Symmetry (biology). The origin of bilateral symmetry in multicellular animals A multicellular organism always has bilateral symmetry

"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 humans? Why aren't we like hydras or starfish? Is it possible in general for such a development of evolution when bodies will have non-bilateral symmetry? We will answer these questions. At the same time, and given 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 severed rays ... Maybe we would have such an ability? ..

Why does bilateral symmetry arise at all?

Answer: This is due to active movement in space. Let us 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 "top-bottom", 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. There is already "up" and "down", but the probability of the appearance of prey or a predator is the same from all sides. This is how radial symmetry arises. Actinia, hydra or jellyfish spreads its tentacles in all directions, the concepts of "right" and "left" for them are nothing.

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

An even more active movement presupposes 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 quickly enough. By exactly following the rule described above, their spherical shell flattens, stretches slightly and becomes bilaterally symmetrical!

And now the MAIN THING:

In a bilaterally symmetrical animal, both halves should develop in the same way.

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

It's simple.

If it were not for the crossing of the 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, the muscles grow, the blood supply to the right hemisphere is better (after all, there is no crossover of nerves).

The more blood, the more nutrition, and the greater the 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 was ruled in half with grief by a tiny left hemisphere. Well, or vice versa ... Agree, a hybrid would be notable - and not survivable.

Therefore, it is more survivable when the right hemisphere controls the left half of the body. Then the stimulation of the right hemisphere will improve the left side of the body! So the growth of one of the two symmetrical body parts, as it were, "pulls up" the other, 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, no matter how sad 🙂

(bilateral symmetry) - mirror reflection symmetry, in which the object has one plane of symmetry, relative to which its two halves are mirror symmetric. If the perpendicular from point A is dropped onto the plane of symmetry and then from point O on the plane of symmetry it is extended to the length AO, then it will reach point A 1, in everything similar to point A. There is no symmetry axis for bilaterally symmetric objects. In animals, bilateral symmetry is manifested in the similarity or almost complete identity of the left and right halves of the body. In this case, there are always random deviations from symmetry (for example, differences in papillary lines, vascular branching and the location of moles on the right and left hands of a person). There are often small but regular differences in the external structure (for example, more developed musculature of the right arm in right-handed people) and more significant differences between the right and left halves of the body in the location of internal organs. For example, the heart in mammals is usually placed asymmetrically, with an offset to the left.

In animals, the appearance of bilateral symmetry in evolution is associated with crawling along the substrate (along the bottom of the reservoir), in connection with which the dorsal and abdominal, as well as the right and left halves of the body appear. In general, among animals, bilateral symmetry is more pronounced in actively mobile forms than in sessile ones. Bilateral symmetry is characteristic of all fairly highly organized animals, except for echinoderms. In other kingdoms of living organisms, bilateral symmetry is inherent in fewer forms. Among protists, it is characteristic of diplomonads (for example, lamblia), some forms of trypanosomes, bodonids, and shells of many foraminifera. In plants, not the whole organism usually has bilateral symmetry, but its individual parts - leaves or flowers. Botany calls bilaterally symmetrical flowers zygomorphic.

Sources

Symmetry (in biology) - article from the Great Soviet Encyclopedia
Beklemishev V.N. Fundamentals of comparative anatomy of invertebrates. In 2 vols. Volume 1. Promorphology. M., Science, 1964.

Wikimedia Foundation. 2010.

See what "Bilateral symmetry" is in other dictionaries:

BILATERAL SYMMETRY, a type of symmetry (see SYMMETRY (in geometry)), in which only one plane of symmetry can be drawn through the animal's body, mirroring it into two identical halves ... encyclopedic Dictionary

BILATERAL SYMMETRY - bilateral symmetry in organisms, expressed in the fact that their body is divided by the middle cavity into the right and left halves, which are, as it were, a mirror image of one another. Typical for stems with two-row leaves or ... ... Dictionary of botanical terms

bilateral symmetry - a type of symmetry in which only two mutually perpendicular planes of symmetry can be drawn through the axis of the organ ... Plant anatomy and morphology

This term has other meanings, see Symmetry (meanings). Symmetry (other Greek συμμετριαι "proportionality") in biology is the regular arrangement of similar (identical) body parts or forms of a living organism, aggregate ... ... Wikipedia

