Rotational symmetry (e) is seen in the ctenophore Beroe, shown swimming open-mouthed. The (b) jellyfish and (c) anemone are radially symmetrical, the (d) butterfly is bilaterally symmetrical. The Ctenophora ( (Figure) e), although they look similar to jellyfish, are considered to have rotational symmetry rather than radial or biradial symmetry because division of the body into two halves along the oral/aboral axis divides them into two copies of the same half, with one copy rotated 180 o, rather than two mirror images.įigure 2. Bilaterally symmetrical animals, like butterflies ( (Figure) d) have only a single plane along which the body can be divided into equivalent halves. 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 many animals in the phyla Cnidaria, including jellyfish and adult sea anemones ( (Figure) b, c). If a radially symmetrical animal is divided in any direction along the oral/aboral axis (the side with a mouth is “oral side,” and the side without a mouth is the “aboral side”), the two halves will be mirror images. 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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