A significant stage in the history of the Earth and the evolution of life was the emergence of multicellularity. This gave a powerful impetus to an increase in the diversity of living beings and their development. Multicellularity made possible the specialization of living cells within a single organism, including the emergence of separate tissues and organs. The first multicellular animals probably appeared in the bottom layers of the oceans at the end of the Proterozoic.
The signs of a multicellular organism are considered to be that its cells must be aggregated, the division of functions between them and the establishment of stable specific contacts are obligatory. A multicellular organism is a rigid colony of cells in which their fixed position is maintained throughout life. In the process of biological evolution, similar cells in the body multicellular organisms specialized in performing certain functions, which led to the formation of tissues and organs. Probably, under the conditions of the Proterozoic World Ocean, which already contained primitive unicellular organisms, spontaneous organization of unicellular organisms into more highly developed multicellular colonies could occur.
One can only guess what the first multicellular organisms of the Proterozoic era were like. The hypothetical ancestor of multicellular organisms could be a phagocytella that swam in the thickness sea ​​water due to the beating of superficial cells - the cilia of the kinoblast.
The phagocytella fed by capturing food particles suspended in the medium and digesting them with the internal cell mass (phagocytoblast). It is possible that it was from the kinoblast and phagocytoblast in the process of evolutionary development that all the variety of forms and tissues of multicellular organisms originated. The phagocytella itself lived in the water column, but had neither a mouth nor an intestine, and its digestion was intracellular. The descendants of phagocytella adapted to the diverse conditions of existence when they settled on the seabed, when moving to the surface, or when their food sources changed. Thanks to this, the first multicellular organisms gradually developed a mouth, intestines and other vital organs.
Another common hypothesis for the origin and evolution of multicellular organisms is the appearance of Trichoplax as the first primitive animal. This flat multicellular organism, resembling a creeping blot, is still considered one of the most mysterious on the planet. It has neither muscles, nor anterior and posterior ends, nor axes of symmetry, nor any complex internal organs, but at the same time it is able to reproduce sexually. Features of the structure and behavior of Trichoplax, crawling on the substrate among microalgae, made it possible to classify it as one of the most primitive multicellular animals on our planet.
Whoever was the ancestor of multicellular animals, the further course of evolution in the Proterozoic led to the appearance of the so-called ctenophores. These are planktonic animals with rows of rowing plates formed by fused cilia. In the Proterozoic, they switched from swimming to crawling along the bottom, therefore their body was flattened, the head section stood out, locomotor system in the form of a skin-muscular sac, respiratory organs, excretory and circulatory systems were formed. Linnaeus, the creator of the first scientific system of the organic world, paid very little attention to ctenophores, mentioning one species of ctenophores in his System of Nature. In 1829, the world's first major work on jellyfish was published. Its author, the German zoologist Eschscholtz, described in it several species of ctenophores known to him. He considered them a special class of jellyfish, which he called ctenophores (Ctenophora). This name has been preserved for them at the present time ”(“ Animal Life, edited by N. A. Gladkov, A. V. Mikheev).
More than 630 million years ago, sponges appeared on Earth, which developed on the seabed, mainly in shallow water, and then sank into deeper waters. The outer layer of the body of sponges is formed by flat integumentary cells, while the inner layer is formed by flagellar cells. At one end, the sponge adheres to any substrate - stones, algae, the surface of the body of other animals.

The first multicellular organisms lived in the bottom layers of the most ancient seas and oceans, where external environmental conditions required them to dismember the body into separate parts, which served either for attachment to the substrate or for nutrition. They fed mainly on organic matter (detritus), which covered the bottom silt. There were practically no predators then. Some multicellular organisms passed through the nutrient-rich upper layers of sea silt or absorbed the living bacteria and algae that lived in it.
Flat and annelid worms slowly swam above the very bottom or crawled among the sediments, while tube worms lay among bottom sediments. In the Proterozoic era, large flat pancake-shaped animals that lived on the muddy bottom, various jellyfish that swam in the water column, and primitive echinoderms were probably widespread in the seas and water basins of the planet. Huge algae bloomed in shallow waters - vendotenii, which reached a length of about one meter and looked like seaweed.
Most living beings on our planet by the end of the Proterozoic era were already represented by multicellular forms. Their life activity has been preserved in the form of prints and casts on the once soft silt. In the deposits of that period, one can observe traces of crawling, subsidence of the soil, and dug minks.
The end of the Proterozoic era was marked by an outbreak of the diversity of multicellular organisms and the appearance of animals, the existence of which was then closely connected with the sea. A huge number of remains of multicellular animals in layers aged 650-700 million years even caused the separation of a special period in the Proterozoic, called the Vendians. It lasted approximately 110 million years and was characterized in comparison with other epochs by the achievement of a significant diversity of multicellular animals.
The emergence of multicellular contributed to a further increase in the diversity of living organisms. It led to an increase in the ability of organisms to create a supply of nutrients in their body and respond to environmental changes.
for the further evolution of the biosphere. Living organisms gradually began to change the shape and composition of the earth's crust themselves, to form a new shell of the earth. It can be said that in the Proterozoic life on the planet became the most important geological factor.