I Symmetry (from the Greek symmetria proportionality) in mathematics, 1) symmetry (in a narrow sense), or reflection (mirror) relative to the plane α in space (relative to a straight line and on a plane), transformation of space ... ... Great Soviet Encyclopedia - Medusa This term has other meanings, see Axial symmetry. Radial symmetry is a form of symmetry in which a body (or ... Wikipedia

Drawing by Leonardo da Vinci, depicting human symmetry Bilateral symmetry Similarity or complete identity of the left and right halves of the body. At the same time, insignificant differences in the external structure and differences in the location of the internal ones are allowed ... Wikipedia

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

For a long time, all hypotheses about the origin of bilaterally symmetric animals were based only on the data of comparative anatomy and embryology. The triple parallelism method (a combination of comparative anatomy, embryology and paleontology) introduced by Haeckel could have been applied after the discovery of the Vendian fauna. It was at this time (between 620 and 545 million years ago) that the Bilateria was formed. In the Vendian fauna, radially symmetric forms prevail over bilaterally symmetric 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 symmetric animals can be divided into several large groups: comparative anatomical, embryological, and paleontological.

Comparative anatomical.Over the course of two centuries, the shit has developed several alternative concepts, which are based on a detailed comparative analysis of the structure of various groups of b / n. 1. Planuloid-turbellar hypotheses. Proponents of these hypotheses (Graf, Beklemishev, Ivanov) assumed that the ancestors of bilateria were organisms resembling parenchymules or planules. 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 ciliated worms (more precisely, the intestinal turbellaria Acoela), from which all other groups of bilateria originated. 2. Architelomatic hypotheses. Supporters - Masterman, Remane, Ultrich - assumed that the ancestors of Bilateria were four-ray coelenterates, 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 the transformations, the whole was subdivided into three sections: pre-oral, perioral and trunk. Among the modern ones, the closest are non-segmented coelomic organisms (sipunculids, brachiopods, phoronids, hemichordates). According to the supporters of this hypothesis, all non-coelomic Bilateria have lost the whole, and some (flatworms) and the through intestines. 3. Metameric hypotheses. Supporters - Beneden, Snodgras, removed Bilateria from multibeam corals. The circular arrangement of the intestinal cavity chambers in radially symmetric forms turned into metamerism of primary Bilateria, and the tentacles located 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 arrangement of cells in a cleaving egg of both types of cleavage obeys 4-beam symmetry. This arrangement of cells allows the formation of a radially symmetric 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 occurs through the growth of one of the gastrula sectors, which will become the dorsal side of the larva. As a result, the relative position of the aboral organ and blastopore changes. The aboral organ is smeared 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 symmetric animals is an extension of the blasoporal side. In other words, Bilateria crawl on an elongated blastopore, while the mouth and anus are porous for the anterior and posterior edges of the blastopore. Paleontological. The organization of some Vendian Bilateria has features of transitional forms from radially symmetric to bilaterally symmetric. Thus, young Dickinsonia have radial symmetry: their segments are located radially around an axis perpendicular to the plane of the body. In adults, Dickinsonian segments are located one after another. In many cases, it is clearly seen that the metameric outgrowths of the intestine are not separated from its median part. Since in Vendian bilaterians it is not possible to find a well-defined mouth and anus, it is possible that in a number of forms the intestine communicated with the environment with an extended mouth. It can be assumed that the Vendian Bilateria still remained coelenterates, although they already possessed bilateral symmetry, which was formed under the influence of a mobile lifestyle at the bottom. Most likely route of originBilateria.The combination of approaches of classical comparative anatomy, embryology, paleontology allows us 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 moved to crawling on the substrate on the oral side. This type of movement determined the formation of bilateral symmetry in these organisms. The Vendian Bilateria, most likely, were not yet three-layered animals, but retained the organization of bilaterally symmetric coelenterates. This means that their intestinal cavity could be connected with the external environment by a long slit mouth, stretching along the ventral side. Such bilaterally symmetric coelenterates became the ancestors of Phanerozoic three-layer animals. In this case, the slit blastopore closed in the middle, and the gastric pockets separated from the central tubular intestine.

A new look at the origin of bilateria

V.V. Malakhov

Vladimir Vasilievich Malakhov, Corresponding Member RAS, head. department Invertebrate Zoology, Lomonosov Moscow State University.

