Morphology of multicellular animals

The body of multicellular organisms consists of a collection of many cells, groups of which specialize in performing certain functions, forming tissues. Tissue complexes form the highest category - organs. The functional activity of organs constitutes an organ system, such as the musculoskeletal system. A complex of systems connected by a single function forms an integral organism of a multicellular animal. With such specialization, individual cells of a multicellular organism cannot exist separately and outside the body.

An idea of ​​the structural features and distribution of functions between cells in a multicellular organism is provided by such tissues as epithelial, muscle, connective, and nervous.

In animals, cells are grouped in such a way that the body can move freely to obtain food or carry out other functions, i.e. they are interconnected into effectively interacting systems.

The number of cells varies among different multicellular organisms. So, for example, in primitive invertebrates $10^2 -10^4$, in highly organized vertebrates the amount ranges from $10^(15)$ to $10^(17)$. The average cell mass weighs about $10^(-8)-10^(-9)$ g.

Cells are characterized by two vital systems:

• A system associated with the reproduction, development, and growth of a cell. Such a cell includes structures that will ensure DNA replication, RNA and protein synthesis.
• The energy supply system for the synthesis of substances and other types of physiological work of the cell.

Both systems are closely interconnected. In addition, cell elements of different origins are characterized by similarity at different levels: atomic - carbon, hydrogen, oxygen, etc., molecular - nucleic acids, proteins, carbohydrates, etc., supramolecular - membrane structures and organelles of cells.

Cells are also characterized by chemical processes: respiration, consumption and transformation of energy, synthesis of macromolecules. All chemical reactions in cells are well ordered and inextricably linked to molecular structures.

Evolutionary features in the morphology of a multicellular organism

Multicellular organisms represent a leap in evolution, since they have greater advantages in organization compared to unicellular organisms.

The main evolutionary features of the structure of multicellular organisms are:

• Multicellularity;
• Body symmetry;
• Cell differentiation;
• The appearance of cells specialized for reproduction.

The prosperity of a group of multicellular animals is directly related to the complication of structure and physiological functions. As a result, the increase in body size of a multicellular organism led to the development of its digestive canal. Developing over time, the musculoskeletal system formed the maintenance of a certain body shape, as well as the protection and support of internal organs.

The large body size of animals led to the emergence of intratransport circulatory systems. Such systems supply nutrients removed from the surface of the body, and also remove metabolic end products from the body. Blood became the main transport system.

Body symmetry

Based on the type of body symmetry, the following groups are distinguished:

1. Radiant, or radially symmetrical;
2. Bilaterally symmetrical.

Radial symmetry is characteristic of animals with a sedentary lifestyle. The organs of such animals are located around the main axis, and pass through the mouth to the oppositely attached pole. Such animals include the Sponge type, the Coelenterate type and the Echinoderm type.

Bilaterally symmetrical animals are mobile. The body is located on one plane, on both sides of which paired organs are located. The body is divided into left and right sides, dorsal and ventral sides, as well as the anterior and posterior ends of the body. Bilaterally symmetrical animals include all other types of animals.

Body cavity

Definition 1

Body cavity- space containing internal organs.

There are primary, secondary and mixed body cavities. The primary body cavity is the presence of a remnant blastula in which mesoderm derivatives develop. Such a cavity is typical for three-layered, low-organized animals such as roundworms.

The secondary body cavity, or coelom, is lined with epithelium from the mesoderm. Such a cavity is characteristic of the types Annelids, Mollusks and Chordata.

With a mixed body cavity, the rudiments of a secondary cavity develop, but this process does not proceed to the end of the formation of the coelom, and eventually merge with the primary body cavity. This type of symmetry is characteristic of the Arthropod phylum.

Note 1

Flatworms generally lack a body cavity; they have a muscular sac filled with parenchyma cells.