The body of multicellular animals consists of a large number of cells, diverse in structure and functions, which have lost their independence, since they constitute a single, integral organism.

Multicellular organisms can be divided into two large groups. Invertebrates are two-layer animals with radial symmetry, the body of which is formed by two tissues: the ectoderm, which covers the body from the outside, and the endoderm, which forms the internal organs - sponges and intestinal cavities. Also included are flat, round, annelids, arthropods, molluscs and echinoderms, bilaterally symmetrical and radial three-layer organisms, which, in addition to the ecto- and endoderm, also have a mesoderm, which in the process of individual development gives rise to muscle and connective tissues. The second group includes all animals that have an axial skeleton: a chord or a vertebral column.

multicellular animals

Coelenterates. Hydra freshwater.

Structure - Radiation symmetry, ectoderm, endoderm, sole, tentacles.
Movement - Contraction of skin-muscle cells, attaching the sole to the substrate.
Food - Tentacles oral cavity intestine is a cavity with digestive cells. Predator. Kills stinging cells with poison.
Respiration - Oxygen dissolved in water permeates through the entire surface of the body.
Reproduction - Hermaphrodites. Sexual: egg cells + sperm = egg. Asexual: budding.
circulatory system no.
Excretion – Food debris is expelled through the mouth.
Nervous system – nerve plexus from nerve cells.

Flatworms. White planaria.

Roundworms. Ascaris human.

Ringed worms. Earthworm.

Structure - Elongated worm-like mucous skin on the outside, internally dissected body cavity, length 10-16 cm, 100-180 segments.
Movement - Contraction of the musculocutaneous sac, mucus, elastic bristles.
Nutrition - Mouth, pharynx, esophagus, goiter, stomach, intestine, anus. It feeds on particles of fresh or decaying plants.
Respiration - Diffusion of oxygen through the entire surface of the body.
Reproduction - Hermaphrodites. Exchange sperm slime with eggs cocoon young worms.
Circulatory system - Closed circulatory system: capillaries annular vessels main vessels: dorsal and abdominal.
Excretion - Body cavity of metanephridia (funnel with cilia) tubule excretory pair.
Nervous system - Nerves ganglions nerve chain peripharyngeal ring. Sensitive cells in the skin.

Soft-bodied. Shellfish. Prudovik ordinary.

Structure - Soft body enclosed in a helical shell = torso + leg.
Movement - Muscular leg.
Nutrition - Mouth pharynx toothed tongue = grater stomach intestines, liver anus.
Breathing - Breathing hole. Lung.
Reproduction - Hermaphrodites. Cross fertilization.
The circulatory system is open. Lung heart vessels body cavity.
Excretion - Kidney.
Nervous system - Periopharyngeal ganglion of nerves.

Arthropods. Crustaceans. Crayfish.

Structure - + belly.
Movement - Four pairs of walking legs, for swimming 5 pairs of ventral legs + caudal fin.
Nutrition - mouth, jaw, pharynx, esophagus, stomach, section with chitinous teeth, filtering apparatus, intestines, food. gland - anus.
Breath - gills.
Reproduction - Dioecious. Caviar on the legs of the abdomen until hatching. With growth, molting of chitin is characteristic. There is a nauplius larval stage.
The circulatory system is open. Heart - vessels - body cavity.
Discharge - Glands with excretory canal at the base of the antennae.
Nervous system - Periopharyngeal ring = supraglottic and subpharyngeal ganglion, abdominal nerve chain. The organ of touch and smell is the base of the short antennae. The organs of vision are two compound eyes.

Arthropods. Arachnids. Spider-cross.

