Associated with bilateral symmetry is the phenomenon of cephalization, which is the evolutionary trend towards the concentration of sensory equipment on the anter ior end; this means that such organisms are directionally sensitive and mobile. Generally the anterior, or cephalized, end is the first to encounter food, danger, or other stimuli. Before bipedal development common in humans and apes , cephalization wa s an adaptation for movement such as crawling, burrowing, or swimming.
Examples of animals that possess bilateral symmetry are: flatworms, common worms "ribbon worms" , clams, snails, octopuses, crustaceans, insects, spiders, brachiopods, sea stars, sea urchins, and vertebrates.
The symmetry of an animal generally fits its lifestyle. For example, many radial animals are sessile forms or plankton and their symmetry equips them to meet their environment equally well from all sides. More active animals are generally bilateral. Th e two forms of symmetry, however, are not absolutely separate. A great deal of radial symmetry is proven to emerge secondarily from a bilateral condition frequently it emerges from animals adapting to a more sedentary lifestyle.
Eucoelomates or coelomates have a true coelom that arises entirely within the mesoderm germ layer and is lined by an epithelial membrane.
This coelomic cavity represents a fluid-filled space that lies between the visceral organs and the body wall. It houses the digestive system, kidneys, reproductive organs, and heart, and it contains the circulatory system. The epithelial membrane also lines the organs within the coelom, connecting and holding them in position while allowing them some free motion.
Annelids, mollusks, arthropods, echinoderms, and chordates are all eucoelomates. The coelom also provides space for the diffusion of gases and nutrients, as well as body flexibility and improved animal motility. The coelom also provides cushioning and shock absorption for the major organ systems, while allowing organs to move freely for optimal development and placement.
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. Mice mutated for the Rere gene show the same retarded somite formation as mice which are deficient in retinoic acid. Their work also showed that the proteins, Nr2f2 and Rere, control the asymmetry of the signalling pathway for retinoic acid. This asymmetry is required to correct interference with the signals which determine the lateralisation of organs.
Hence, this study improves our understanding of how the general symmetry of the body can be reconciled with the lateralisation of some organs. In man, the anomalies in symmetric development of the somites could be responsible for vertebral symmetry disorders such as scoliosis. A defect in the regulation of functions performed by RERE or Nr2f2 on the retinoic acid signalling pathway may be implicated in the occurrence of these frequent, and sometimes acute, diseases. Materials provided by CNRS.
Note: Content may be edited for style and length. Animals can be classified by three types of body plan symmetry: radial symmetry, bilateral symmetry, and asymmetry. At 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. 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.
0コメント