Cnidarians are aquatic animals characterized by presenting various morphological forms according to their vital stage, that is, according to what phase or stage of their life they are. They include both the jellyfish that occupy our coasts in summer, as well as the aesthetic corals that make up the amazing and colorful reefs of Australia, including the stinging tentacle anemones.
If you want to discover more about the cnidarians: characteristics and examplesContinue reading this Green Ecologist article in which we are going to explain everything you need to know about this particular animal phylum.
Are aquatic organisms, most of them marine, and make up the phylum Cnidaria. More than 10,000 living species of cnidarians are known and there are also fossil species that date back to the Ordovician. These are the main characteristics of cnidarians:
There are four types or classes of cnidarians: hydrozoa, cubozoa, siphozoa and anthozoa.
It includes small predatory animals that live in fresh water (hydras) or marine environments. Most generate calcite shells. Some live in isolation and others in colonies. They have a non-cellular mesoglea, lack tentacles in the gastric cavity, and do not have an esophagus. The hydroid phase predominates in their life cycles, although, in some species, the medusoid phase is more important.
It groups the box jellyfish, which are related to the siphomedusae but differ from them in that they are smaller, have a primitive nervous system and eyes, as well as a characteristic cube-shaped morphology. Box jellyfish stings can be fatal to humans. Opinions diverge as to whether they are to be considered as an order or a class. When they reproduce, they give rise to a relatively long-lived polypoid state called scyphistoma, which can reproduce asexually giving rise to other scyphistomas, through the formation of lateral buds. At the end of this larval stage of scyphistoma, the organism passes into the form of a cubomedusa, characterized by having four edges on the edges of the body where one or more tentacles with a flattened base are placed, which make up the so-called pedalium. In addition to the pedaliums, they also present ropalios, which are conglomerates of sensory organs, photoreceptors, and neurons that also appear in sciphomedusae.
It is the class of "Real jellyfish". All its specimens live in the ocean. They have a short phase in the form of a polyp (scifopolyp or scyphistoma), while most of their lives are spent in the form of a jellyfish (sciphomedusa). They are larger in size than the hydrozoan jellyfish, and can reach up to two meters in length, although it is normal for them to be between 2 and 40 centimeters in diameter. They are characterized by presenting a cellular mesoglea, as well as tentacles in their gastric cavity. There are jellyfish in which fertilization takes place in the manubrium, where the embryos develop to give rise to the "planula larva", which remains free and, at the end of its swimming life, goes to the bottom and gives rise to sphistoma. Scyphistoma is like a polyp that can live for a few months and, through budding or strobilation processes, gives rise to new individuals. Through strobilation, scyphistoma generates ephrae, small discs that remain free in the water and develop into the shape of a jellyfish. The time that each phase occupies is highly variable: there are scyphistomas with a longer life, while, in other species, it is practically non-existent and the planula larva directly gives rise to an ephira …
If you want to know more about the so-called "real jellyfish", here you can learn how jellyfish reproduce.
They include anemones, corals, and sea feathers. It is the largest class of cnidarians, with more than 6,000 known species (there are many fossil specimens) in all seas, even at great depths. They only present a polyp form, not a medusoid, and live in marine environments. They can live alone or in colonies. Individuals are column-shaped with an aboral end where they are attached to the substrate and an oral end where the mouth is surrounded by tentacles. In anthozoans, the mouth continues through a pharynx that occupies a good part of the gastrovascular cavity. In the area where the pharynx is located, anthozoans present complete or incomplete septa or mesenteries that increase the absorption and digestion surface. The most important group of anthozoans is the subclass Hexacorallia, which mainly contains anemones and stony corals, which form the reefs.
Once we have explained what cnidarians are and their characteristics, we now present some examples of cnidarians indicating of each class.
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