Functions of the body cavity:

1. Free arrangement of organs;
2. Support;
3. Transport of nutrients;
4. Sexual.

The living world is filled with a dizzying array of living creatures. Most organisms consist of only one cell and are not visible to the naked eye. Many of them become visible only under a microscope. Others, such as the rabbit, elephant or pine tree, as well as humans, are made of many cells, and these multicellular organisms also inhabit our entire world in huge numbers.

Building Blocks of Life

The structural and functional units of all living organisms are cells. They are also called the building blocks of life. All living organisms are made up of cells. These structural units were discovered by Robert Hooke back in 1665. There are about one hundred trillion cells in the human body. The size of one is about ten micrometers. The cell contains cellular organelles that control its activity.

There are unicellular and multicellular organisms. The former consist of a single cell, such as bacteria, while the latter include plants and animals. The number of cells depends on the type. Most plant cells and animal cells are between one and one hundred micrometers in size, so they are visible under a microscope.

Unicellular organisms

These tiny creatures are made up of a single cell. Amoebas and ciliates are the oldest forms of life, existing around 3.8 million years ago. Bacteria, archaea, protozoa, some algae and fungi are the main groups of single-celled organisms. There are two main categories: prokaryotes and eukaryotes. They also vary in size.

The smallest are about three hundred nanometers, and some can reach sizes of up to twenty centimeters. Such organisms usually have cilia and flagella that help them move. They have a simple body with basic functions. Reproduction can be either asexual or sexual. Nutrition is usually carried out through the process of phagocytosis, where food particles are absorbed and stored in special vacuoles that are present in the body.

Multicellular organisms

Living things made up of more than one cell are called multicellular. They are made up of units that are identified and attached to each other to form complex multicellular organisms. Most of them are visible to the naked eye. Organisms such as plants, some animals and algae emerge from a single cell and grow into multi-chain organizations. Both categories of living things, prokaryotes and eukaryotes, can exhibit multicellularity.

Mechanisms of multicellularity

There are three theories to discuss the mechanisms by which multicellularity could arise:

• The symbiotic theory states that the first cell of a multicellular organism arose due to the symbiosis of different species of unicellular organisms, each of which performed different functions.
• The syncytial theory states that a multicellular organism could not have evolved from single-celled creatures with multiple nuclei. Protozoa such as ciliates and slimy fungi have multiple nuclei, thereby supporting this theory.
• Colonial theory states that the symbiosis of many organisms of the same species leads to the evolution of a multicellular organism. It was proposed by Haeckel in 1874. Most multicellular formations occur due to the fact that cells cannot separate after the process of division. Examples that support this theory are the algae Volvox and Eudorina.

Benefits of Multicellularity

Which organisms - multicellular or unicellular - have more advantages? This question is quite difficult to answer. The multicellularity of an organism allows it to exceed size limits and increases the complexity of the organism, allowing the differentiation of numerous cell lineages. Reproduction occurs primarily sexually. The anatomy of multicellular organisms and the processes that occur in them are quite complex due to the presence of different types of cells that control their vital functions. Let's take division for example. This process must be precise and coordinated to prevent abnormal growth and development of a multicellular organism.

Examples of multicellular organisms

As mentioned above, multicellular organisms come in two types: prokaryotes and eukaryotes. The first category includes mainly bacteria. Some cyanobacteria, such as Chara or Spirogyra, are also multicellular prokaryotes, sometimes also called colonial. Most eukaryotic organisms are also composed of many units. They have a well-developed body structure and have specialized organs to perform specific functions. Most well-developed plants and animals are multicellular. Examples include almost all types of gymnosperms and angiosperms. Almost all animals are multicellular eukaryotes.

Features and characteristics of multicellular organisms

There are many signs by which you can easily determine whether an organism is multicellular or not. Among them are the following:

• They have a rather complex body organization.
• Specialized functions are performed by various cells, tissues, organs, or organ systems.
• The division of labor in the body can be at the cellular level, at the level of tissues, organs and the level of organ systems.
• These are mainly eukaryotes.
• Injury or death of some cells does not globally affect the body: the affected cells will be replaced.
• Thanks to multicellularity, an organism can reach large sizes.
• Compared to single-celled organisms, they have a longer life cycle.
• The main type of reproduction is sexual.
• Cell differentiation is characteristic only of multicellular organisms.

