Structures on one side of an axis of symmetry mirror structures on the opposite side. Asymmetrical body plans are relatively rare in the animal kingdom. Some notable examples of body plan asymmetry can be found in sponges phylum Porifera; Fig. Most animals have either bilateral or radial symmetry. Radial symmetry occurs when two or more axes of symmetry can be drawn through the center of the organism Fig.
Radially symmetrical organisms are typically cylinder-shaped with body structures arranged around the center of the organism. Perfect radial symmetry is relatively rare but does occur in some sponges and cnidarians like anemones, corals and jellyfish phylum Cnidaria; Fig. Sea stars, urchins, sea cucumber, and other animals in the phylum Echinodermata typically have five axes of symmetry Fig.
Radially symmetrical aquatic animals typically have an oral mouth surface and an aboral surface on the opposite side Fig. Sensory and feeding structures are often concentrated around the center point. From an evolutionary perspective, this would be advantageous because these organisms will be encountering stumuli and food from many directions. Bilateral symmetry occurs when an object has only one axis of symmetry Fig.
Most animal phyla have bilaterial symmetry. Examples of bilaterally symmetrical animals include worms, insects, and molluscs.
These organisms will typically have a front end known as the anterior and a back end known as the posterior. They also have left and right sides that mirror each other.
Bilateral symmetry is typically associated with organisms that have locomotion or can move under their own power. Many bilaterally symmetrical animals have evolved feeding and sensory structures located at the front end of their bodies Fig. Cephalization is the evolutionary development of an anterior head with concentrated feeding organs and sensory tissues in animals.
Bilaterally symmetrical organisms typically move towards their environment at the anterior end. Cephalization likely evolved because it was advantageous to have feeding structures at the anterior end where food would be encountered as an organism moved forward. Similarly, it would be important to concentrate external sensory structures like eyes and antennae at the anterior end.
Symmetry is a relatively approximate measure. Not all organisms will show an exact mirror image match when comparing each side of an axis of symmetry. For example humans are considered bilaterally symmetrical because we have an axis of symmetry that bisects our body from our head to our feet Fig. However, these are adaptations that have been built on a bilaterally symmetrical body plan. The presence of true tissue allows for complexity and increased body size within the animal kingdom.
Tissue is an aggregation of similar cells that perform a specific function. For example, muscle tissue is made up of muscle cells that function to produce motion.
Only a few animal phyla lack true tissue. Sponges phylum Porifera lack true tissue but are able to increase size through intricate branching and folding patterns. In animals that contain true tissue, the tissue layers in the adult are derived from embryonic tissue layers called germ layers.
Germ layers are the tissues that occur after a fertilized egg has gone through several stages of cleavage, and cell aggregations are beginning to form tissue layers. This process in the embryo is called gastrulation Fig. During the gastrulation process, two germ layers develop: the ectoderm and the endoderm. The ectoderm is the germ layer that forms on the outside of the developing embryo Fig. The endoderm is the layer that develops on the inside of the embryo Fig.
The science of embryology , or developmental biology , examines how these germ layers develop into certain tissue types in the adult organism. Understanding how these germ layers are positioned in the embryo provides insight into how the adult organism will be constructed.
The ectoderm tissue always develops into the outer skin layer and nervous system. The endoderm always develops into the lining of the adult digestive system.
Diploblastic animals only have two germ layers: the inner endoderm and the outer ectoderm. Animals in the phyla Cnidaria and Ctenophora are diploblastic. Triploblasts that do not develop a coelom are called acoelomates: their mesoderm region is completely filled with tissue. Flatworms in the phylum Platyhelminthes are acoelomates. 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. 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. This is termed secondary radial symmetry. They are believed to have evolved from bilaterally symmetrical animals; thus, they are classified as bilaterally symmetrical.
Only members of the phylum Porifera sponges have no body plan symmetry. There are some fish species, such as flounder, that lack symmetry as adults. However, the larval fish are bilaterally symmetrical. Learning Objectives Differentiate among the ways in which animals can be characterized by body symmetry. 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.
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