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.
To eat, a hydra extends its tube shaped body and traps prey in its tentacles. It feeds on a range of other small invertebrates, with waste products from the digestion process being secreted through the mouth opening. The creatures can reproduce sexually or asexually, depending on their environment, and they also exhibit hermaphroditic tendencies that allow them to produce eggs and then fertilize them.
Depending on the extent of the damage, these animals are able to partially regenerate themselves after injuries; in the 1800s, biologists mistakenly believed that they could force a hydra through a sieve and they individual pieces would regenerate. While this is not, in fact, the case, they are remarkably hardy. Unlike other animal species, they also do not age; a 1998 paper by Daniel Martinez detailed extensive research on this topic, and other researchers have since followed suit.
The largest examples are still so small that observers need microscopes to discern their features. Along with numerous other tiny aquatic organisms, hydras demonstrate the incredibly diverse life that can be found on every corner of the Earth. While these creatures might seem extremely bizarre to humans, they have survived for millions of years, enduring changing environments and animal populations with remarkable adaptability.