Cnidaria is one of the 30 or so recognized phyla of the animal kingdom. The term cnidaria comes from the Greek word cnidos, which means "stinging nettle." Cnidaria is a diverse phylum, but its constituents are united by the presence of stinging cells called nematocysts, or cnidocysts, that the animals use for predation and protection. All cnidaria are aquatic; the vast majority are saltwater organisms. Some scientists combine the phyla cnidaria and ctenophora (comb jellies) into a single phylum coelenterata since organisms of both phyla contain a single gastrointestinal cavity called a coelenteron. Separate phyla are preferred by most taxonomists.
Cnidaria is a significant phylum for a variety of reasons. Cnidaria are among the oldest animals in the fossil record. Cnidarian fossils from the Vendian Period of the Palezoic Era (650 to 544 million years ago) have been recovered. Coral reefs, sometimes called the "rainforests of the ocean," are among the most prolific ecological regions of the earth. Jellyfish are downright beautiful--a section of the Monterey Bay Aquarium is devoted to jellyfish art.
The life cycles of cnidaria differ among the classes. A typical life cycle can include a sessile polyp phase and a mobile medusa phase. Cnidaria obtain energy by two methods. One method is to use tentacles lined with nematocysts to capture prey. Another source of energy (common in class anthozoa) is the photosynthetic material created by symbiotic zooxanthellae (dinoflagellates) that live in cnidarian tissues.
Classes of phylum Cnidaria
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