![]() ![]() 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 âoral side,â and the side without a mouth is the âaboral sideâ), 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 animalâs 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. 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 donât.Which of the following statements is false? The Ctenophora and Porifera are both considered to be basal because of the absence of Hox genes in this group, but how they are related to the âParahoxozoaâ (Placozoa + Eumetazoa) or to each other, continues to be a matter of debate. The phylogenetic tree of animals is based on morphological, fossil, and genetic evidence. 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. 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 ârulesâ governing animal classification ( Figure). ![]()
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