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The name Cnidaria comes from the Greek word "condos", which means stinging nettle. When you touch the nematocysts it releases barbed threads tipped with poison. There are thousands of cnidarian species in the worlds oceans. There are even some species that are found in rivers and fresh water lakes. Cnidarians have four main groups. One of the main groups is the Anthozoa, it includes true corals, anemones, and sea pens.
The Anthozoans are the most famous cnidarians. They have a long fossil record that extends back at least 550 million years. Polyp-like and sea pen-like fossils are most likely the oldest entozoon. In the Cambrian period the first mineralized coral-like organisms appeared.
The true corals, the kind that is living in our time today, did not appear until the middle Triassic era. Another one of the main groups of cnidarians are Cubozoa. The Cubozoa are known sometimes as the box jellies. Box jellies are similar in form to the normal "jellyfish." But it is easy to tell the two groups apart from each other, Cubozoa are square shaped when viewed from above. They have well developed eyes and usually have four tentacles that are evenly spaced apart. The 3 rd main group is Hydrozoan.
The Hydrozoan is the most diverse group with siphonophores, hydroids, fire corals, and many medusae. These organisms may exist as either polyps or medusae. Many species pass through both forms in their life cycles and in others, one form or the other is suppressed. The majority of species are marine and colonial, but many species are solitary and some live in fresh water. Hydrozoans include such diverse forms as hydra, which are a group of species of solitary, fresh-water forms that live as polyps throughout their lives. Reproduction by hydras can be the typical form for polyps, asexual by budding, or by sexual means in which temporary gonads appear on the polyp itself.
Hydrozoan medusae all have a distinctive structure called a velum. This is a shelf or rim that projects inward around the margin of the bell, partially closing the opening. Hydrozoan medusae swim by alternately constricting and relaxing muscles in the bell. This causes water to shoot out the constricted opening of the bell, moving the animal by a sort of "jet propulsion. " The Class Scyphozoa includes most of the larger jellyfish. The medusae of members of this class lack a velum. In many species, the rim of the bell contains sense organs, including statocyst's that sense balance and orientation and photoreceptors that are sensitive to light.
Scyphozoans are often amply covered with nematocysts, so that a swimmer's encounter with one can be an unpleasant experience. They feed in a variety of ways, but often involving prey capture by nematocysts on the arms and transport of food items to the gastrovascular cavity by means of cilia. As in other cnidarians, digestion is intracellular.
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