What are single-celled animals? Who discovered single-celled organisms? Classes and types

The world around us amazes with the diversity of life forms - from invisible viruses and millimeter-thick duckweed to a giant 30-meter blue whale. Representatives of the Animal kingdom alone account for about 1.5 million species. They all have close common features: cannot create organic substances on their own, lead an active lifestyle and have sensory organs that allow them to navigate in space.

The most elementary representatives of animals are the simplest single-celled organisms. Almost 70 thousand varieties of these creatures are known. Like bacteria, they consist of a single cell with a more complex structure. Since the cell has a nucleus, they are classified as eukaryotes. These are entirely self-sufficient organisms that have organelles that perform digestive, regulatory and excretory functions.

Depending on the type of food, there are:

Autotrophs - can independently synthesize the necessary organic substances from inorganics.
Heterotrophs - get necessary substances from other ingested organisms.
Mixotrophs - capable of photosynthesis like plants, while simultaneously exhibiting the ability to absorb organic substances, like animals.

Most protozoan organisms are heterotrophs, choosing either plants or other heterotrophs or their remains as a food source.

The methods of movement of protozoa are quite diverse and serve as the basis for division into classes, which include: flagellates, rhizopods, sporozoans, ciliates, sunfish, radiolarians.

Spreading

Protozoa live in any humid environment: sea, river, swamp water, in puddles after rain and in damp soil, even in moss. And this is an incomplete list of places where protozoa live. They can also be found in cells, blood plasma, and intestines of multicellular organisms.

Under unfavorable conditions (lack of moisture, oxygen or food), these organisms create a cyst around themselves. So, they can remain alive for a long time when low temperatures or at complete absence moisture. In this state, protozoa live from one and a half to 17 years.

Dry cysts are easily picked up by the wind, which carries them over long distances. They attach themselves to birds and insects that carry uninvited passengers into different places. As soon as the cysts enter good conditions life, then their inhabitants will immediately leave the shelter and quickly restore their activity.

The diversity of nutrition and the ability to be transported over long distances have led to the widespread distribution of protozoa across the globe.

Classes and types

Class Flagellates

This class includes animals that move with the help of one or more flagella or whips - thin cytoplasmic processes. Their quantity is 1–8 pieces. In some representatives, the scourge runs along the body, joining it with an outgrowth of the cytoplasm. This outgrowth makes wave movements and serves as an auxiliary organ.

Flagellates live in both freshwater and sea water, much less common in soil. They are important for bodies of water. They reproduce both by longitudinal cell division in half and by the formation of gametes. Prominent representatives are green euglena and volvox.

Euglena green– a typical representative of a solitary form, a resident of freshwater bodies of water. It has a spindle-shaped body shape, constant due to the compaction of the outer layer of protoplasm. Possessing a single flagellum, it floats on top during daylight hours, since during this period the ability for photosynthesis appears. In the dark, the animal becomes heterotrophic and seeks liquid organic food. With a favorable diet, the body stores nutrients, similar in composition to starch.

It reproduces asexually - first the nucleus divides, then the entire body of the protozoan. One daughter cell receives the old flagellum, or there may be no transfer of it at all and a new one grows in both cells. During winter, euglena forms a cyst, discarding the flagellum, and lives in this state until it warms up.

Volvox- a colonial form of flagellates, which includes thousands of individuals. Lives in stagnant fresh water. Colonies are formed in the form of balls reaching 1 mm. In each ball there are many cells similar in structure to euglena. However, unlike it, Volvox cells have 2 flagella. A colony is a gelatinous substance in which cells are immersed in such a way that the moving flagella are exposed. So, the Volvox rolls on the water.

When it's time to reproduce, several cells dive deeper into the substance and divide, forming new young colonies, which then find their way out. And also sexual forms can be formed from macro and microgametes.

Freshwater amoeba lives in puddles and small ponds. It feeds on algae and particles of organic substances, digesting them in digestive vacuoles. Reproduction is asexual - first the cell nucleus divides, and then the cytoplasm. The body is pierced by pores through which pseudopods protrude.

Sunfishes and radiolarians are also classified as rhizopods or sarcodae.

Sunfish live in fresh water bodies. They feed on algae and microorganisms, invertebrate larvae and bacteria. They differ from other protozoa by the skeleton on the cell surface.

