Multicellular organism. Multicellular animals. General characteristics of the main classes

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

Multicellular organisms can be divided into two large groups. Invertebrate animals 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 internal organs– sponges and coelenterates. It also includes flat, round, annelids, arthropods, mollusks and echinoderms, bilaterally symmetrical and radial three-layered organisms, which in addition to ecto- and endoderm also have 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: notochord or vertebral column.

Multicellular animals

Coelenterates. Freshwater hydra.

Structure – Radial symmetry, ectoderm, endoderm, sole, tentacles.
Movement – ​​Contraction of skin-muscle cells, attachment of the sole to the substrate.
Food – Tentacles oral cavity intestine cavity with digestive cells. Predator. Kills stinging cells with poison.
Breathing – Oxygen dissolved in water penetrates the entire surface of the body.
Reproduction - Hermaphrodites. Sexual: egg cells + sperm = egg. Asexual: budding.
Circulatory system - No.
Elimination - Food remains are removed through the mouth.
Nervous system – Nerve plexus from nerve cells.

Flatworms. White planaria.

Roundworms. Human roundworm.

Annelids. Earthworm.

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

Soft-bodied. Shellfish. Common pondweed.

Structure – Soft body enclosed in a helical shell = torso + leg.
Movement – ​​Muscular leg.
Nutrition – Mouth, pharynx, tongue with teeth = grater, stomach, intestines, liver, anus.
Breathing - Breathing hole. Lung.
Reproduction - Hermaphrodites. Cross fertilization.
The circulatory system is not closed. Light heart vessels body cavity.
Excretion – Kidney.
Nervous system – Peripharyngeal cluster of nerve nodes.

Arthropods. Crustaceans. Crayfish.

Structure – + belly.
Movement – ​​Four pairs of walking legs, 5 pairs of ventral legs + caudal fin for swimming.
Nutrition - jaw mouth, pharynx, esophagus, stomach, section with chitinous teeth, filtering apparatus, intestines, food. gland - anus.
Breathing - gills.
Reproduction – Dioecious. Eggs on abdomen legs before hatching. During growth, chitin shedding is characteristic. There is a nauplius larval stage.
Circulatory system – Unclosed. Heart – blood vessels – body cavity.
Excretion - Glands with an excretory canal at the base of the antennae.
Nervous system – Periopharyngeal ring = suprapharyngeal and subpharyngeal node, ventral nerve cord. The organ of touch and smell is the base of the short antennae. The organs of vision are two compound eyes.

Arthropods. Arachnids. Cross spider.

Structure – Cephalothorax + abdomen.
Movement - Four pairs of legs, 3 pairs of arachnoid warts on the belly, arachnoid glands for weaving a fishing net.
Nutrition – Mouth = jaws with venom and claws. Poison is pre-digestion outside the body. Esophagus – stomach, intestines, anus.
Respiration - In the abdomen there are a pair of pulmonary sacs with folds. Two bundles of trachea respiratory openings.
Reproduction – Dioecious. Eggs in a cocoon - young spiders
Circulatory system – Unclosed. Heart – blood vessels – body cavity
Excretion – Malpischian vessels
Nervous system – Pairs of ganglia + ventral chain. The organs of vision are simple eyes.

Arthropods. Insects. Chafer.

Structure – Head + chest + abdomen (8 segments)
Movement – ​​3 pairs of legs with hard claws, a pair of wings, a pair of elytra
Food – Mouth = upper lip+ 4 jaws + lower lip esophagus, stomach with chitinous teeth, intestines, anus
Breathing – Spiracles on the abdominal segments of the trachea, all organs and tissues
Reproduction – Females: ovaries, oviducts, spermatic receptacles.
Males: 2 testes, vas deferens, canal, complete metamorphosis.
The circulatory system is not closed. Heart with valves, vessels, body cavity.
Excretion – Malpish vessels in the body cavity, fat body.
Nervous system – Circumpharyngeal ring + ventral chain. Brain. 2 compound eyes, olfactory organs - 2 antennae with plates at the end.

Echinoderms.

