![]() ![]() The (b) jellyfish and (c) anemone are radially symmetrical, and the (d) butterfly is bilaterally symmetrical. Radial symmetry equips these sea creatures (which may be sedentary or only capable of slow movement or floating) to experience the environment equally from all directions. This form of symmetry marks the body plans of animals in the phyla Ctenophora and Cnidaria, including jellyfish and adult sea anemones ( Figure bc). The two halves of a radially symmetrical animal may be described as the side with a mouth or “oral side,” and the side without a mouth (the “aboral side”). It results in animals having top and bottom surfaces but no left and right sides, or front or back. Radial symmetry is the arrangement of body parts around a central axis, as is seen in a drinking glass or pie. All types of symmetry are well suited to meet the unique demands of a particular animal’s lifestyle. Only a few animal groups display radial symmetry. Asymmetry is a unique feature of Parazoa ( Figure a). Arthropods are more closely related to nematodes than they are to annelids.Īnimal Characterization Based on Body SymmetryĪt a very basic level of classification, true animals can be largely divided into three groups based on the type of symmetry of their body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical.Acoela and Cnidaria both possess radial symmetry.Lophotrochozoa and Ecdysozoa are both Bilataria.Eumetazoans have specialized tissues and parazoans don’t.Which of the following statements is false? ![]() ![]() The phylogenetic tree of animals is based on morphological, fossil, and genetic evidence. Additional characteristics include the number of tissue layers formed during development, the presence or absence of an internal body cavity, and other features of embryological development, such as the origin of the mouth and anus. This means that their distribution of body parts is balanced along an axis. One of the most prominent features of the body plan of true animals is that they are morphologically symmetrical. Animals are primarily classified according to morphological and developmental characteristics, such as a body plan. Scientists have developed a classification scheme that categorizes all members of the animal kingdom, although there are exceptions to most “rules” governing animal classification ( Figure). ![]()
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