Structure - cephalothorax + abdomen.
Movement - Four pairs of legs, on the belly 3 pairs of arachnoid warts, arachnoid glands for weaving a trapping net.
Nutrition – Mouth = venomous jaws and toe tentacles. Poison - preliminary digestion outside the body. Esophagus - stomach, intestines, anus.
Respiration - In the abdomen, a pair of lung sacs with folds. Two bundles of tracheas are respiratory openings.
Reproduction - Dioecious. Eggs in a cocoon - young spiders
The circulatory system is open. Heart - vessels - body cavity
Isolation - Malpishian vessels
Nervous system - Pairs of ganglia + abdominal chain. The organs of vision are simple eyes.

Arthropods. Insects. Chafer.

Structure - Head + thorax + abdomen (8 segments)
Movement - 3 pairs of legs with hard claws, a pair of wings, a pair of elytra
Nutrition - Mouth \u003d upper lip + 4 jaws + lower lip esophagus, stomach with chitinous teeth, intestines, anus
Respiration - spiracles on the abdominal segments of the trachea all organs and tissues
Reproduction - Females: ovaries, oviduct, seminal receptacle.
Males: 2 testicles, vas deferens, canal, complete metamorphosis.
The circulatory system is open. Heart with valves vessels body cavity.
Isolation - Malpishian vessels in the body cavity, fat body.
Nervous system - Opharyngeal ring + abdominal chain. Brain. 2 compound eyes, olfactory organs - 2 antennae with plates at the end.

Echinoderms.

Structure - Star-shaped, spherical or human-shaped body. Underdeveloped skeleton. Two layers of integument - outer - single layer, inner - fibrous connective tissue with elements of the calcareous skeleton.
Movement - Move slowly with the help of the limbs, musculature is developed.
Nutrition - Mouth opening short esophagus intestine anus.
Respiration - Skin gills, integuments of the body with the participation of the water-vascular system.
Reproduction - Two annular vessels. One surrounds the mouth, the other the anus. There are radial vessels.
Circulatory system - No special. The excretion occurs through the walls of the channels of the water-vascular system.
Isolation - The genitals have a different structure. Most echinoderms are dioecious, but there are hermaphrodites. Development occurs with a series of complex transformations. The larvae swim in the water column; in the process of metamorphosis, the animals acquire radial symmetry.
Nervous system - The nervous system has a radial structure: radial nerve cords depart from the peripharyngeal nerve ring according to the number of people in the body.

multicellular organism- a non-systematic category of living organisms, the body of which consists of many cells, most of which (except for stem cells, such as, for example, cambium cells in plants) are differentiated, that is, they differ in structure and functions.

Differences from coloniality

The oldest metazoans currently known are worm-like organisms up to 12 cm long, discovered in 2010 in the Francevillian B Formation in Gabon. Their age is estimated at 2.1 billion years. About 1.9 billion years old are Grypania spiralis, presumably eukaryotic algae up to 10 mm long, found in sediments of the Negauni iron formation in the Empire Mine. near Marquette Michigan.

In general, multicellularity arose in different evolutionary lines of the organic world several dozen times. For reasons that are not entirely clear, multicellularity is more characteristic of eukaryotes, although the rudiments of multicellularity are also found among prokaryotes. So, in some filamentous cyanobacteria, three types of clearly differentiated cells are found in the filaments, and when the filaments move, they show high level integrity. Multicellular fruiting bodies are characteristic of myxobacteria.

According to modern data, the main prerequisites for the emergence of multicellularity, namely:

• intercellular space filler proteins, varieties of collagen and proteoglycan;
• "molecular glue" or "molecular rivets" for connecting cells;
• signaling substances to ensure interaction between cells,

arose long before the advent of multicellularity, but performed other functions in unicellular organisms. "Molecular rivets" were allegedly used by single-celled predators to capture and hold prey, and signal substances were used to attract potential prey and scare away predators.

The reason for the emergence of multicellular organisms is the evolutionary expediency of increasing the size of individuals, which allows them to more successfully resist predators, as well as absorb and digest a larger prey. However, the conditions for the mass appearance of multicellular organisms appeared only in the Ediacaran period, when the level of oxygen in the atmosphere reached a value that made it possible to cover the increasing energy costs for maintaining multicellularity.

Ontogenesis

The development of many multicellular organisms begins with a single cell (for example, zygotes in animals or spores in the case of gametophytes of higher plants). In this case, most of the cells of a multicellular organism have the same genome. During vegetative propagation, when an organism develops from a multicellular fragment of the mother organism, as a rule, natural cloning also occurs.

In some primitive multicellular organisms (for example, cellular slime molds and myxobacteria), the emergence of multicellular stages of the life cycle occurs in a fundamentally different way - cells, often having very different genotypes, are combined into a single organism.

