A multicellular organism always has bilateral symmetry. Symmetry (biology)

Asymmetry- (Greek α- - "without" and "symmetry") - lack of symmetry. Sometimes the term is used to describe organisms that lack symmetry in the first place, as opposed to dissymmetry- secondary loss of symmetry or its individual elements.

The concepts of symmetry and asymmetry are alternative. The more symmetrical an organism is, the less asymmetric it is, and vice versa. The body structure of many multicellular organisms reflects certain forms of symmetry, radial or bilateral. A small number of organisms are completely asymmetric. In this case, one should distinguish between the variability of shape (for example, in an amoeba) and the lack of symmetry. In nature and, in particular, in living nature, symmetry is not absolute and always contains some degree of asymmetry. For example, symmetrical plant leaves do not exactly match when folded in half.

Elements of symmetry

Among the elements of symmetry, the following are distinguished:

plane of symmetry - a plane dividing an object into two equal (mirror-symmetrical) halves;
axis of symmetry - a straight line, when rotated around it at some angle less than 360 o, the object coincides with itself;
center of symmetry - a point dividing in half all straight lines connecting similar points of an object.

Usually, axes of symmetry pass through the center of symmetry, and planes of symmetry pass through the axis of symmetry. However, there are bodies and figures that, in the presence of a center of symmetry, have neither axes nor planes of symmetry, and in the presence of an axis of symmetry, there are no planes of symmetry (see below).

In addition to these geometric symmetry elements, biological ones are distinguished:

Symmetry types

Biological objects have the following types of symmetry:

spherical symmetry - symmetry with respect to rotations in three-dimensional space through arbitrary angles.
axial symmetry (radial symmetry, rotational symmetry of an indefinite order) - symmetry with respect to rotations through an arbitrary angle around an axis.
- rotational symmetry of the nth order - symmetry with respect to rotations through an angle of 360 ° / n around any axis.
bilateral (bilateral) symmetry - symmetry with respect to the plane of symmetry (mirror reflection symmetry).
translational symmetry - symmetry with respect to shifts of space in any direction for a certain distance (its special case in animals is metamerism (biology)).
triaxial asymmetry - lack of symmetry along all three spatial axes.

Classification of symmetry types of plant flowers

Symmetry types of plant flowers

Symmetry type	Planes of symmetry	Synonyms	Examples
Ancient asymmetry or haplomorphy	Not	Actinomorphy, radial, regular	Magnolia (Magnoliaceae), Nymphea (Nymphaceae)
Actinomorphy or radial symmetry	Usually more than two (polysymmetric)	Regular, pleomorphy, stereomorphy, multisymmetry	Primula (Primulaceae), Narcissus (Amaryllidaceae), Pyrola (Ericaceae)
Dissymmetry	Two (dissymmetric)	Bilateral symmetry	Dicentra (Fumariaceae)
zygomorphy	One (monosymmetric)	Bilateral, irregular, medial zygomorphy
medial zygomorphy or bilateral symmetry			Salvia (Lamiaceae), Orchid (Orchidaceae), Scrophularia (Scrophulariaceae)
transverse (top-bottom) zygomorphy			Fumaria and Corydalis (Fumariaceae)
diagonal zygomorphy		obligate zygomorphy	Aesculus (Hippocastanaceae) found in Malpighiaceae, Sapindaceae
Acquired asymmetry	Not	Irregular, asymmetric
new asymmetry		Irregular, asymmetric	Centranthus (Valerianaceae), found in Cannaceae, Fabaceae, Marantaceae, Zingiberaceae
enantiomorphy mono-enantiomorphy di-enantiomorphy		Enantiostyly, unequal lateral	Cassia (Caeasalpinaceae), Cyanella (Tecophilaeceae), Monochoria (Pontederiaceae), Solanum (Solanaceae), Barberetta and Wachendorffia (Haemodoraceae)

spherical symmetry

Radial symmetry

Bilateral symmetry

The evolution of symmetry

Signs of symmetry are determined by the external environment. A completely isotropic ecological niche corresponds to the maximum degree of symmetry of organisms. The first organisms on Earth, unicellular floating in the water column, may have had the maximum possible symmetry - spherical, they appeared about 3.5 billion years ago.

