At a very simple level the classification, true animals can it is in largely split into 3 groups based on the form of symmetry of your body plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Asymmetry is viewed in two modern-day clades, the Parazoa (Figurea) and Placozoa. (Although we have to note that the genealogical fossils of the Parazoa apparently exhibited bilateral symmetry.) One clade, the Cnidaria (Figureb,c), exhibits radial or biradial symmetry: Ctenophores have rotational symmetry (Figuree). Bilateral the opposite is watched in the largest of the clades, the Bilateria (Figured); yet the Echinodermata room bilateral together larvae and also metamorphose secondarily right into radial adults. All species of symmetry are well suitable to accomplish the distinctive demands of a certain animal’s lifestyle.

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Radial symmetry is the arrangement of body parts approximately a central axis, as is viewed in a bike wheel or pie. It outcomes in pets having top and bottom surfaces yet no left and also right sides, nor front or back. If a radially symmetrical animal is separated in any type of direction along the oral/aboral axis (the side through a mouth is “oral side,” and the side without a mouth is the “aboral side”), the 2 halves will certainly be winter images. This kind of symmetry marks the human body plans the many pets in the phyla Cnidaria, consisting of jellyfish and also adult sea anemones (Figureb, c). Radial the contrary equips these sea creatures (which may be sedentary or only qualified of slow motion or floating) to experience the environment equally from every directions. Bilaterally symmetry animals, favor butterflies (Figured) have only a single plane along which the body deserve to be divided into identical halves. The Ctenophora (Figuree), although castle look similar to jellyfish, are taken into consideration to have actually rotational symmetry quite than radial or biradial the opposite because department of the body right into two halves along the oral/aboral axis divides them right into two copies of the same half, through one copy rotated 180o, fairly than two mirror images.

Symmetry in animals. The (a) sponge is asymmetrical. The (b) jellyfish and also (c) anemone are radially symmetrical, the (d) butterfly is bilaterally symmetrical. Rotational the opposite (e) is watched in the ctenophore Beroe, presented swimming open-mouthed. (credit a: change of job-related by Andrew Turner; credit b: alteration of job-related by Robert Freiburger; credit transaction c: change of job-related by Samuel Chow; credit transaction d: modification of occupational by Cory Zanker; credit e: modification of occupational by NOAA)

Bilateral symmetry requires the division of the pet through a midsagittal plane, resulting in two superficially mirror images, right and left halves, such together those the a butterfly (Figured), crab, or human being body. Pets with bilateral symmetry have a “head” and also “tail” (anterior vs. Posterior), prior and earlier (dorsal vs. Ventral), and right and left sides (Figure). Every Eumetazoa except those with secondary radial symmetry space bilaterally symmetrical. The advancement of bilateral the contrary that enabled for the formation of anterior and also posterior (head and also tail) ends supported a phenomenon referred to as cephalization, which refers to the collection of an organized nervous system at the animal’s anterior end. In contrast to radial symmetry, which is ideal suited because that stationary or limited-motion lifestyles, bilateral symmetry permits for streamlined and directional motion. In evolutionary terms, this simple type of the opposite promoted energetic and managed directional mobility and increased sophistication the resource-seeking and predator-prey relationships.

Bilateral symmetry. The bilaterally symmetrical person body have the right to be split by several planes.

Animals in the phylum Echinodermata (such as sea stars, sand dollars, and sea urchins) screen modified radial symmetry together adults, yet as we have noted, your larval step (such as the bipinnaria) at first exhibit bilateral symmetry till they metamorphose in animals with radial the contrary (this is termed an additional radial symmetry). Echinoderms developed from bilaterally symmetry animals; thus, they room classified as bilaterally symmetrical.

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Watch this video to see a fast sketch of the different types of body symmetry.