One of the mysteries of the development of life on Earth is the “Cambrian explosion”. This is how paleontologists call the almost simultaneous appearance of almost all types of the animal kingdom about 500 million years ago. It is surprising that among the Cambrian fossil organisms there are many complexly organized - arthropods, molluscs, brachiopods, echinoderms, and even chordates. All of them belong to bilaterally symmetric animals, or bilateria ( Bilateria), which predominate in the animal kingdom during the entire Phanerozoic - from the Cambrian to the present day. Nevertheless, biologists believe that all bilaterally symmetric animals evolved from coelenterates - organisms with radial symmetry ( Radiata). In modern fauna, these include corals, jellyfish and the well-known hydra.

Intestinal in comparison with bilateria, indeed, are much simpler, they have neither a through intestine (there is a mouth, but there is no anus), nor a circulatory and excretory system, they are devoid of a secondary body cavity. In addition, at the early stages of embryogenesis (cleavage, blastula and gastrula), some bilateria obey radial symmetry and only at later stages does bilateral appear. According to the so-called biogenetic law discovered by E. Haeckel and F. Müller, embryonic development in a brief form repeats the evolutionary (phylogenetic) path of the origin of organisms. Hence, in the evolution of the animal kingdom, radial symmetry preceded bilateral symmetry.

Haeckel entered the history of biology not only as the author of the biogenetic law, but also as the founder of the famous evolutionary triad. It was Haeckel who first proposed using three approaches simultaneously to solve the problems of the origin of certain groups of animals and plants: comparative anatomy, embryology, and paleontology. However, in the problem of the origin of bilaterally symmetric animals, the method of the Haeckel triad could not be fully applied because for a long time no remains of animals of the Precambrian time were found.

The situation changed only in the second half of the twentieth century, when Russian paleontologists discovered the richest fauna of the Vendian - the last period of the Proterozoic era, preceding the Cambrian and lasting about a hundred million years [,]. Imprints of various Vendian organisms were found on the shores of the White Sea, as well as in other regions of the planet (Australia, Canada, Eastern Siberia, etc.). In the Vendian fauna, there were much more radially symmetric animals than bilaterally symmetric ones, and among them there were many transitional forms. Apparently, bilateria formed approximately between 620 and 535 million years ago.

In the last decades of the last century, evolutionary molecular biology began to flourish. And now, in order to determine the relationship of organisms and reconstruct the paths of their evolution, they began to compare the nucleotide sequences in separate parts of the genome or even entire genomes. Studies of genes that control the processes of embryonic development play a special role in the study of the phylogenetic relationships of multicellular animals. That is why in our time it is more correct to speak not about the evolutionary triad, but about the tetrad, i.e. a combination of methods of comparative anatomy, embryology, paleontology and molecular biology. What does each of these sciences separately and their combination give for solving the question of the origin of bilateria?

Comparative anatomy

Over the past 150 years, specialists in comparative anatomy have developed several dozen hypotheses about the origin of bilaterally symmetric animals. All these hypotheses can be divided into three groups: planuloid-turbellar, archicelomate, and metameric.

Diagrams reflecting the different views of comparative anatomists on the origin of bilaterally symmetric animals. According to the supporters of the planuloid-turbellar hypotheses ( up), the ancestors of bilateria were organisms resembling the larvae of modern coelenterates (planules). According to one of the versions (upper row), the ventral side of the primary bilateria was formed from the lateral sector of the planuloid ancestor, according to the other, from the oral surface.
According to archelomatic hypotheses ( at the bottom), bilateria evolved from four-rayed coral polyps, the intestinal cavity of which is divided into four chambers, and metameric ( below all) - from multibeam corals (top and side views, as well as from the side of the oral surface).

According to the planuloid-turbellar hypotheses, all or at least some of the bilaterally symmetrical animals evolved from organisms that resemble the larvae of modern coelenterates - planules. At the posterior end of such ancestral forms there was a mouth, and at the opposite (aboral organ) there was an accumulation of ciliary sensitive cells. At first, the planuloid organisms swam in the water column, then sank to the bottom and proceeded to crawl over the substrate, which contributed to the formation of bilateral symmetry. According to the supporters of these hypotheses, primary bilateria were very simple organisms, they did not have a through intestine and coelom. Ciliary worms became their descendants ( Turbellaria), from which other bilaterally symmetrical animals originate.

In understanding exactly how the ventral side of bilateria originated, different authors were not so unanimous. For example, domestic zoologists (VN Beklemishev, AV Ivanov and YV Mamkaev and others) are convinced that in ciliary worms it is homologous to the lateral sector (antimere) of the planuloid ancestor [,]. The American researcher L. Hyman believed that the ancestors of bilaterally symmetrical animals began to crawl on the oral surface, which was transformed into the abdominal side.