Evolution of symmetry in animals and protists

Asymmetrization in animals along the "up-down" axis occurred under the action of the field gravity. This led to the appearance of the ventral (lower) and dorsal (upper) side in the vast majority of mobile animals (both with radial and bilateral symmetry). Some radially symmetrical sessile animals do not have a dorsal and ventral side; the aboral pole usually corresponds to the lower side of the body, while the oral (oral) pole corresponds to the upper side.

Asymmetrization along the anterior-posterior axis occurred when interacting with spatial field, when a quick movement was needed (to escape from a predator, to catch up with a prey). As a result, the main receptors and the brain were in the front of the body.

Bilaterally symmetrical multicellular animals have dominated for the last 600-535 Ma. They became finally dominant in the fauna of the Earth after the "Cambrian explosion". Prior to this, among the representatives of the Vendian fauna, radially symmetrical forms and peculiar animals that possessed the “symmetry of sliding reflection”, for example, charnia, prevailed.

Among modern animals, only sponges and ctenophores seem to have primary radial symmetry; although cnidarians are radially symmetrical animals, the symmetry in coral polyps is usually bilateral. According to modern molecular data, the symmetry in cnidarians was probably initially bilateral, and the radial symmetry inherent in medusozoans is secondary.

V. N. Beklemishev in his classic work gave a detailed analysis of the symmetry elements and a detailed classification of the symmetry types of protists. Among the body forms characteristic of these organisms, he distinguished the following:

anaxon - for example, in amoeba (complete asymmetry);
spherical (spherical symmetry, there is a center of symmetry in which an infinite number of axes of symmetry of an infinitely large order intersect) - for example, in many spores or cysts;
indefinitely polyaxon (there is a center of symmetry and a finite, but indefinite number of axes and planes) - many sunflowers;
correct polyaxon (a strictly defined number of symmetry axes of a certain order) - many radiolarians;
stauraxon (monaxon) homopolar (there is one axis of symmetry with equivalent poles, that is, intersected in the center by a plane of symmetry, in which at least two additional axes of symmetry lie) - some radiolarians;
monaxon heteropolar (there is one axis of symmetry with two unequal poles, the center of symmetry disappears) - many radiolarians and

"and subsection" "" we published the article "Why do right-handers exist? » Today we will continue the topic and consider an even more global issue - why bilateral symmetry in higher animals and man? Why aren't we like hydras or starfish? Is such a development of evolution possible at all, when the bodies will have non-bilateral symmetry? These are the questions we will answer. At the same time, to the question asked in the previous article “Why is the right hemisphere responsible for the left side of the body, and the left for the right?”

Why bilateral symmetry? You probably know hundreds of examples of such bodies - these are horses, dogs, frogs, cats - almost any vertebrates that you take will be bilaterally symmetrical. But why? It would be nice to have five-ray symmetry, like a starfish... They say that a new individual can grow from one of its torn off rays... Maybe we would have such an ability?..

Why does bilateral symmetry occur at all?

Answer: This is due to active movement in space. Let's explain in detail:

Some unicellular and multicellular creatures live in the water column. Strictly speaking, for them there are no concepts of "right-left" and "up-down", because the force of gravity is negligible, and the environment is the same. Therefore, they look like a sphere - needles and outgrowths stick out in all directions to increase buoyancy. An example is radiolaria:

Primitive multicellular organisms attached to the bottom live differently. The "up" and "down" are already there, but the probability of the appearance of prey or predator is the same from all sides. This creates radial symmetry. An anemone, a hydra or a jellyfish spreads its tentacles in all directions, the concepts of "right" and "left" are nothing to them.

With more active movement, the concepts of "front" and "behind" arise. All the main sense organs go forward, because the probability of an attack or prey is greater in front than behind, and everything that has already been indifferently crawled past, swam, ran and flew by, is not so significant.