How do multicellular organisms grow?

All creatures, from small plants and insects to large elephants, giraffes and even humans, begin their journey as single simple cells called fertilized eggs. To grow into a large adult organism, they go through several specific developmental stages. After fertilization of the egg, the process of multicellular development begins. Throughout the entire path, individual cells grow and divide multiple times. This replication ultimately creates the final product, which is a complex, fully formed living entity.

Cell division creates a series of complex patterns determined by genomes that are virtually identical in all cells. This diversity results in gene expression that controls the four stages of cell and embryo development: proliferation, specialization, interaction, and movement. The first involves the replication of many cells from a single source, the second has to do with the creation of cells with isolated, defined characteristics, the third involves the dissemination of information between cells, and the fourth is responsible for the placement of cells throughout the body to form organs, tissues, bones and others. physical characteristics of developed organisms.

A few words about classification

Among multicellular creatures, two large groups are distinguished:

• invertebrates (sponges, annelids, arthropods, mollusks and others);
• Chordates (all animals that have an axial skeleton).

An important stage in the entire history of the planet was the emergence of multicellularity in the process of evolutionary development. This served as a powerful impetus for increasing biological diversity and its further development. The main feature of a multicellular organism is a clear distribution of cellular functions, responsibilities, as well as the establishment and establishment of stable and strong contacts between them. In other words, it is a large colony of cells that is able to maintain a fixed position throughout the entire life cycle of a living creature.

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 the 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.
Nutrition - Tentacles, mouth, intestines, 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 of 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 of the skin-muscle sac, 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. Lung 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
Nutrition – 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 - the outer one is single-layer, the inner one is 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.
Discretion – The genital organs have different structures. Most echinoderms are dioecious, but some are hermaphrodites. Development occurs through a series of complex transformations. The larvae swim in the water column; during 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.

All living organisms are divided into subkingdoms of multicellular and unicellular creatures. The latter are one cell and belong to the simplest, while plants and animals are those structures in which a more complex organization has developed over the centuries. The number of cells varies depending on the variety to which the individual belongs. Most are so small that they can only be seen under a microscope. Cells appeared on Earth approximately 3.5 billion years ago.

Nowadays, all processes occurring with living organisms are studied by biology. This science deals with the subkingdom of multicellular and unicellular organisms.

Unicellular organisms

Unicellularity is determined by the presence in the body of a single cell that performs all vital functions. The well-known amoeba and slipper ciliates are primitive and, at the same time, the most ancient forms of life that are representatives of this species. They were the first living creatures to live on Earth. This also includes groups such as Sporozoans, Sarcodaceae and bacteria. They are all small and mostly invisible to the naked eye. They are usually divided into two general categories: prokaryotic and eukaryotic.

Prokaryotes are represented by protozoa or some species of fungi. Some of them live in colonies, where all individuals are the same. The entire process of life is carried out in each individual cell in order for it to survive.

Prokaryotic organisms do not have membrane-bound nuclei and cellular organelles. These are usually bacteria and cyanobacteria, such as E. coli, salmonella, nostoca, etc.

All representatives of these groups vary in size. The smallest bacterium is only 300 nanometers long. Unicellular organisms usually have special flagella or cilia that are involved in their movement. They have a simple body with pronounced basic features. Nutrition, as a rule, occurs during the process of absorption (phagocytosis) of food and is stored in special cell organelles.

Single-celled organisms have dominated as a form of life on Earth for billions of years. However, evolution from the simplest to the more complex individuals changed the entire landscape, as it led to the emergence of biologically evolved connections. In addition, the emergence of new species has created new environments with diverse ecological interactions.