The adherents of the archicelomate hypotheses assumed that the ancestors of bilateria were four-ray polyps, the intestinal cavity of which is divided by partitions into four chambers [-]. Such polyps sank to the ground and began to move on the oral surface, which later turned into the ventral side. The primary mouth stretched out and became slit-like (like in modern coral polyps), and then closed in the middle so that only two openings remained: one at the front end of the body and became the mouth, and the other at the back and transformed into the anus. The intestinal pockets separated from the central part of the intestine and gave rise to five chambers of the secondary body cavity - a whole, dissected into three sections. According to the supporters of the archicelomate hypotheses, all non-coelomic bilaterally symmetrical organisms (for example, flat and round worms) have lost their secondary body cavity.

Early stages of embryonic development in some groups of bilaterally symmetric animals. The blastopore and its transformations in echinoderms, ciliary and annelids to the ventral side, and in chordates, to the dorsal side are highlighted in red. Explanations in the text.

Proponents of metameric hypotheses also derived bilaterally symmetric animals from corals, but not from four-rayed polyps, but multi-ray polyps, in which the intestinal cavity is divided into many chambers by numerous septa-septa, for example, as in the six-rayed coral Ceriantharia [-]. As in the archicelomate hypotheses, it was assumed that such corals began to crawl on the mouth side, the slit mouth closed in the middle, and the intestinal chambers separated from the central part of the intestine and gave rise to numerous coelomic chambers. Only in this case, the circular arrangement of the intestinal cavity chambers (cyclomeria) in radially symmetric forms turned into metamerism of primary bilateria, and the tentacles located in a circle - into lateral metameric appendages (parapodia), which serve as organs of movement. Thus, primary bilateria were not only coelomic, but also segmented organisms with metameric appendages - limb rudiments.

Classical embryology

In primitive bilaterally symmetric animals (marine invertebrates that have retained external fertilization and ciliary larvae floating in the water column), as already mentioned, the early stages of embryonic development - cleavage, blastula and gastrula - obey radial symmetry. This happens by the growth of one of the sectors of the gastrula - exactly the one that will later become the dorsal side of the larva. As a result, the relative position of the primary nerve center (aboral organ) and the opening of the primary intestinal cavity (blastopore) changes. The aboral organ moves forward, and the blastoporal side becomes the ventral side of the future larva. This process is typical for the overwhelming majority of groups of bilaterally symmetric animals. The only exception is chordates, in which the blastoporal side becomes the dorsal part of the body. There is an explanation for this: chordates are inverted bilateria.

Varieties of slit blastopore in embryos of bilaterally symmetric animals.

In many groups of bilaterally symmetric animals, the blastopore elongates and acquires a slit-like shape. This form of blastopore is characteristic of annelids, mollusks, roundworms, arthropods, lower chordates, etc. In arthropods, the slit-like blastopore is represented by a deep longitudinal groove running along the ventral side of the embryo and connecting the rudiments of the mouth and anus. Closure of the slit blastopore leads to the formation of a tubular intestine. In primitive cases, the blastopore first closes in the middle, dividing into two holes, one at the front and the other at the posterior end of the embryo. One of these holes corresponds to the mouth and the other to the anus.

Thus, the embryological data helped to clarify, firstly, that the ventral side of bilaterally symmetric animals is an extension of the blastoporal side, and secondly, that their mouth and anus are derivatives of the anterior and posterior ends of the slit blastopore.

Paleontology

The formation of bilateral symmetry and the initial radiation of the main evolutionary trunks of bilateria, which gave rise to the rich and diverse fauna of the early Phanerozoic, apparently took place in the Vendian. The remains of the organisms inhabited at that time are represented by imprints of various structures, but mostly rather large forms, from one or two centimeters to a meter [,]. Most of them did not have a solid mineral skeleton, which sharply separates the Vendian biota from the communities of subsequent Phanerozoic periods.

It is difficult to say why the Vendian multicellular animals were skeletal. It is possible that they lived in cold water basins outside the carbonate belt controlled by cyanobacterial communities. Be that as it may, one thing is obvious: the natural environment of the Vendian, in which the evolution of primary multicellular animals proceeded, was very different from the conditions of all subsequent periods of the Phanerozoic, from the Cambrian to the present.

