The hydra is a small coelenterate that is found in freshwater (sea anemones and jellyfish are more familiar marine coelenterates). It is usually in the range of a centimeter long, but hard to see as it is translucent.

A hydra attach itself to a substrate with a sticky substances, but is capable of moving, using the tentacles. The tentacles which contain stinging cells (nematocycts) are also used for catching food and even for defense. The hydra can reproduce by forming a bud (see frame 2 of the photograph) that will eventually be pinched off to become free living.

The green hydra gets its color from symbiotic algae, but it also feeds on a wide variety of small aquatic invertebrates.