Structure – Star-shaped, spherical or human-shaped body shape. Underdeveloped skeleton. Two layers of integument - outer - single layer, inner - fibrous connective tissue with elements of a calcareous skeleton.
Movement – ​​Move slowly with the help of limbs, muscles are developed.
Nutrition - Mouth opening, short esophagus, intestine, anus.
Respiration - Skin gills, body coverings with the participation of the water-vascular system.
Reproduction – Two ring vessels. One surrounds the mouth, the other the anus. There are radial vessels.
Circulatory system – No special ones. Excretion occurs through the walls of the canals of the water-vascular system.
Discharge – The genitals have different structure. Most echinoderms are dioecious, but some are hermaphrodites. Development happens at speed complex transformations. The larvae swim in the water column; during the process of metamorphosis, the animals acquire radial symmetry.
Nervous system - The nervous system has a radial structure: radial nerve cords extend from the peripharyngeal nerve ring according to the number of people in the body.

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 increasing the diversity of living beings and their development. Multicellularity made it possible for the specialization of living cells within one organism, including the emergence of individual tissues and organs. The first multicellular animals probably appeared in the bottom layers of the world's oceans at the end of the Proterozoic.
The signs of a multicellular organism are that its cells must be aggregated, division of functions and the establishment of stable specific contacts between them are obligatory. A multicellular organism is a rigid colony of cells in which their position remains fixed throughout life. During the process of biological evolution, similar cells in the body of multicellular organisms specialized to perform 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, which floated in the thickness sea ​​water due to the beating of surface cells - cilia of the kinoblast.
The phagocytella fed by capturing food particles suspended in the medium and digesting them with the internal cell mass (phagocytoblast). Perhaps it was from the kinoblast and phagocytoblast that in the process of evolutionary development all the diversity 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 diverse living conditions 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 crawling 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 is capable of reproducing sexually. The structural features and behavior of Trichoplax, crawling along 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 paddle plates formed by fused cilia. In the Proterozoic, they switched from swimming to crawling along the bottom, their body was therefore flattened, the head section, the locomotor system in the form of a skin-muscular sac, respiratory organs were formed, the excretory and circulatory system. Linnaeus, creator of the first scientific system organic world, paid very little attention to ctenophores, mentioning one species of ctenophores in his “System of Nature”. In 1829, the world's first big job dedicated to jellyfish. 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 ctenophora. This name has been retained by them to this day” (“The Life of Animals,” 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 waters, 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 grows to some substrate - stones, algae, the surface of the body of other animals.

The first multicellular organisms lived in the bottom layers of the 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) that covered the bottom silt. There were practically no predators then. Some multicellular organisms passed through the nutrient-filled upper layers of sea mud or absorbed the living bacteria and algae that lived in it.
Flat and annelid worms slowly swam above the bottom or crawled among the sediments, and tube worms lay among the 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 the shallow waters - Vendothenia, which reached a length of about one meter and looked like seaweed.
By the end of the Proterozoic era, most living creatures on our planet were already represented by multicellular forms. Their vital activity was preserved in the form of imprints and casts on the once soft silt. In the deposits of that period one can observe traces of crawling, subsidence, and dug burrows.
The end of the Proterozoic era was marked by an explosion in the diversity of multicellular organisms and the appearance of animals, the existence of which was then closely connected with the sea. The huge number of remains of multicellular animals in layers aged 650-700 million years even served as the reason for the identification of a special period in the Proterozoic, called the Vendian. It lasted approximately 110 million years and was characterized, in comparison with other eras, by the achievement of a significant diversity of multicellular animals.
The emergence of multicellularity contributed to a further increase in the diversity of living organisms. It has led to an increase in the ability of organisms to create a supply of nutrients in their bodies 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, forming a new shell of the earth. We can say that in the Proterozoic, life on the planet became the most important geological factor.

Multicellular organism- an extra-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 most ancient multicellular organisms currently known are worm-like organisms up to 12 cm long, discovered in 2010 in the sediments of the Francevillian B formation in Gabon. Their age is estimated at 2.1 billion years. The approximately 1.9-billion-year-old organism Grypania spiralis, a presumably eukaryotic algae up to 10 mm long, was discovered in sediments of the Negaunee Ferrous Formation at the Empire Mine. near the city of Marquette (English)Russian, pcs. Michigan.

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

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

• intercellular space filler proteins, types 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 presumably used by single-celled predators to capture and hold prey, and signaling substances were used to attract potential prey and scare away predators.

