It results in animals having top and bottom surfaces but no left and right sides, nor front or back. Radial symmetry is the arrangement of body parts around a central axis, as is seen in a bicycle wheel or pie. All types of symmetry are well suited to meet the unique demands of a particular animal’s lifestyle. Bilateral symmetry is seen in the largest of the clades, the Bilateria ( (Figure) d) however the Echinodermata are bilateral as larvae and metamorphose secondarily into radial adults. (Although we should note that the ancestral fossils of the Parazoa apparently exhibited bilateral symmetry.) One clade, the Cnidaria ( (Figure) b,c), exhibits radial or biradial symmetry: Ctenophores have rotational symmetry ( (Figure) e). Asymmetry is seen in two modern clades, the Parazoa ( (Figure) a) and Placozoa. Developmental characteristics include the number of germ tissue layers formed during development, the origin of the mouth and anus, the presence or absence of an internal body cavity, and other features of embryological development, such as larval types or whether or not periods of growth are interspersed with molting.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. Symmetrical animals can be divided into roughly equivalent halves along at least one axis. The major feature of the body plan is its symmetry: how the body parts are distributed along the major body axis. Animals have been traditionally classified according to two characteristics: body plan and developmental pathway. –>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)). Compare and contrast the embryonic development of protostomes and deuterostomes.Explain the differences in animal body plans that support basic animal classification.By the end of this section, you will be able to do the following:
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