Hydra

For almost three billion years, the only living things on earth were one-celled creatures, either prokaryotes or eukaryotes. Even today, the vast majority of the living things on earth are one-celled creatures, or microbes. But about 600 million years ago, a few one-celled creatures began to group together and cooperate to make multi-celled creatures. The first ones were probably sponges, and then Hydra are the next earliest and simplest creatures with more than one cell.

A hydra eating a water flea

One problem that any multi-celled creature has to solve is how to get food to all of the cells. All of the hydras lived in the water, like all other living things at this time, so the food normally just floated by and the cell pulled it inside through its lipid

A hydra is a small multicellular organism in the genus of the same name. These tiny animals are found in fresh water all over the world, and they have a number of distinctive traits that make them interesting to scientists. People who are interested in seeing one for themselves can try taking a sample of some local fresh water and looking at it under a microscope; in addition to hydras, they may see an assortment of interesting aquatic creatures including water bears, diatoms, and rotifers.
The body of a hydra is formed in the shape of a tube, and the organisms demonstrate radial symmetry, meaning that they are symmetrical along multiple planes when viewed head on. One end has a foot called a basal disc; the animals secrete an adhesive substance to attach themselves to substrates like rocks and plants. The mouth opening is on the other end of the tube, and it is surrounded by tentacles that have small stinging cells for stunning prey. These cells can be found in many members of the Cnidaria phylum; jellyfish are perhaps the most famous stinging representatives.