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Jun 11, 2024 · Bilateral symmetry: There is an axis; on both sides of the axis the organism looks roughly the same; Spherical symmetry: If the organism is cut through its center, the resulting parts look the same. In nature and biology, symmetry is approximate. For example, plant leaves, while considered symmetric, will rarely match up exactly when folded in ...
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symmetry, in biology, the repetition of the parts in an animal or plant in an orderly fashion. Specifically, symmetry refers to a correspondence of body parts, in size, shape, and relative position, on opposite sides of a dividing line or distributed around a central point or axis. With the exception of radial symmetry, external form has little rel...
Certain animals, particularly most sponges and the ameboid protozoans, lack symmetry, having either an irregular shape different for each individual or else one undergoing constant changes of form. The vast majority of animals, however, exhibit a definite symmetrical form. Four such patterns of symmetry occur among animals: spherical, radial, biradial, and bilateral.
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In spherical symmetry, illustrated only by the protozoan groups Radiolaria and Heliozoia, the body has the shape of a sphere and the parts are arranged concentrically around or radiate from the centre of the sphere. Such an animal has no ends or sides, and any plane passing through the centre will divide the animal into equivalent halves. The spherical type of symmetry is possible only in minute animals of simple internal construction, since in spheres the interior mass is large relative to the surface area and becomes too large for efficient functioning with increase in size and complexity.
In radial symmetry the body has the general form of a short or long cylinder or bowl, with a central axis from which the body parts radiate or along which they are arranged in regular fashion. The main axis is heteropolar—i.e., with unlike ends, one of which bears the mouth and is termed the oral, or anterior, end, and the other of which, called the aboral, or posterior, end, forms the rear end of the animal and may bear the anus. The main axis is hence termed the oral-aboral, or anteroposterior, axis. Except in animals having an odd number of parts arranged in circular fashion (as in the five-armed sea stars), any plane passing through this axis will divide the animal into symmetrical halves. Animals having three, five, seven, etc., parts in a circle have symmetry that may be referred to, respectively, as three-rayed, five-rayed, seven-rayed, etc.; only certain planes through the axis will divide such animals into symmetrical halves. Radial symmetry is found in the cnidarians (including jellyfish, sea anemones, and coral) and echinoderms (such as sea urchins, brittle stars, and sea stars).
In biradial symmetry, in addition to the anteroposterior axis, there are also two other axes or planes of symmetry at right angles to it and to each other: the sagittal, or median vertical-longitudinal, and transverse, or cross, axes. Such an animal therefore not only has two ends but also has two pairs of symmetrical sides. There are but two planes of symmetry in a biradial animal, one passing through the anteroposterior and sagittal axes and the other through the anteroposterior and transverse axes. Biradial symmetry occurs in the comb jellies.
The concept of symmetry is also applied in botany. A flower is considered symmetrical when each whorl consists of an equal number of parts or when the parts of any one whorl are multiples of that preceding it. Thus, a symmetrical flower may have five sepals, five petals, five stamens, and five carpels, or the number of any of these parts may be a multiple of five.
The number of parts in the pistillate (female) whorl is frequently not in conformity with that in the other whorls, but in such cases the flower is still called symmetrical, provided that the other whorls are normal. A flower in which the parts are in twos is dimerous; in threes, fours, or fives, trimerous, tetramerous, or pentamerous, respectively. Trimerous symmetry is the rule in the monocotyledons, pentamerous the most common in the dicotyledons, although dimerous and tetramerous flowers also occur in the latter group.
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Feb 13, 2017 · Bilateral Symmetry Definition. Bilateral symmetry refers to organisms with body shapes that are mirror images along a midline called the sagittal plane. The internal organs, however, are not necessarily distributed symmetrically. The vast majority of animals display bilateral symmetry; also known as plane symmetry, this is a trait that applies ...
- Asymmetrical Symmetry: In some animals there are no body axis and no plane of symmetry, hence the animals are called asymmetrical. The amoeboid forms (e.g., Amoeba) and many sponges have irregular growth pattern of the body and cannot be divided into two equal halves (Fig.
- Spherical Symmetry: In spherical symmetry the shape of the body is spherical and lack any axis. The body can be divided into two identical halves in any plane that runs through the organism’s centre.
- Radial Symmetry: In radial symmetry the body can be divided into two roughly equal halves by any one of many vertical planes passing through the central axis (Fig.
- Biradial Symmetry: The body of animals which exhibits biradial symmetry, represents a combination of both radial and bilateral symmetry. The organs are arranged radially and the body can be divided into two by a mid-longitudinal plane.
Flexi Says: Animals exhibit three types of symmetry: Radial Symmetry: In radial symmetry, any line through the central axis divides the organism into two identical halves. This type of symmetry is observed in animals like starfish and sea anemones. Bilateral Symmetry: In bilateral symmetry, the organism can be divided into identical halves only ...
Mar 13, 2021 · Radial vs. Bilateral Symmetry. In Biology, symmetry refers to the arrangement of body parts of an organism around a central point or axis. The two main types of symmetry are radial symmetry (in which body parts are arranged around a central axis) and bilateral symmetry (in which organisms can be divided into two near-identical halves along a ...
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Aug 4, 2022 · Symmetry in biology is the balanced arrangement of body parts or shapes around a central point or axis. That is, the size, shape, and relative location on one side of a dividing line mirrors the size, shape, and relative location on the other side. Animals can be categorized on the basis of their symmetry.
