Hydra can be found attached to water-plants, stones etc., with its cylindrical body hanging downwards, with the tentacles gently moving. When disturbed it will rapidly contract into a blob of jelly, making it difficult to find. Hydrae feed on small animals, such as water-fleas. These are caught by structures called nematocysts. There are different types of nematocysts, some stinging and paralysing the prey, whilst others discharge threads which coil round the prey and hold it. Most commonly used coelenterate. Each container is enough for a class of 30 students.