Radiolarians) live in the salty water of the seas and oceans of tropical and subtropical zones. They have an internal skeleton, the rays of which serve to strengthen the pseudopodia that capture prey. They reproduce by division. After dying, they accumulate in the form of silt, which is subsequently transformed into minerals.

Sporozoan class

These are the most highly organized protozoa, living in water bodies, soil and in foreign organisms. They move with the help of cilia. There are about 5 thousand species. A prominent representative is the ciliate slipper. The common one lives in stagnant bodies of water and reproduces asexually and sexually. It feeds on bacteria and algae, which in turn serves as food for fish.

The simplest single-celled organisms play both negative and positive roles in nature and human life:

Has a long history. It all started approximately 4 billion years ago. The Earth's atmosphere does not yet have an ozone layer, the concentration of oxygen in the air is very low and nothing can be heard on the surface of the planet except erupting volcanoes and the noise of the wind. Scientists believe that this is what our planet looked like when life began to appear on it. It is very difficult to confirm or refute this. Rocks that could provide more information to people were destroyed a long time ago, thanks to the geological processes of the planet. So, the main stages of the evolution of life on Earth.

Evolution of life on Earth. Unicellular organisms.

Life began with the appearance of the simplest forms of life - single-celled organisms. The first unicellular organisms were prokaryotes. These organisms were the first to appear after the Earth became suitable for life. would not allow even the simplest forms of life to appear on its surface and in the atmosphere. This organism did not require oxygen for its existence. The concentration of oxygen in the atmosphere increased, which led to the appearance eukaryotes. For these organisms, oxygen became the main thing for life; in an environment where the oxygen concentration was low, they did not survive.

The first organisms capable of photosynthesis appeared 1 billion years after the appearance of life. These photosynthetic organisms were anaerobic bacteria . Life gradually began to develop and after the content of nitrogenous organic compounds fell, new living organisms appeared that were able to use nitrogen from the Earth’s atmosphere. Such creatures were blue-green algae. The evolution of single-celled organisms occurred after terrible events in the life of the planet and all stages of evolution were protected under magnetic field land.

Over time, the simplest organisms began to develop and improve their genetic apparatus and develop methods of reproduction. Then, in the life of single-celled organisms, a transition occurred to the division of their generative cells into male and female.

Evolution of life on Earth. Multicellular organisms.

After the emergence of single-celled organisms, more complex forms of life appeared - multicellular organisms. The evolution of life on planet Earth has acquired more complex organisms, characterized by a more complex structure and complex transitional stages of life.

First stage of life - Colonial unicellular stage. The transition from unicellular organisms to multicellular ones, the structure of organisms and the genetic apparatus becomes more complex. This stage is considered the simplest in the life of multicellular organisms.

Second stage of life - Primary differentiated stage. A more complex stage is characterized by the beginning of the principle of “division of labor” between organisms of one colony. At this stage, specialization of body functions occurred at the tissue, organ and systemic organ levels. Thanks to this, a nervous system began to form in simple multicellular organisms. The system did not yet have a nerve center, but there was a coordination center.

Third stage of life - Centrally differentiated stage. During this stage, the morphophysiological structure of organisms becomes more complex. Improvement of this structure occurs through increased tissue specialization. The nutritional, excretory, generative and other systems of multicellular organisms become more complex. U nervous systems a well-defined nerve center appears. Reproduction methods are improving - from external to internal fertilization.

The conclusion of the third stage of life of multicellular organisms is the appearance of man.

Vegetable world.

The evolutionary tree of the simplest eukaryotes was divided into several branches. Multicellular plants and fungi appeared. Some of these plants could float freely on the surface of the water, while others were attached to the bottom.

Psilophytes- plants that first mastered land. Then other groups of terrestrial plants arose: ferns, mosses and others. These plants reproduced by spores, but preferred an aquatic habitat.

Plants reached great diversity during the Carboniferous period. Plants developed and could reach a height of up to 30 meters. During this period, the first gymnosperms appeared. The most widespread species were lycophytes and cordaites. Cordaites resembled coniferous plants in their trunk shape and had long leaves. After this period, the surface of the Earth was diversified with various plants that reached 30 meters in height. Later a large number of Over time, our planet became similar to the one we know now. Now there is a huge variety of animals and plants on the planet, and man has appeared. Man, as a rational being, after he got “on his feet”, devoted his life to studying. Riddles began to interest people, as well as the most important thing - where did man come from and why does he exist. As you know, there are still no answers to these questions, there are only theories that contradict each other.

