Viruses also exhibit radial symmetry by arranging the protein molecules in their coats symmetrically. Types of the viral coats containing the radial symmetry are icosahedrons, polyhedrons, spheres and ovoids. Bodies of some organisms form segments around the central axis, creating variations in the radial symmetry.
Several variations in the radial symmetry can be identified such as tetramerism, pentamerism, hexamerism and octamerism. Figure 1: Radial symmetry of the purple sea urchin. In bilateral symmetry, the body of the organism is divided into two sides as left and right by a basic body plane. Thus, bilateral symmetry is also called the plane symmetry. The plane, which divides the body bilaterally is referred to as the sagittal plane. The two sides generated here are the mirror image of each other.
Hence, they exhibit a mirror symmetry in the sagittal plane. The sagittal plane divides the body into left and right vertically. Internal organs may not be symmetrically distributed, but the sense organs and the limb pair can be divided in bilateral symmetry. Since the head is the body part which is located in front of a moving organism, most sensory organs such as eyes and the mouth are concentrated around the head.
But, the left and right are difficult to distinguish. A majority of organisms, including the human, are bilaterally symmetric. The phylum Echinodermata also contains bilateral symmetry at their larval stage. In plants, some flowers such as orchid and pea families consist of bilateral symmetry.
Figure 2: The bilateral symmetry of Orchid. Bilateral Symmetry: The body of the organism generates two sides as left and right along the sagittal plane. Only a few animal groups display radial symmetry, while asymmetry is a unique feature of phyla Porifera sponges.
Radial symmetry is the arrangement of body parts around a central axis, like rays on a sun or pieces in a pie. Radially symmetrical animals have top and bottom surfaces, but no left and right sides, or front and back.
This form of symmetry marks the body 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 be sedentary or only capable of slow movement or floating, to experience the environment equally from all directions. Bilateral 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, crab, or human body.
All true animals, except those with radial symmetry, are bilaterally symmetrical. In contrast to radial symmetry, which is best suited for stationary or limited-motion lifestyles, bilateral symmetry allows for streamlined and directional motion. The pseudocoelomates have a coelom derived partly from mesoderm and partly from endoderm. Although still functional, these are considered false coeloms. The phylum Nematoda roundworms is an example of a pseudocoelomate.
Bilaterally symmetrical, tribloblastic eucoelomates can be further divided into two groups based on differences in their early embryonic development. These two groups are separated based on which opening of the digestive cavity develops first: mouth protostomes or anus deuterostomes.
Early embryonic development in eucoelomates : Eucoelomates can be divided into two groups based on their early embryonic development. In protostomes, part of the mesoderm separates to form the coelom in a process called schizocoely.
In deuterostomes, the mesoderm pinches off to form the coelom in a process called enterocoely. The coelom of most protostomes is formed through a process called schizocoely, when a solid mass of the mesoderm splits apart and forms the hollow opening of the coelom.
Deuterostomes differ in that their coelom forms through a process called enterocoely, when the mesoderm develops as pouches that are pinched off from the endoderm tissue. These pouches eventually fuse to form the mesoderm, which then gives rise to the coelom.
Protostomes undergo spiral cleavage: the cells of one pole of the embryo are rotated and, thus, misaligned with respect to the cells of the opposite pole.
This spiral cleavage is due to the oblique angle of the cleavage. Protostomes also undergo determinate cleavage: the developmental fate of each embryonic cell is pre-determined.
Deuterostomes undergo radial cleavage where the cleavage axes are either parallel or perpendicular to the polar axis, resulting in the alignment of the cells between the two poles. Unlike protostomes, deuterostomes undergo indeterminate cleavage: cells remain undifferentiated until a later developmental stage. This characteristic of deuterostomes is reflected in the existence of familiar embryonic stem cells, which have the ability to develop into any cell type.
Privacy Policy. Skip to main content. Introduction to Animal Diversity. Search for:. Features Used to Classify Animals. Animal Characterization Based on Body Symmetry Animals can be classified by three 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. Key Takeaways Key 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.
Only sponges phylum Porifera have asymmetrical body plans. Some animals start life with one type of body symmetry, but develop a different type as adults; for example, sea stars are classified as bilaterally symmetrical even though their adult forms are radially symmetrical.
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