Which invertebrate is not bilaterally symmetrical

Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3. Disease Defences 4. 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. Some animals, such as t he sea urchin, are radially symmetrical, but their embryonic development and internal anatomy show that they arose from a bilaterally symmetrical ancestor.

Porifera no symmetry Cnidaria radial Annelida bilateral Chordata Arthropoda: Dragonfly Crayfish Mollusca bilateral An example of the "midplane" in bilateral symmetry.

Bilateral and radial symmetry are also found in the Plant kingdom; symmetry in ggeneral, however, is less significant here that among animals. 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. 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.

Key Terms sagittal plane : divides the body into right and left halves radial symmetry : a form of symmetry wherein identical parts are arranged in a circular fashion around a central axis bilateral symmetry : having equal arrangement of parts symmetry about a vertical plane running from head to tail.

Animal Characterization Based on Features of Embryological Development Animals may be characterized by the presence of a coelom, formation of the mouth, and type of cell cleavage during embryonic development.

Learning Objectives Explain the ways in which animals can be characterized by features of embryological development. Key Takeaways Key Points Diploblasts contain two germ layers inner endoderm and outer ectoderm , while triploblasts contain three germ layers endoderm, mesoderm, and ectoderm. The endoderm becomes the digestive and respiratory tracts; the ectoderm becomes the outer epithelial covering of the body surface and the central nervous system; and the mesoderm becomes all muscle tissues, connective tissues, and most other organs.

Bilaterally symmetrical, tribloblastic eucoelomates can be divided into protostomes, those animals that develop a mouth first, and deuterstomes, those animals that develop an anus first and a mouth second.

In protostomes, the coelom forms when the mesoderm splits through the process of schizocoely, while in deuterostomes, the coelom forms when the mesoderm pinches off through the process of enterocoely.

Protostomes undergo spiral cleavage, while deuterostomes undergo radial cleavage.