An even more active movement implies a uniform interest in both what is on the left and what is on the right. There is a need for bilateral symmetry. An example that explains the dependence of the pace of movement and symmetry is sea urchins. Slowly crawling species have, like all echinoderms, ray symmetry.

However, some species have mastered life in the sea sand, in which they dig and move quite quickly. Exactly in accordance with the rule described above, their spherical shell is flattened, stretched a little and becomes bilaterally symmetrical!

And now the MAIN THING:

In a bilaterally symmetrical animal, both halves should develop equally.

After all any bias in one direction or another is harmful.

Everything is simple.

If there were no crossing of nerves, and the right hemisphere was responsible for the right side of the body:

The degree of development of each of the halves depends on the load. Imagine: by chance, the right side of the animal's body moves more, muscles grow, blood supply to the right hemisphere is better (after all, there is no decussation of nerves).

The more blood, the more nutrition, and the more development of the right half of the brain. Hence, if there was no crossing of nerves, there would be a huge right half of the body and a huge right hemisphere. Whereas the frail left half of the body with grief in half was controlled by the tiny left hemisphere. Well, or vice versa ... Agree, a hybrid would be noble - and non-survival.

Therefore, it is more survivable when the right hemisphere controls the left half of the body. Then stimulation of the right hemisphere will improve the left side of the body! So the growth of one of the two symmetrical parts of the body, as it were, "pulls" the other behind it, thereby ensuring their uniform coordinated development.

General conclusion:

Active movement creates bilateral symmetry.

Therefore, if we lived in other bodies (hydras, jellyfish, starfish, etc.), and led the same active lifestyle, then we would again have bilateral symmetry.

So here it is, no matter how sad 🙂

Most zoologists believe that all bilaterally symmetrical organisms are descended from Radiata. The point is that the early stages of development of bilaterally symmetrical organisms (stages of cleavage, blastula, and gastrula) are usually subject to radial symmetry, and bilateral symmetry is formed only later in development. In addition, radially symmetrical animals are simpler than bilaterally symmetrical ones: ctenophores and cnidarians do not have a through intestine, circulatory, and excretory system.

For a long time, all hypotheses about the origin of bilaterally symmetrical animals were based only on the data of comparative anatomy and embryology. The method of triple parallelism (a combination of comparative anatomy, embryology and paleontology) introduced by Haeckel could be applied after the discovery of the Vendian fauna. It was at this time (between 620 and 545 million years ago) that the formation of Bilateria took place. In the Vendian fauna, radially symmetrical forms predominate over bilaterally symmetrical ones, and among the latter there are many that can be considered as transitional forms between Radiata and Bilateria. Theories of the origin of bilaterally symmetrical animals can be divided into several large groups: comparative anatomical, embryological, paleontological.