A mysterious group of microscopic single-celled organisms, considered as a subkingdom of the kingdom Animalia, and sometimes separated into a separate kingdom.

Protozoa unicellular

People first learned about the existence of protozoa in the 7th century from the discovery of a Dutch naturalist; he was the first to observe them in a drop of water, using a microscope he invented.

Over many years of the development of biology, with the advent of electron microscopy and genetics, this group of organisms was increasingly studied and its systematics underwent significant changes.

Today they are increasingly defined as a separate kingdom, since among the simplest unicellular organisms there are organisms that have characteristics different from those of animals. For example, Euglena greena has the ability for photosynthesis, characteristic of plants. Or, for example, the Labyrinthula type - previously classified as mushrooms.

The cell of the simplest unicellular organism has an organization common to eukaryotic cells. But most protozoa also have specific organelles:

contractile vacuoles, which serve to remove excess fluid and maintain the desired osmotic pressure;
various organelles of movement: flagella, cilia and pseudopodia (pseudopods). Pseudopaedes, as the name implies, are not real organelles; they are just protrusions of the cell.

Subkingdom (or kingdom) Protozoa unicellular represented by 7 main types:

Let's look at the types in more detail

Type of Sarcomastigophora

It is divided into three subtypes: Flagellates, Opalines, Sarcodaceae.

Flagellates- a group of organisms, as the name suggests, they are characterized by common organelles of movement - flagella.

Habitats: fresh waters, seas, soils. There are flagellates that live in multicellular organisms. Flagellates are characterized by maintaining a constant body shape, thanks to the pellicle, or shell.

They reproduce mainly asexually: by longitudinal division in two.

Types of nutrition: heterotrophic, autotrophic, mixotrophic.

Let's look at the structure using an example Euglena green.

It is characterized by a mixotrophic (mixed) type of nutrition.
There are special organelles - chlorophyll-containing chromatophores, in which the process of photosynthesis occurs, similar to the photosynthesis of plants.
Due to the ability to photosynthesize, Euglena greena has a light-sensitive organelle - stigma, it is also sometimes called a light-sensitive ocellus.
Removal of excess fluid occurs due to the work of the contractile vacuole.

Some types of trypanosomes cause sleeping sickness. The carrier of African trypanosomiasis (as this disease is scientifically called) is the tsetse fly. This is a blood-sucking insect.

Trypanosomes. They float and cause a dangerous disease.

Giardia. Looks like a pear. Mnemonic rule: Giardia is in the shape of a pear, so to avoid getting infected, you need to wash the pear.

Sarcodidae are protozoans that do not have a constant body shape.

The organelles of movement are pseudopodia (pseudopods). Previously, sarcodaceae and flagellates were classified as two different types, contrasting their organelles of movement: pseudopodia and flagella. But it turned out that at some stages of development, sarcodids have flagella, and some organisms have characteristics of both flagella and sarcodids.

The Sarcodidae subtype includes the following classes: Roothoppers, Radiolarians (Radiants), Solarians.

Roots. This class includes the orders: Amoebas, testate amoebas, foraminifera.

Amoebas feed by phagocytosis. A digestive vacuole forms around the captured piece of food.
They reproduce by dividing in two.
If Euglena green moves towards the light (since it needs it for photosynthesis), then Amoeba vulgaris, on the contrary, moves away from the light. The amoeba also avoids other irritants.

Usually the following experiment is considered: a salt crystal is placed on one side of a drop of water with an amoeba, and one can observe the movement of the amoeba in the opposite direction.

Testate amoebas. They have a similar structure to amoebas, only they have a shell with a hole (mouth) from which pseudopodia “look out.” All testate amoebae are free-living and live in fresh waters. Since the shell cannot split in two, division occurs in a special way: a daughter individual is formed, but it is not immediately separated from the mother. A new shell is formed around the daughter shell. Then the amoebas separate.

Foraminifera are one of the most numerous orders of the simplest unicellular organisms - rhizomes. They are part of marine plankton. Foraminifera, like testate amoebae, have a shell.

Radiolarians very interesting microorganisms that are part of marine plankton. They are characterized by the presence of an internal skeleton. Radiolarians have the largest number of chromosomes of any living creature.

Radiolarians, Foraminifera and testate amoebae die, leaving behind shells and internal skeletons. The accumulation of all this goodness forms deposits of limestone, chalk, quartz and other things.