The reason for the appearance of multicellular organisms is considered to be the evolutionary expediency of enlarging the size of individuals, which allows them to more successfully resist predators, as well as absorb and digest larger prey. However, conditions for the mass appearance of multicellular organisms appeared only in the Ediacaran period, when the level of oxygen in the atmosphere reached a level that made it possible to cover the increasing energy costs of 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 cells of a multicellular organism have the same genome. During vegetative propagation, when an organism develops from a multicellular fragment of the maternal 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 began to flourish. 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 biosphere mechanism of interaction between unicellular and multicellular organisms arose - the former became a food product for the latter. Plankton, abundant in cold waters, using light energy, became 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 the process of increasing complexity in 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 sponge body consists of supporting structures based on silicates or calcium carbonate, intertwined with collagen fibers.

At the beginning of the 20th century, Henry van Peters Wilson conducted a classic experiment in which he rubbed the body of a sponge through a fine sieve, separating it into individual cells. Placed in glass dishes 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. Restoration of the sponge organism also occurred if the cup contained only a portion of the original number of cells.

Choanoflagellates

Choanoflagellates- single-celled organisms shaped like glasses 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 fallacy of this view was established only after deciphering the genomes of both organisms. Choanoflagellates have elements of molecular cascades that provide cell-to-cell interactions in metazoans, as well as several types of molecular rivets and proteins like collagen and proteoglycan.

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

Bacteria

In many bacteria, for example, steptococci, proteins are found that are similar to collagen and proteoglycan, but do not form ropes and sheets, as in animals. Sugars that are part of the proteoglycan complex that forms cartilage have been found in the walls of bacteria.

Evolutionary experiments

Yeast

Experiments on the evolution of multicellularity conducted in 2012 by University of Minnesota researchers led by William Ratcliffe and Michael Travisano used baker's yeast as a model object. These single-celled 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 may 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 at which clusters settled to the bottom of the tank. The clusters that passed the selection filter were again cultivated, and the largest ones were again selected.

Over time, the yeast clusters began to behave like single organisms: after the juvenile stage, when cell growth occurred, there followed a reproduction stage, during which the cluster was divided into large and small parts. In this case, the cells located at the border died, allowing the parent and daughter 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 have inherited some 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, multicellular clusters with synchronization developed in one of the cultures life cycles individual cells. Remaining together for several hours, the clusters then dispersed into individual cells, which, remaining inside the common mucous membrane, 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 dozen generations, primitive multicellularity appears in them. However, unlike yeast clusters, which remained a single organism during the budding process, chlamydomonas clusters are divided into separate cells during reproduction. This indicates that the mechanisms of multicellularity could arise independently in various groups unicellular and vary from case to case.

Artificial multicellular organisms

Currently, there is no information about the creation of truly multicellular artificial organisms, but experiments are being conducted to create artificial colonies of unicellular ones.

In 2009, Ravil Fakhrullin from Kazan (Volga) state university(Tatarstan, Russia) and Vesselin Paunov from the University of Hull (Yorkshire, UK) obtained new biological structures called “cellosomes” and representing artificially created colonies of single-celled organisms. 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 template used. In the resulting cellosomes, the yeast cells retained their activity and template shape.

Animals.

Lower, simply structured multicellular organisms usually move by bending the body, that is, by crawling. However, most multicellular organisms move using limbs, such as legs, wings, and fins. The limbs are moved by the muscles associated with them. In order for muscles to move limbs, they must be attached at one end to something stationary 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, or human). The skeleton serves not only as a place for muscle attachment, 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 to obtain energy inside the cell. Cells are provided with oxygen through the respiratory system. Main bodies respiratory system multicellular animals have lungs or gills. The lungs are used for breathing in the air, and the gills are used to extract oxygen from the sea or fresh water. Gas exchange occurs in the lungs and gills: oxygen penetrates into the blood, and what is not needed by the body is removed from the blood. carbon dioxide. Some multicellular animals carry out gas exchange through skin, and also through the trachea.

Circulation

cm. Blood circulation in animals

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

Selection

cm. Excretion (secretion) in animals Material from the site

During the life processes 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 bodies, forming the excretory system. In different animals, these organs are structured differently, but the nature of their work is similar. By passing body fluids (for example, blood) through themselves, they extract unnecessary substances from them and remove them out. Typically, the excretory system has its own external excretory opening. Sometimes the excretory opening is combined with the anal and genital openings: a cloaca is formed.

Reproduction

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

Origin

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