cnidarians

A class of marine animals which include sea anemones, corals, hydroidshydroids Hydroids, or sea-firs, are plant-like animals. They resemble fir-like plants, but are actually the simplest of stinging-celled animals. The cells of these animals contain capsules (called cnidae), and these capsules contain a long hollow, coiled thread which uncoils and shoots out under water pressure when the cell is triggered by touch or water pressure. These threads, whose precise characteristics vary according to species, capture the prey., sea pens and sea firs, and jellyfish. Cnidarianscnidarians A class of marine animals which include sea anemones, corals, hydroids, sea pens and sea firs, and jellyfish. Cnidarians are distinguished by the fact that they possess stinging cells which are used both for defence and the capture of prey. These stinging cells contain a long, hollow, coiled thread which shoots out and uncoils under water pressure. Discharge is triggered by touch or chemical stimulus. The threads have different functions. Some simply entangle their prey, whilst others stick to it or inject poison. Most cnidarians have a two-stage life cycle. In one of these the animal takes the form of a polyp where it has a tubular body fastened to hard surfaces whilst during the alternate, known as the medusa stage, the animal is free-floating and swims in the sea. Sea anemones, for example, are usually encountered in their polyp stage; whereas jellyfish are usually encountered in their medusa stage. are distinguished by the fact that they possess stinging cells which are used both for defence and the capture of prey. These stinging cells contain a long, hollow, coiled thread which shoots out and uncoils under water pressure. Discharge is triggered by touch or chemical stimulus. The threads have different functions. Some simply entangle their prey, whilst others stick to it or inject poison. Most cnidarians have a two-stage life cycle. In one of these the animal takes the form of a polyp where it has a tubular body fastened to hard surfaces whilst during the alternate, known as the medusa stage, the animal is free-floating and swims in the sea. Sea anemones, for example, are usually encountered in their polyp stage; whereas jellyfish are usually encountered in their medusa stage.