Evolution

Six hundred million years ago, in the Late Precambrian (Vendian), multicellular organisms flourished. The diversity of the Vendian fauna is surprising: different types and classes of animals appear as if suddenly, but the number of genera and species is small. In the Vendian, a biospheric mechanism of interconnection between unicellular and multicellular organisms arose - the former became a food product for the latter. Abundant in cold waters, plankton using light energy has become food for floating and bottom microorganisms, as well as for multicellular animals. Gradual warming and an increase in oxygen content led to the fact that eukaryotes, including multicellular animals, began to populate the carbonate belt of the planet, displacing cyanobacteria. The beginning of the Paleozoic era brought two mysteries: the disappearance of the Vendian fauna and the "Cambrian explosion" - the appearance of skeletal forms.

The evolution of life in the Phanerozoic (the last 545 million years of earth's history) is a process of complication of the organization of multicellular forms in the plant and animal world.

The line between unicellular and multicellular

There is no clear line between unicellular and multicellular organisms. Many unicellular organisms have the means to create multicellular colonies, while individual cells of some multicellular organisms have the ability to exist independently.

Sponges

Sponges- the simplest of multicellular creatures. A significant part of the body of the sponge is made up of supporting structures based on silicates or calcium carbonate, intertwined with collagen fibers.

At the beginning of the 20th century, Henry van Peters Wilson set up a classic experiment, during which he rubbed the body of a sponge through a fine sieve, dividing it into individual cells. Placed in a glass dish and left to their own devices, these amoeboid cells began to group into shapeless reddish lumps, which then acquired structure and turned into a sponge organism. The restoration of the organism of the sponge also occurred if only a part of the initial number of cells was placed in the cup.

Choanoflagellates

Choanoflagellates- single-celled organisms resembling glasses in shape with flagella in the middle. In their anatomy, they are so similar to the cells of the inner surface of sponges that for some time they were considered degenerate sponges that had lost other types of cells. The erroneousness of this view was established only after deciphering the genomes of both organisms. Choanoflagellates have elements of molecular cascades that provide communication between cells in multicellular organisms, as well as several types of molecular rivets and proteins like collagen and proteoglycan.

A detailed study of choanoflagellates was undertaken by Nicole King of the University of California at Berkeley.

bacteria

Many bacteria, for example, streptococci, have proteins similar to collagen and proteoglycan, but do not form ropes and layers, as in animals. In the walls of bacteria, sugars were found that are part of the proteoglycan complex that forms cartilage.

evolutionary experiments

Yeast

In experiments on the evolution of multicellularity, conducted in 2012 by researchers at the University of Minnesota led by William Ratcliffe and Michael Travisano, baker's yeast served as a model object. These unicellular fungi reproduce by budding; when the mother cell reaches a certain size, a smaller daughter cell separates from it and becomes an independent organism. Daughter cells can also stick together to form clusters. The researchers carried out an artificial selection of cells included in the largest clusters. The selection criterion was the rate of cluster settling to the bottom of the reservoir. The clusters that passed the selection filter were cultivated again, and the largest ones were again selected among them.

Over time, yeast clusters began to behave as single organisms: after the juvenile stage, when cells grew, the reproduction stage followed, during which the cluster was divided into large and small parts. At the same time, the cells located at the border died, allowing the parent and child clusters to disperse.

The experiment took 60 days. The result was individual clusters of yeast cells that lived and died as a single organism.

The researchers themselves do not consider the experiment pure, since yeast in the past had multicellular ancestors, from which they could inherit some of the mechanisms of multicellularity.

Algae Chlamydomonas reinhardtii

In 2013, a group of researchers at the University of Minnesota led by William Ratcliffe, previously known for evolutionary experiments with yeast, conducted similar experiments with single-celled algae. Chlamydomonas reinhardtii. 10 cultures of these organisms were cultivated for 50 generations, centrifuging them from time to time and selecting the largest clusters. After 50 generations, one of the cultures developed multicellular clusters with synchronization of the life cycles of individual cells. After remaining together for several hours, the clusters then diverged into individual cells, which, remaining inside the common mucosa, began to divide and form new clusters.

Unlike yeast, chlamydomonas never had multicellular ancestors and could not inherit the mechanisms of multicellularity from them, however, as a result of artificial selection over several tens of generations, primitive multicellularity also appears in them. However, unlike yeast clusters, which remained a single organism during budding, chlamydomonas clusters divide into separate cells during reproduction. This indicates that the mechanisms of multicellularity could arise independently in different groups of unicellular organisms and vary from case to case.

