As nematocysts fire, barbs unfold and become embedded in the tissue of the prey. For example, a nematocyst may respond only if there is mechanical stimulation from physical contact with the prey as well as chemical stimulation signaling the presence of suitable prey. Most nematocysts require stimulation in more than one sensory mode before they will fire. The nematocysts are springing barbs with small hairlike triggers that are activated by contact with prey. Ĭnidarians feed using tentacles that are embedded with stinging nematocysts. This opening is surrounded by tentacles and leads to an internal digestive cavity called the gastrovascular cavity. The mesoglea of cnidarians is not as highly developed as the mesoderm of other animal groups, being primarily gelatinous with only a few fibrous or amoeba-like cells.Ĭnidarians possess only one digestive opening, which serves as both the mouth and the anus. Both are involved in feeding.Ĭnidarians are characterized by the presence of three tissue layers, an outer protective epidermis, a middle layer called the mesoglea, and an inner layer called the gastrodermis, whose function is primarily digestive. Most cnidarians possess tentacles, and many also have nematocysts (specialized stinging cells). Although cnidarians are more advanced than sponges (phylum Porifera) in that they possess distinct tissue layers, they lack many of the features of more advanced animal phyla, such as internal organs and central nervous systems. Characteristics of CnidariansĪll cnidarians are characterized by radially symmetric body plans, rather than the bilaterally symmetric body plans that are found in most other animal phyla. Most cnidarians are marine, although a few, such as the well-known hydra, are freshwater species. It includes aquatic organisms such as jellyfish, sea anemones, corals, and hydras. Cnidaria is one of the more primitive animal phyla.
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