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Before entering fully into the establishment of the meaning of the term cnidarians, it is necessary to know its etymological origin. In this case, we can state that it is the result of the sum of two distinct parts:
-The Greek noun "knide", which can be translated as "nettle".
-The Latin “-arium”, which is used to indicate “membership”.

The cnidarians are celestial animals what are they presenting cells stinging and they usually live fixed at the bottom of the sea, in colonies or in plankton communities. The jellyfish and the corals They belong to this edge.

Returning to the definition, we can say that the cnidarians are celentéreos since they exhibit a radiated symmetry and have a unique gastrovascular cavity with exit to the outside, which works as an anus and mouth simultaneously. On the other hand, stinging cells have the ability to generate itching.

All cnidarians are aquatic and mostly sailors . his body resembles a coat and have one or more tentacles around the mouth. The cnidarians do not have a circulatory system, respiratory system or excretory system: all these functions satisfy them through the aforementioned gastrovascular cavity and the ectodermis (the outer layer of the body).

In the ectodermis there are different kinds of cells, such as basal cells, sensory cells and neurons. The anatomy of the cnidarians also includes the gastrodermis (the inner body layer, which delimits the gastrovascular cavity) and the mesoglea (It is located between the ectodermis and the gastrodermis).

Beyond the common characteristics, it is possible to differentiate between about 10,000 species of cnidarians. Some, like the polyps , they are sessile: they do not have organs or limbs to support your body.

On the edge of the cnidarians one can also distinguish between several lessons , Like the spifozoans , the hydrozoans , the cubozoos and the anthozoans . It is believed that all classes derived from anthozoans, which are the oldest cnidarians.

In addition to the above, we can establish another series of relevant aspects about the cnidarians that are worth knowing:
-In that group are also jellyfish, hydras or anemones.
-They have two layers of cells: the inner one, which responds to the name of endoderm, and the outer one, which is called ectoderm.
-From a reproductive point of view, we can establish that they reproduce, both sexually and asexually, through what is known as budding. However, we must not forget the fact that others have what responds to the name of alternating reproduction. This is what takes place when they come to alternate asexual reproduction, which is that of the polyp phase, with the sexual one, in the jellyfish phase.
-The hydrozoans are identified because throughout their life they will alternate what is the polyp phase with the jellyfish.
-The anthozoans, on the other hand, come to be corals and anemones. They always have the form of polyps and have a kind of pharynx, which is divided into several clearly defined parts.
-Shphifozoans become jellyfish.

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