Comparative anatomical. Over the course of two centuries, the Sran has developed several alternative concepts, which are based on a detailed comparative analysis of the structure of various b / n groups. 1. Planuloid-turbelar hypotheses. Supporters of these hypotheses (Graf, Beklemishev, Ivanov) assumed that the ancestors of bilateria were organisms resembling parenchymuls or planulas. Such organisms first swam in the water column with the help of cilia, then sank to the bottom. An active benthic lifestyle contributed to the formation of bilateral symmetry. Primary Bilateria were very simple organisms: they did not have a through intestine and coelom. Among those living today, these are ciliary worms (more precisely, intestinalless turbellaria Acoela), from which all other groups of bilateria originated. 2. Archicelomatic hypotheses. Supporters - Masterman, Remane, Ultrich - assumed that the ancestors of Bilateria were four-beam intestinal cavities, the gastric cavity of which was divided by partitions into 4 chambers. Such polyps began to crawl on the mouth, which turned into the ventral side. In the course of transformations, the whole turned out to be subdivided into three sections: preoral, perioral, and trunk. Among modern organisms, the closest are unsegmented coelomic organisms (sipunculids, brachiopods, phoronids, hemichordates). According to the supporters of this hypothesis, all non-coelomic Bilateria lost the whole, and some (flatworms) also the through intestine. 3. Metameric hypotheses. Supporters - Beneden, Snodgrass, bred Bilateria from multi-beam corals. The circular arrangement of the chambers of the intestinal cavity in radially symmetrical forms turned into the metamerism of the primary Bilateria, and the tentacles arranged in a circle turned into lateral metameric appendages - parapodia or limbs. Embryological. Bilateria have two types of cleavage: spiral (annelids, molluscs) and radial (Lophophorata and Deuterostomia). The location of cells in a crushing egg in both types of crushing is subject to 4-beam symmetry. This arrangement of cells allows the formation of a radially symmetrical blastula, which is the first larval stage of primitive Bilateria. The formation of bilateral symmetry begins in primitive Bilateria only after the gastrula stage. This happens through the growth of one of the sectors of the gastrula, which will become the dorsal side of the larva. As a result, the relative position of the aboral organ and the blastopore changes. The aboral organ slides forward, and the blastoporal side turns out to be ventral (this is typical for most Bilateria, except for chordates, in which the blastoporal side becomes the back, since chordates are inverted Bilateria). Thus, the data of classical embryology indicate that the ventral side of bilaterally symmetrical animals is an outgrowth of the blastoporal side. In other words, Bilateria crawl on an elongated blastopore, while the mouth and anus are derivatives of the anterior and posterior edges of the blastopore. Paleontological. The organization of some Vendian Bilateria bears features of transitional forms from radially symmetrical to bilaterally symmetrical. Thus, young Dickinsonia have radial symmetry: their segments are arranged radially around an axis perpendicular to the plane of the organism. In adult Dickinsonia, the segments are arranged one after the other. In many cases, it is clearly seen that the metameric outgrowths of the intestine are not separated from its median part. Since a well-defined mouth and anus cannot be found in Vendian bilaterians, it is possible that in a number of forms the intestines communicated with the environment by an elongated mouth. It can be assumed that the Vendian Bilateria still remained coelenterates, although they already had bilateral symmetry, which was formed under the influence of a mobile lifestyle at the bottom. Most likely originBilateria. The combination of the approaches of classical comparative anatomy, embryology, and paleontology makes it possible to present the origin of Bilateria as follows. In the Vendian period, there was an extensive fauna of radially symmetric coelenterates, some of whose representatives switched to crawling along the substrate on the oral side. This nature of locomotion determined the formation of bilateral symmetry in these organisms. The Vendian Bilateria, most likely, were not yet three-layer animals, but retained the organization of bilaterally symmetrical coelenterates. This means that their intestinal cavity could be connected to the external environment by a long slit-like mouth extending along the ventral side. Such bilaterally symmetrical coelenterates became the ancestors of the Phanerozoic three-layered animals. At the same time, the slit-like blastopore closed in the middle, and the gastric pockets separated from the central tubular intestine.

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Among the elements of symmetry, the following are distinguished:

plane of symmetry - a plane dividing an object into two equal (mirror-symmetrical) halves;
axis of symmetry - a straight line, when rotated around which at some angle less than 360 o, the object coincides with itself;
center of symmetry - a point dividing in half all straight lines connecting similar points of an object.

In addition to these geometric symmetry elements, biological ones are distinguished:

Symmetry types

Biological objects have the following types of symmetry:

spherical symmetry - symmetry with respect to rotations in three-dimensional space through arbitrary angles.
axial symmetry (radial symmetry, rotational symmetry of an indefinite order) - symmetry with respect to rotations through an arbitrary angle around an axis.
- rotational symmetry of the nth order - symmetry with respect to rotations through an angle of 360 ° / n around any axis.
bilateral (bilateral) symmetry - symmetry relative to the plane of symmetry (mirror reflection symmetry).
translational symmetry - symmetry with respect to shifts of space in any direction for a certain distance (its particular case in animals is metamerism (biology)).
triaxial asymmetry - lack of symmetry along all three spatial axes.