Which Type Of Symmetry Is Most Often Seen In An Animal That Is Mobile And Exhibits Cephalization?

27.2A: Animal Characterization Based on Body Symmetry

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13693

Animals can exist classified by iii types of body plan symmetry: radial symmetry, bilateral symmetry, and asymmetry.

Learning Objectives

• Differentiate among the ways in which animals can be characterized by body symmetry

Primal Points

• Animals with radial symmetry have no right or left sides, only a top or bottom; these species are usually marine organisms like jellyfish and corals.
• Most animals are bilaterally symmetrical with a line of symmetry dividing their body into left and right sides along with a "head" and "tail" in add-on to a top and bottom.
• Only sponges (phylum Porifera) accept asymmetrical trunk plans.
• Some animals start life with i blazon of body symmetry, only develop a dissimilar type as adults; for example, sea stars are classified equally bilaterally symmetrical fifty-fifty though their adult forms are radially symmetrical.

Primal Terms

• sagittal airplane: divides the torso into right and left halves
• radial symmetry: a form of symmetry wherein identical parts are bundled in a circular manner around a central axis
• bilateral symmetry: having equal arrangement of parts (symmetry) well-nigh a vertical plane running from caput to tail

Animal Characterization Based on Torso Symmetry

At a very basic level of classification, true animals can be largely divided into three groups based on the blazon of symmetry of their trunk plan: radially symmetrical, bilaterally symmetrical, and asymmetrical. Only a few animal groups display radial symmetry, while asymmetry is a unique characteristic of phyla Porifera (sponges). All types of symmetry are well suited to meet the unique demands of a particular creature's lifestyle.

Radial Symmetry

Radial symmetry is the arrangement of body parts effectually a central axis, like rays on a sunday or pieces in a pie. Radially symmetrical animals have tiptop and bottom surfaces, just no left and correct sides, or front end and back. The two halves of a radially symmetrical animal may be described as the side with a oral cavity ("oral side") and the side without a oral fissure ("aboral side"). This form of symmetry marks the torso plans of animals in the phyla Ctenophora (comb jellies) and Cnidaria (corals, sea anemones, and other jellies). Radial symmetry enables these sea creatures, which may exist sedentary or only capable of boring movement or floating, to experience the environment as from all directions.

Figure \(\PageIndex{1}\): Radial symmetry: Some organisms, similar sea anemones (phylum Cnidaria), have radial symmetry.

Bilateral Symmetry

Bilateral symmetry involves the division of the animal through a sagittal plane, resulting in two mirror-epitome, right and left halves, such every bit those of a butterfly, crab, or human being trunk. Animals with bilateral symmetry take a "head" and "tail" (anterior vs. posterior), front and back (dorsal vs. ventral), and correct and left sides. All true animals, except those with radial symmetry, are bilaterally symmetrical. The development of bilateral symmetry and, therefore, the germination of anterior and posterior (head and tail) ends promoted a phenomenon chosen cephalization, which refers to the collection of an organized nervous arrangement at the animal's inductive end. In dissimilarity to radial symmetry, which is best suited for stationary or limited-move lifestyles, bilateral symmetry allows for streamlined and directional motion. In evolutionary terms, this elementary form of symmetry promoted active mobility and increased composure of resources-seeking and predator-casualty relationships.

Figure \(\PageIndex{one}\): Bilateral symmetry: This monarch butterfly demonstrates bilateral symmetry down the sagittal airplane, with the line of symmetry running from ventral to dorsal and dividing the body into 2 left and correct halves.

Animals in the phylum Echinodermata (such every bit sea stars, sand dollars, and sea urchins) display radial symmetry as adults, simply their larval stages exhibit bilateral symmetry. This is termed secondary radial symmetry. They are believed to have evolved from bilaterally symmetrical animals; thus, they are classified as bilaterally symmetrical.

Figure \(\PageIndex{1}\): Secondary radial symmetry in echinoderms: The larvae of echinoderms (sea stars, sand dollars, and sea urchins) accept bilateral symmetry as larvae, but develop radial symmetry equally full adults.

Disproportion

Only members of the phylum Porifera (sponges) take no torso programme symmetry. There are some fish species, such as flounder, that lack symmetry equally adults. Withal, the larval fish are bilaterally symmetrical.

Source: https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book:_General_Biology_%28Boundless%29/27:_Introduction_to_Animal_Diversity/27.02:_Features_Used_to_Classify_Animals/27.2A:_Animal_Characterization_Based_on_Body_Symmetry

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