Solnechniki - a small group of protozoa. They got their name because of the similarity appearance pseudopodium with sun rays. Such pseudopodia are called axopodia.

Type of Ciliates

Characteristics:

constant body shape due to the presence of pellicle;
Some ciliates are characterized by specific protective organelles;
nuclear dualism, i.e. the presence of two nuclei: a polyploid macronucleus ( vegetative nucleus) and diploid micronucleus (generative nucleus). This situation with the nuclei is necessary for the sexual process to occur: . And direct reproduction is only asexual: by longitudinal division in two.
The organelles of movement are cilia. The structure of cilia is the same as that of flagella.

Let's look at the structure using the example of the slipper ciliate. This is a classic, you need to know this.

The slipper ciliate is a predator. Feeds on bacteria. The prey is captured by specialized cilia and directed into the cell mouth, followed by the cell pharynx, then the digestive vacuole. Undigested residues are released through the powder into the external environment.

IN digestive system Ruminant animals are inhabited by symbiotic ciliates that help digest fiber:

Trumpeter ciliate

Suvoiki are ciliates leading an attached lifestyle.

Type Apicomplexa

For example, the protozoa of the genus Plasmodium cause dangerous disease- malaria.

Type Labyrinthula

Protozoa are single-celled, free-living colonial protozoa that live on seaweed. Previously classified as mushrooms. They got this name because the colony really resembles a labyrinth.

Type Ascetosporidia

Type Myxosporidium

Microsporidia type

So, we have looked at the types of kingdom (sub-kingdom) of the simplest single-celled organisms. To consolidate all the knowledge, let's look at the taxonomy:

Despite their small size, the simplest unicellular organisms are of great importance:

protozoa are included in food chains;
form plankton;
act as saprophytes, absorbing decaying remains;
protozoa clean water bodies not only of rotting residues, but also of bacteria;
participate in the formation of soils and deposits of chalk and limestone.
are good indicators of water purity.
autotrophic and mixotrophic protozoa, together with plants, perform a very important mission - replenishing the atmosphere with oxygen.

Organisms whose body contains only one cell are classified as protozoa. They may have different shapes and all kinds of transportation methods. Everyone knows at least one name that the simplest living organism has, but not everyone realizes that it is exactly such a creature. So, what are they, and what types are the most common? And what kind of creatures are these? Like the most complex and coelenterate organisms, unicellular organisms deserve detailed study.

Subkingdom unicellular

Protozoa are the smallest creatures. Their bodies have all the functions necessary for life. Thus, the simplest single-celled organisms are capable of showing irritability, moving and reproducing. Some have a constant body shape, while others constantly change it. The main component of the body is the nucleus surrounded by cytoplasm. It contains several types of organelles. The first are general cellular. These include ribosomes, mitochondria, the Galgi apparatus, and the like. The second ones are special. These include digestive and almost all protozoan single-celled organisms can move without much difficulty. In this they are helped by pseudopods, flagella or cilia. A distinctive feature of organisms is phagocytosis - the ability to capture solid particles and digest them. Some can also carry out photosynthesis.

How do unicellular organisms spread?

Protozoa can be found everywhere - in fresh water, soil or sea. High degree Their survival is ensured by the ability to encyst. This means that under unfavorable conditions the body enters a resting stage, becoming covered with a dense protective shell. The creation of a cyst promotes not only survival, but also proliferation - this way the organism can find itself in a more comfortable environment where it will receive nutrition and the opportunity to reproduce. Protozoan organisms accomplish the latter by dividing into two new cells. Some also have the ability to reproduce sexually, and there are species that combine both.

Amoeba

It is worth listing the most common organisms. Protozoa are often associated with this particular species - amoebas. They do not have a permanent body shape, and use pseudopods for movement. With them, the amoeba captures food - algae, bacteria or other protozoa. Surrounding it with pseudopods, the body forms a digestive vacuole. From it, all substances obtained enter the cytoplasm, and undigested substances are thrown out. The amoeba carries out respiration throughout the body using diffusion. Excess water is removed from the body by the contractile vacuole. The process of reproduction occurs through nuclear division, after which two cells are produced from one cell. Amoebas are freshwater. Protozoa are found in humans and animals, in which case they can lead to a variety of diseases or worsen the general condition.

