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Cnidarians Review 1. What are some examples of cnidarians? In which environments can these animals be found? Jellyfish, hydra, corals and sea anemones are good examples. All of them are aquatic and most live in the sea. Image Diversity: jellyfish hydra corals sea anemone 2. What are the two main morphological patterns of cnidarians? Concerning locomotion how do these forms differentiate from each other? Morphologically, cnidarians classify as polyps or medusae. Polyps are cylindrical and medusae are circular convex, like an umbrella. Both shapes have tentacles. In general polyps are sessile but some species, like hydra, can move by alternating contact points on the substrate and performing somersaults. Medusae can move expelling water jets by contraction of the body. Some cnidarians alternate polypoid and medusoid forms in their life cycle. Image Diversity: polyps medusae 3. Concerning tissue complexity how different are cnidarians from poriferans? Cnidarians have true tissue differentiation, they present distinct organized tissues in the body. Poriferans present only some dispersed specialized cells with no tissue differentiation. 4. Which are the germ layers present in cnidarians? Which tissues of the animal do they originate? These beings present ectoderm and endoderm, two germ layers. Animals with only two germ layers are called diploblastic animals. The ectoderm gives birth to the epidermis and the endoderm originates the covering of the digestive cavity. Image Diversity: germ layers 5. Why is the digestive system of these animals called incomplete? Incomplete digestive system is that in which the digestive cavity has only one opening. 6. What is the type of digestion that occurs in cnidarians? These animals have a digestive cavity and they make extracellular and intracellular digestion. The extracellular digestion takes place within the digestive cavity. 7. What are cnidocytes? What is the name of the capsule inside the cnidocyte? What are the biological functions of this structure? Cnidocytes are specialized cells present in coelenterates. They are found in the epidermis and contain toxic substances that can hurt, paralyze or even kill other animals. Each cnidocyte has an internal capsule know as nematocyst where the actual urticating substance is stored. When a cnidocyte is excited it causes the nematocyst to expose a filament containing the toxic chemical. Cnidocytes and their nematocysts have the biological functions of defending the individual from external aggression and of helping to capture their prey. Image Diversity: cnidocytes 8. How is the nervous tissue distributed in cnidarians? Their nervous system is diffuse, there are no brain or ganglia. 9. What are the types of reproduction presented by cnidarians? They present asexual and sexual reproduction. 10. What is the type of asexual reproduction that occurs in hydras? Hydras reproduce asexually by budding. 11. What is metagenesis? What are the other names of this process? Metagenesis is the type of life cycle in which there are two different forms of individuals of the same species, one haploid and the other diploid. In one of these stages gametogenesis occurs and fecundated gametes give birth to the zygote that then develops into the other form. Metagenesis is also known as alternation of generations or as diplobiont life cycle. (All plants, for example, present metagenesis.) 12. In the metagenesis of Aurelia and Obelia what is the form that produces gametes? What is the form that reproduces asexually? In the metagenesis of some coelenterates, like Aurelia and Obelia, there is alternation between polypoid and medusoid forms. The polyps reproduce asexually originating medusae that then liberate gametes. The male and female gametes unite under water to form the zygote that then develops into larva and later originates another polyp. The form that reproduces asexually is the polyp. Polyps give birth to medusae by budding in Obelia and by strobilization in Aurelia. Image Diversity: metagenesis of Aurelia metagenesis of Obelia 13. What is the name of the larva of corals and sea anemones? What is the biological importance of that larval stage? Sexual reproduction in corals and sea anemones have a larval stage and the larva is called planula. Many marine animals are sessile or practically sessile, like sponges, corals and sea anemones. The mobile larval stage of their life cycle provides better spatial distribution of these species. Image Diversity: planula 14. What are the main classes into which the phylum is divided? What are some examples of each and in which form (polyp or medusae) are they found? Coelenterates are divided into three main classes: hydrozoans, scyphozoans and anthozoans. In hydrozoans the polypoid form predominates and examples are hydras, by-the-wind-sailors and Obelia. In scyphozoans the main phase is the medusoid and the best known example is the common jellyfish (Aurelia). In anthozoans there is only the polypoid form and corals and sea anemones are notable in this group. Image Diversity: hydrozoans scyphozoans anthozoans 15. What does radial symmetry means? What is the type of symmetry found in chordates? Which are other phyla of the animal kingdom that present species with radial symmetry? Radial symmetry means (biologically) that the animal structures are situated in a radial or circular pattern around a center point with nonexistence of sides, like right or left. An alternative type of symmetry in which structures are placed equally in the sides of a longitudinal axis is the lateral symmetry (the symmetry present in human beings, for example). Chordates present lateral symmetry. Besides cnidarians another animal phylum with species presenting secondary radial symmetry is the phylum Echinodermata. (It is considered that the simplicity of poriferans does not characterize any symmetry.) 16. What are corals? Corals are characterized by their polypoid shape, sessility and slow growth and secretion of a solid skeleton made of calcium carbonate. Numerous coral individuals associate in intraspecific harmonic ecological interaction forming colonies with hundreds, thousands and even millions of beings. Water-permeated solid structures of these colonies, known as reefs, work as true ecosystems for other living beings. The biggest known coral colony is the Great Barrier Reef in the northeast coast of Australia. There are however many coral species whose individuals live alone and do not form colonies. Image Diversity: Great Barrier Reef 17. Cnidarian identity card. How are they characterized according to examples of representing beings, basic morphology, type of symmetry, germ layers and coelom, digestive system, respiratory system, circulatory system, excretory system, nervous system and types of reproduction? Examples of representing beings: jellyfish, corals, sea anemones, hydra. Basic morphology: polyp or medusa. Type of symmetry: radial. Germ layers and coelom: diploblastics, acoelomate. Digestive system: incomplete. Respiratory system: nonexistent. Circulatory system: nonexistent. Excretory system: nonexistent. Nervous system: diffuse. Types of reproduction: asexual and sexual with larval stage and metagenesis. |