Forms with radial or axial symmetry are very numerous in the Vendian and in many localities undoubtedly dominate over bilaterally symmetric forms. But it is among the Vendian organisms that we first meet the imprints of true bilateria. Their body, as a rule, was dismembered into a different number of repeating segments - metameres located along the longitudinal axis or plane of symmetry, which allowed these organisms to actively move along the bottom. However, the metamerism of some Vendian bilateria is rather unusual: their segments were not strictly symmetrical, but offset. A striking example of "abnormal" segmentation - Dickinsonia, in which identical elements of the right and left sides of the body are shifted relative to each other. On the prints of this animal, like most Vendian bilateria, the median groove (or ridge) corresponding to the central part of the intestine is clearly visible, as well as lateral branches extending from it - outgrowths from the central intestinal region.

Drawings of prints of Vendian bilateria.

Dickinsonia is also an example of transitional forms, of which, I note, there are quite a few among Vendian organisms that combine the features of both radial and bilateria. Young Dickinsoniathere was essentially radial symmetry: the segments (a kind of antimeres) diverged radially from the axis perpendicular to the plane of the organism. In adult forms, the segments followed each other, although at the anterior and posterior ends, as in the young, they were located radially. Symmetry Dickinsonia is close to the bilateral symmetry of some modern and fossil corals.

Vendian bilaterally symmetric organisms.
Reconstructions by M.A. Fedonkin.

Like modern coelenterates, their aboral (i.e., opposite to the mouth) side was covered with a thickened protective cover - the cuticle. True, in Vendian bilaterians this cover was organic, while in modern coelenterates it can be both organic and calcareous. Unlike modern coelenterates, which are attached to the substrate, the Vendian bilaterally symmetric coelenterates were mobile organisms that crawled on the oral surface.

Molecular biology

In the last two decades, evolutionary biology has received a powerful new tool for studying homology - a comparative analysis of the structure and expression of genes that control the formation of spatial organization during embryonic development. These genes contain homeoboxes - specific sequences of approximately 180 base pairs. For the first time, they were found in homeotic genes, mutations in which lead to the transformation of one part of the adult organism into another, completely different structure. Genes containing homeoboxes regulate the development of axial structures, segmentation, laying of extremities, and other most fundamental processes in the embryogenesis of both invertebrates and vertebrates. It is assumed that the system of such genes arose as a result of the multiplication and subsequent differentiation of one gene in the ancestors of multicellular organisms.

Diagram of the distribution of the expression zones of genes containing homeoboxes,
in coelenterates ( left) and bilaterian embryos.

This system was found in all multicellular animals, including modern coelenterates [-]. They have, in particular, homologues of the Brachiury, goosecoid, and fork head genes. In the embryos of bilaterally symmetric animals, these genes are expressed during gastrulation along the slit-like blastopore at the anterior and posterior edges or around the products of its division - the mouth and anus. They play an important role in the formation of the anterior and posterior mesodermal primordia in vertebrates and invertebrates. In coelenterates, homologues of these genes are expressed in the annular region around the oral opening [-]. Obviously, the embryonic organizers of the head and trunk-caudal structures, which in bilateria are associated with the anterior and posterior ends of the slit-like blastopore, originated from a single annular organizer of coelenterates. Interestingly, some genes, such as the fork head, are involved in the anterior and posterior organizers (sometimes also in a series of regions along the blastopore closure line), while others, such as the goosecoid and Brachiury, are segregated: the goosecoid in the anterior, Brachiury in the posterior. Such a splitting of the originally single organizer could arise only as a result of lengthening the primary oral opening, closing it in the middle and transforming the openings at its ends into definitive mouth and anus bilaterium, as was assumed in the constructions of classical comparative anatomy.

The most probable way of origin of bilateria

We will try to propose a theory of the origin of bilateria in accordance with the current level of development of evolutionary biology, combining the approaches of classical comparative anatomy, embryology, paleontology and molecular developmental biology.

In the Vendian period, there was an extensive fauna of radially symmetric coelenterates, some of which moved to crawling on the substrate on the oral surface. This type of movement determined the formation of bilateral symmetry in these organisms. The intestinal cavity of the Vendian bilateria could still be connected with the external environment by a long slit-like mouth extending along the ventral (oral) side. It is possible that the metamerically located pockets of the intestinal cavity were also not yet separated from its central part. The slit blastopore closed in the middle, and the pockets separated from the central tubular intestine. It was these bilaterally symmetric coelenterates that became the ancestors of the Phanerozoic three-layer animals.

Origin of bilaterally symmetrical animals.
The aboral nerve center is highlighted in red.