Euglena green

Another organism common in fresh water bodies is also a protozoa. Euglena green has a spindle-shaped body with a dense outer layer of cytoplasm. The anterior end of the body ends with a long flagellum, with the help of which the body moves. In the cytoplasm there are several oval chromatophores in which chlorophyll is located. This means that in the light, euglena feeds autotrophically - not all organisms can do this. Protozoa navigate with the help of an eye. If the euglena stays in the dark for a long time, the chlorophyll will disappear and the body will switch to a heterotrophic method of nutrition with the absorption of organic substances from the water. Like amoebas, these protozoa reproduce by division and also breathe throughout the body.

Volvox

Among unicellular organisms there are also colonial organisms. A protozoan called volvox lives this way. They have a spherical shape and gelatinous bodies formed by individual members of the colony. Each Volvox has two flagella. The coordinated movement of all cells ensures movement in space. Some of them are capable of reproduction. This is how daughter Volvox colonies arise. The simplest algae known as Chlamydomonas also have the same structure.

Ciliate slipper

This is another common inhabitant of fresh water. The ciliates get their name from the shape of their own cell, which resembles a shoe. The organelles used for movement are called cilia. The body has a constant shape with a dense shell and two cores, small and large. The first is necessary for reproduction, and the second controls all life processes. Ciliates use bacteria, algae and other single-celled organisms as food. Protozoa often create a digestive vacuole; in slippers it is located in a specific place near the mouth opening. To remove undigested residues, powder is present, and excretion is carried out using a contractile vacuole. This is typical for ciliates, but it can also be accompanied by the union of two individuals to exchange nuclear material. This process is called conjugation. Among all freshwater protozoa, the slipper ciliate is the most complex in its structure.

Ministry of Higher and Secondary Education of the Russian Federation

Moscow State University Food Production

Institute of Economics and Entrepreneurship

Abstract on the topic:

Unicellular organisms are the most simple shapes life

Completed by a student

Groups 06 E-5

Pantyukhina O.S.

Checked by Prof. Butova S.V.

Moscow 2006

1. Introduction. . . . . . . . . . . .3

2. Protozoa. . . . . . . . . . . 4-5

3. Four main classes of protozoa. . . . .5-7

4. Reproduction is the basis of life. . . . . . . . . 8-9

5. The great role of small protozoa. . . . . 9-11

6. Conclusion. . . . . . . . . . . . .12

7. List of references. . . . . . .13

Introduction

Single-celled organisms perform the same functions as multicellular organisms: they feed, move and reproduce. Their cells should be<<мастером на все руки>> to do all this that other animals do have special organs. Therefore, single-celled animals are so different from the rest that they are separated into separate subkingdoms of protozoa.

Protozoa

The body of a protozoan consists of only one cell. The body shape of protozoa is varied. It can be permanent, have radial, bilateral symmetry (flagellates, ciliates) or not have a permanent shape at all (amoeba). The body sizes of protozoa are usually small - from 2-4 microns to 1.5 mm, although some large individuals reach 5 mm in length, and fossil shell rhizomes had a diameter of 3 cm or more.

The body of protozoa consists of cytoplasm and nucleus. The cytoplasm is limited by the outer cytoplasmic membrane; it contains organelles - mitochondria, ribosomes, endoplasmic reticulum, and Golgi apparatus. The simplest have one or several nuclei. The form of nuclear division is mitosis. There is also the sexual process. It involves the formation of a zygote. Organelles of movement of protozoa are flagella, cilia, pseudopods; or there are none at all. Most protozoa, like all other representatives of the animal kingdom, are heterotrophic. However, among them there are also autotrophic ones.

The ability of protozoa to tolerate unfavorable conditions environment- consists of the ability incis tidy up , i.e. form cyst . When a cyst is formed, the movement organelles disappear, the volume of the animal decreases, it acquires a rounded shape, and the cell is covered with a dense membrane. The animal goes into a state of rest and, when favorable conditions occur, returns to active life.

The reproduction of protozoa is very diverse, from simple division (asexual reproduction) to a rather complex sexual process - conjugation and copulation.

The habitat of protozoa is varied - the sea, fresh water, moist soil.

Four main classes of protozoa

1 – flagella (Flagellata, or Mastigophora);

2 – sarcodaceae (Sarcodina, or Rhizopoda);

3 – sporozoa (Sporozoa);

4 – ciliates (Infusoria, or Ciliata).

