- The most ancient animals are the parazoa, which includes the phylum Porifera, or sponges. The parazoa have no true tissue, although there is cooperation among the cells in sponges. - The phylum Placozoa is poorly understood. It may represent a clue to the evolution of animals. - The most ancient animals with true tissues are the Cnidarians and the Ctenophores. Members of both of these phyla have two cell layers (diploblastic) and have radial symmetry. They are known as the radiata. - Although sponges don’t have true organ systems, they do take in food and circulate nutrients. They behave like multicellular animals because they have cells that work together cooperatively. - Specialized cells form silica or calcium carbonate spicules, which hold the sponge together. Other specialized cells produce spongin, an elastic fiber that gives the sponge flexibility. Ameobocytes crawl through the sponge, digesting food and secreting nutrients for other cells. Collar cells or choanocytes are flagellated cells that produce a feeding and circulation current through the sponge. - The Placozoa probably evolved between the radiata and the bilateria, but this is debatable. There are only two known species of Placozoa. They are motile, multicellular, very simple animals. They are significant because they may represent a very early form of multicellular animal; their anatomy is similar to a blastula that has been flattened out. - The Cnidarians include the jellyfish, sea anemones, corals, and gorgonians. They are the first animals that have true tissues. They were the dominant organisms about 600 million years ago. All cnidarians display radial symmetry. They have a gastrovascular cavity that has one opening. The cnidarians have a life cycle that alternates between a sessile polyp and a floating medusa stage. Cnidarians are characterized by a specialized cell called a cnidocyte that is used for predation and defense. - Ctenophores are also known as comb jellies. They display radial symmetry and are diploblastic. Ctenophores are characterized by rows of cells that contain fused cilia and are used for locomotion.

Which of the following is a characteristic of both Cnidarians and Ctenophores?

- diploblastic - radial symmetry - both A and C

11.10.1 Introduction to Animals: Parazoa and Radiata Flashcards by Irina Soloshenko

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11.10.1 Introduction to Animals: Parazoa and Radiata Flashcards

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