A prominent difference between the two classes is the arrangement of tentacles. Class 1. One unanswered question relates to their evolutionary position among the lower Metazoa (a division of the animal kingdom that includes all phyla except the Protozoa). Cnidarians contain specialized cells known as cnidocytes (“stinging cells”) containing organelles called nematocysts (stingers). Hydros- water, zoon- animal): Either only polyps are found or polyps and medusae are present. The key difference between Cnidaria and Ctenophora is that the cnidaria show alteration of generation between medusa and polyp, while Ctenophora does not show alteration of generation; only medusa form is present.. Phylum Coelenterata is a subdivision of Kingdom Animalia. Most Cnidarians have radial symmetry. Cubozoans display overall morphological and anatomical characteristics that are similar to those of the scyphozoans. Animals in the phylum Chordata share four key features that appear at some stage during their development: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail (Figure 2). Jellyfish, any planktonic marine member of the class Scyphozoa (phylum Cnidaria), a group of invertebrate animals composed of about 200 described species, or of the class Cubozoa (approximately 20 species). Cnidarians share several basic characteristics. Hydrozoa (Gk. Cnidarians contain specialized cells known as cnidocytes (“stinging cells”) containing organelles called nematocysts (stingers). Nematocysts – and their enclosing Cnidocytes – come in about 24 different forms. Cnidarian species are found throughout the world and are quite diverse, but they share many similar characteristics. ADVERTISEMENTS: In this article we will discuss about the interrelationships between the different classes of phylum Cnidaria. Characteristics of Phylum Cnidaria. It consists of two main phyla Cnidaria and Ctenophora and approximately 15000 species included. Phylum Cnidaria. Phylum Cnidaria includes animals that show radial or biradial symmetry and are diploblastic, that is, they develop from two embryonic layers.Nearly all (about 99 percent) cnidarians are marine species. Cnidarians only have two body layers, the ectoderm and endoderm, separated by a jelly-like layer called the mesoglea. Phylum Cnidaria is a pretty cool group of animals. When damaged, some cnidarians can regenerate their body parts, making them effectively immortal. This is because cnidarians are things like corals, anemones, jellies, and hydras. They all have tentacles with stinging cells called nematocysts that they use to capture food. Characteristics of Chordata. The differences play a functional role in the classification of the phylum. A Cnidocyte is … One of the most important distinguishing characteristics of the phylum cnidaria are the Nematocysts. In some groups, some of these are present only during embryonic development. Class Hydrozoa represents the most primitive forms amongst the Cnidarians. The Cnidaria (Cnidaria spp.) This is the most venomous group of all the cnidarians (Figure 4). The relationship between Hydrozoa and Scyphozoa is not very clear. Cnidarian - Cnidarian - Critical appraisal: The classification of living cnidarians is relatively stable and generally accepted. In spite of […] The phylum Cnidaria contains more than 10,000 living species, among them, mostly are marine animals whereas only 20 species inhabit in freshwater. All Cnidaria are aquatic, mostly marine, organisms. is the phylum of animals that contains corals, jellyfish (sea jellies), sea anemones, sea pens, and hydrozoans. Classes in the Phylum Cnidaria. These species may achieve sizes of 15–25 cm. The lowest hydrozoan form is exhibited by Hydra and the hydrula stage of some Hydrozoa. 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