1. About 1000 species, mainly with an elongated oval or pear-shaped body, make up the class of flagellates ( Flagellatata or Mastigophora). The organelles of movement are flagella, of which different representatives of the class can have from 1 to 8 or more. Flagellum- a thin cytoplasmic outgrowth consisting of the finest fibrils. Its base is attached to basal body or kinetoplast . Flagellates move forward with a cord, creating vortex whirlpools with their movement and, as it were, “screwing in” the animal

into the surrounding liquid environment.

Way nutrition : Flagellates are divided into those that have chlorophyll and feed autotrophically, and those that do not have chlorophyll and feed, like other animals, heterotrophically. Heterotrophs on the front side of the body have a special depression - cytostome , through which, when the flagellum moves, food is driven into the digestive vacuole. A number of flagellate forms feed osmotically, absorbing dissolved organic substances from the environment over the entire surface of the body.

Methods reproduction : Reproduction most often occurs by dividing in two: usually one individual gives rise to two daughters. Sometimes reproduction occurs very quickly, with the formation of countless individuals (nightlight).

2. Representatives of the class of sarcodes, or rhizomes ( Sarcodina or Rhizopoda), move with the help of pseudopods - pseudo-similarities.

The class includes a variety of aquatic unicellular organisms: amoebae, sunfish, and rayfish. Among amoebas, in addition to forms that do not have a skeleton or shell, there are species that have a house.

Most sarcodae are inhabitants of the seas; there are also freshwater ones that live in the soil.

Sarcodidae are characterized by an inconsistent body shape. Breathing is carried out over its entire surface. Nutrition is heterotrophic. Reproduction is asexual; there is also a sexual process.

in vertebrates - mammals, fish, birds. Coccidia toxoplasmosis causes the human disease toxoplasmosis. It can be contracted from any member of the cat family.

4. Representatives of the ciliate class ( Infusorians or Ciliata) have organelles of movement - cilia, usually in large numbers. So, at the shoe ( Paramecium caudatum) the number of cilia is more than 2000. Cilia (like flagella) are special complex cytoplasmic projections. The body of ciliates is covered with a membrane permeated with tiny pores through which cilia emerge.

The type of ciliates includes the most highly organized protozoa. They are the pinnacle of the achievements made by evolution in this sub-realm. Ciliates lead a free-swimming or attached lifestyle. They live like

All ciliates have at least two nuclei. The large core regulates all life processes. The small nucleus plays a major role in the sexual process.

Ciliates reproduce by division (across the axis of the body). In addition, they periodically undergo sexual intercourse - conjugation . Ciliate “ shoe” is shared daily, some others - several times a day, and “ trumpeter" - once

in a few days.

Food enters the animal’s body through the cellular “mouth”, where it is driven by the movement of the cilia; are formed at the bottom of the pharynx digestive vacuoles . Undigested residues are excreted.

Many ciliates feed only on bacteria, while others are predators. For example, the most dangerous enemies “ shoes” – didinia ciliates. They are smaller than her, but, attacking in twos or fours, they surround her from all sides.” shoe” and kill her by throwing a special “ stick" Some didinia eat up to 12 “shoes” per day.

Organelles of secretion of ciliates are two contractile vacuoles; in 30 minutes they remove from the ciliate an amount of water equal to the volume of its entire body.

Reproduction is the basis of life

Asexual reproduction - cell division : Most often found in protozoa asexual reproduction. It occurs through cell division. First the nucleus divides. The development program of an organism is located in the cell nucleus in the form of a set of DNA molecules. Therefore, even before cell division, the nucleus doubles so that each of the daughter cells receives its own copy of the hereditary text. Then the cell divides into two approximately equal parts. Each of the descendants receives only half of the cytoplasm with organelles, but a complete copy of the maternal DNA and, using the instructions, builds itself into a whole cell.

Asexual reproduction is simple and quick way increase the number of your descendants. This method of reproduction is essentially no different from cell division during the growth of the body of a multicellular organism. The whole difference is that the daughter cells of unicellular organisms eventually disperse as independent organisms.

During cell division, the parent individual does not disappear, but simply turns into two twin individuals. This means that when asexual reproduction an organism can live forever, repeating itself exactly in its descendants. Indeed, scientists managed to preserve a culture of protozoa with the same hereditary properties for several decades. But, firstly, in nature the number of animals is strictly limited by food supplies, so that only a few descendants survive. Secondly, absolutely identical organisms may soon turn out to be equally unadapted to changing conditions and all will die. The sexual process helps to avoid this catastrophe.