Artificial multicellular organisms

Currently, there is no information on the creation of truly multicellular artificial organisms, however, experiments are being carried out to create artificial colonies of unicellular organisms.

In 2009, Ravil Fakhrullin from Kazan (Volga Region) State University (Tatarstan, Russia) and Vesselin Paunov from the University of Hull (Yorkshire, UK) obtained new biological structures, called cellosomes, which were artificially created colonies unicellular. A layer of yeast cells was applied to aragonite and calcite crystals using polymer electrolytes as a binder, then the crystals were dissolved with acid and hollow closed cellosomes were obtained that retained the shape of the used template. In the obtained cellosomes, the yeast cells retained their activity and the shape of the template.

Animals.

The lower, simply arranged multicellular ones usually move by bending the body, that is, by crawling. However, most multicellular organisms move with the help of limbs, for example, legs, wings, fins. The limbs are driven by the muscles associated with them. In order for the muscles to move the limbs, they must be attached at one end to something immovable and solid, that is, to the skeleton. The skeleton is the hard frame of an animal's body. For animals that move with the help of limbs, the presence of a skeleton is mandatory. It can be external (for example, the shell of crayfish or insects) or internal (for example, the spine of a fish, bird, human). The skeleton serves not only as a place of attachment of muscles, but also protects internal organs from mechanical damage.

Nutrition and digestion

cm. Nutrition in animals

Breath

Withm. Breathing in animals

Each living cell needs oxygen. It is necessary for obtaining energy inside the cell. Cells are provided with oxygen thanks to the respiratory system. The main organs of the respiratory system of multicellular animals are the lungs or gills. The lungs are used for breathing in the air, and the gills are used to extract oxygen from sea or fresh water. In the lungs and gills, gas exchange occurs: oxygen enters the blood, and carbon dioxide unnecessary for the body is removed from the blood. Some multicellular animals carry out gas exchange through the skin, as well as through the trachea.

Circulation

cm. Circulation in animals

Most multicellular animals have blood - a fluid that bathes the internal organs. The main task of blood is to provide communication between these organs, to supply them with nutrients and remove harmful waste products. Usually, blood flows through special tubes - blood vessels. The movement of blood is facilitated by a kind of muscular pump - the heart. Heart, blood vessels and blood form the circulatory system.

Selection

cm. Excretion (excretion) in animals material from the site

In the process of vital activity of cells and organs of multicellular animals, substances that are unnecessary or even harmful to the body are constantly formed. To remove them, most animals have special organs that form the excretory system. In different animals, these organs are arranged differently, but the nature of their work is similar. By passing body fluids (for example, blood) through them, they extract unnecessary substances from them and bring them out. Usually, the excretory system has its own external excretory opening. Sometimes the excretory opening is combined with the anal and genital: a cloaca is formed.

reproduction

A special organ system of a multicellular animal is associated with reproduction. This is the reproductive system. In female animals, it is represented by the ovaries, which produce female germ cells - haploid eggs. Male sex cells - haploid spermatozoa - are formed in the testes - the genital organs of males. From the fusion of the egg and sperm, a diploid fertilized egg, or egg, is formed, giving rise to a new organism. The very process of fusion of female and male germ cells is called fertilization.

Origin

Questions about this item:

Type of protozoa

Sarcomastigophores

Sarcode

Proteus amoeba (common), dysenteric amoeba, radiolaria

Flagella

Euglena green, Volvox, African trypanosoma, Leishmania, Trichomonas, Giardia hepatic

spores

coccidiae

Malarial Plasmodium

ciliates

Eyelash

Infusoria-balantidia, infusoria-shoe, infusoria-trumpeter

Trichofriosis

Used Books:
1. Biology: a complete guide to preparing for the exam. / G.I. Lerner. - M.: AST: Astrel; Vladimir; VKT, 2009 2. Biology: Animals: textbook. for 7-8 cells. general education institutions. - 7th ed. - M.: Education, 2000. 3. Biology: study guide / A.G. Lebedev. M.: AST: Astrel. 2009. 4. Biology. Full course of secondary school: textbook for schoolchildren and applicants / M.A.Valovaya, N.A.Sokolova, A.A. Kamensky. - M.: Exam, 2002. 5. Biology for university applicants. Intensive course / G.L. Bilich, V.A. Kryzhanovsky. - M.: Onyx Publishing House, 2006.
Used Internet resources: