, made of just a few layers of cells. But they still respond to chemical signals sent out by genes as they grow into a tubelike body and tentacle-encircled maw.