![]() In evolutionary terms, this simple form of symmetry promoted active mobility and increased sophistication of resource-seeking and predator-prey relationships. In contrast to radial symmetry, which is best suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. The evolution of bilateral symmetry that allowed for the formation of anterior and posterior (head and tail) ends promoted a phenomenon called cephalization, which refers to the collection of an organized nervous system at the animalâs anterior end. All true animals except those with radial symmetry are bilaterally symmetrical. ventral), and right and left sides ( Figure). Animals with bilateral symmetry have a âheadâ and âtailâ (anterior vs. ![]() (credit a: modification of work by Andrew Turner credit b: modification of work by Robert Freiburger credit c: modification of work by Samuel Chow credit d: modification of work by Cory Zanker)īilateral symmetry involves the division of the animal through a sagittal plane, resulting in two mirror image, right and left halves, such as those of a butterfly ( Figure d), crab, or human body. 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 âoral side,â and the side without a mouth (the âaboral sideâ). 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 animalâs 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 donât.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 ârulesâ governing animal classification ( Figure).
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