One should not imagine the origin of bilateria as a result of detachment from the substrate and the transition to crawling of a certain polyp. In the larvae of modern coelenterates, just as in bilaterally symmetrical animals, the aboral nerve center is preserved. The larvae of modern coelenterates attach to the substrate; therefore, in polyps, the aboral nerve center is reduced. In modern bilaterians, it is not only not lost, but on its basis in some groups (for example, annelids and mollusks), a cerebral ganglion is formed. The retention of the aboral nerve center in the larvae of modern bilateria suggests that their radially symmetric ancestors were not sessile forms. Most likely, they led a pelagic lifestyle and went on to a bottom existence, since they were mobile organisms capable of crawling on the oral surface.

It is possible that Vendian bilaterally symmetric coelenterates were the ancestors of not only three-layer bilateria, but also Phanerozoic coelenterates. Bilateral symmetry, for example, is characteristic of sedentary corals, which are known to promote the development of radial symmetry. The manifestation of bilateral symmetry in modern and fossil corals appears to be a legacy of ancestral symmetry that is gradually being lost as a result of an attached lifestyle.

Some representatives of the Cambrian bilaterally symmetric animals.

It is no less important that the probable ancestors of three-layer bilateria turned out to be morphologically very complex organisms with a through intestine, metamerism, and a segmented secondary body cavity. This greatly changes our understanding of the main directions of evolution of bilaterally symmetric animals. Now, when discussing the directions of evolution of the animal kingdom, we must consider not the theory of the origin of the coelom and metamerism, but, on the contrary, the causes, ways and consequences of the loss of the coelom and metamerism.

The great morphological complexity of the Vendian ancestors of bilateria explains the so-called Cambrian explosion, i.e. almost simultaneous appearance in the Cambrian period of almost all types of bilaterally symmetric animals. It is very characteristic that among the Cambrian organisms there are many that resemble modern mollusks or arthropods. Their abundance in the Cambrian was as if prepared by the fact that already in the Vendian period there were many complex metameric organisms that gave rise to the main trunks of bilaterally symmetrical animals.

Literature

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2. Sokolov B.S. Essays on the formation of the Vendian. M., 1997.

3. Beklemishev V.N. Fundamentals of Comparative Invertebrate Anatomy. M., 1952.

4. Ivanov A.V., Mamkaev Yu.V.Ciliary worms (Traces of independent movement - the final proof of the animal nature of Ediacaran organisms // Evolution of life on Earth: Proceedings of the II International Symposium. Tomsk, 2001. Pp. 133-137.

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(bilateral symmetry) - mirror reflection symmetry, in which the object has one plane of symmetry, relative to which its two halves are mirror symmetric. If the perpendicular from point A is dropped onto the plane of symmetry and then from point O on the plane of symmetry it is extended to the length AO, then it will reach point A 1, in everything similar to point A. There is no symmetry axis for bilaterally symmetric objects. In animals, bilateral symmetry is manifested in the similarity or almost complete identity of the left and right halves of the body. In this case, there are always random deviations from symmetry (for example, differences in papillary lines, vascular branching and the location of moles on the right and left hands of a person). Often there are small, but regular differences in the external structure (for example, more developed musculature of the right arm in right-handed people) and more significant differences between the right and left halves of the body in the location of internal organs. For example, the heart in mammals is usually placed asymmetrically, with an offset to the left.

In animals, the appearance of bilateral symmetry in evolution is associated with crawling over the substrate (along the bottom of the reservoir), in connection with which the dorsal and abdominal, as well as the right and left halves of the body appear. In general, among animals, bilateral symmetry is more pronounced in actively mobile forms than in sessile ones. Bilateral symmetry is characteristic of all fairly highly organized animals, except for echinoderms. In other kingdoms of living organisms, bilateral symmetry is inherent in fewer forms. Among protists, it is characteristic of diplomonads (for example, lamblia), some forms of trypanosomes, bodonids, and shells of many foraminifera. In plants, not the entire organism usually has bilateral symmetry, but its individual parts - leaves or flowers. Botany calls bilaterally symmetrical flowers zygomorphic.

Sources

Symmetry (in biology) // Great Soviet Encyclopedia: [in 30 volumes] / Ch. ed. A.M. Prokhorov... - 3rd ed. - M. : Soviet Encyclopedia, 1969-1978.
Beklemishev V.N. Fundamentals of comparative anatomy of invertebrates. In 2 vols. Volume 1. Promorphology. M., Science, 1964.

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