Multicellular organisms

The main characteristic of the metazoan subkingdom is the presence of a large number of cells in one individual. They are fastened together, thereby creating a completely new organization, which consists of many derivative parts. The majority of them can be seen without any special equipment. Plants, fish, birds and animals emerge from a single cell. All creatures included in the subkingdom of multicellular organisms regenerate new individuals from embryos that are formed from two opposite gametes.

Any part of an individual or a whole organism, which is determined by a large number of components, is a complex, highly developed structure. In the subkingdom of multicellular organisms, the classification clearly separates the functions in which each of the individual particles performs its task. They engage in vital processes, thereby supporting the existence of the entire organism.

The subkingdom Multicellular in Latin sounds like Metazoa. To form a complex organism, cells must be identified and joined to others. Only a dozen protozoa can be seen individually with the naked eye. The remaining nearly two million visible individuals are multicellular.

Pluricellular animals are created by the union of individuals through the formation of colonies, filaments, or aggregation. Pluricellular organisms developed independently, like Volvox and some flagellated green algae.

A sign of the subkingdom metazoans, that is, its early primitive species, was the absence of bones, shells and other hard parts of the body. Therefore, no traces of them have survived to this day. The exception is sponges, which still live in the seas and oceans. Perhaps their remains are found in some ancient rocks, such as Grypania spiralis, whose fossils were found in the oldest layers of black shale dating back to the early Proterozoic era.

In the table below, the multicellular subkingdom is presented in all its diversity.

Complex relationships arose as a result of the evolution of protozoa and the emergence of the ability of cells to divide into groups and organize tissues and organs. There are many theories explaining the mechanisms by which single-celled organisms may have evolved.

Theories of origin

Today, there are three main theories of the origin of the multicellular subkingdom. A brief summary of the syncytial theory, without going into details, can be described in a few words. Its essence is that a primitive organism, which had several nuclei in its cells, could eventually separate each of them with an internal membrane. For example, several nuclei contain mold fungus, as well as slipper ciliates, which confirm this theory. However, having several nuclei is not enough for science. To confirm the theory of their multiplicity, it is necessary to demonstrate the transformation of the simplest eukaryote into a well-developed animal.

Colony theory says that symbiosis, consisting of different organisms of the same species, led to their change and the emergence of more advanced creatures. Haeckel was the first scientist to introduce this theory in 1874. The complexity of the organization arises because cells stay together rather than separate as they divide. Examples of this theory can be seen in such protozoan multicellular organisms as green algae called Eudorina or Volvaxa. They form colonies of up to 50,000 cells, depending on the species.

Colony theory proposes the fusion of different organisms of the same species. The advantage of this theory is that during times of food shortage, amoebas have been observed to group into a colony, which moves as one unit to a new location. Some of these amoebas are slightly different from each other.

However, the problem with this theory is that it is unknown how the DNA of different individuals can be included in a single genome.

For example, mitochondria and chloroplasts can be endosymbionts (organisms within a body). This happens extremely rarely, and even then the genomes of endosymbionts retain differences among themselves. They separately synchronize their DNA during mitosis of host species.

The two or three symbiotic individuals that make up a lichen, although dependent on each other for survival, must reproduce separately and then recombine, again creating a single organism.

Other theories that also consider the emergence of the metazoan subkingdom:

• GK-PID theory. About 800 million years ago, a small genetic change in a single molecule called GK-PID may have allowed individuals to move from a single cell to a more complex structure.
• The role of viruses. It has recently been recognized that genes borrowed from viruses play a crucial role in the division of tissues, organs, and even in sexual reproduction, during the fusion of egg and sperm. The first protein, syncytin-1, was found to be transmitted from a virus to humans. It is found in the intercellular membranes that separate the placenta and brain. A second protein was identified in 2007 and named EFF1. It helps form the skin of nematode roundworms and is part of the entire FF family of proteins. Dr. Felix Rey at the Pasteur Institute in Paris built a 3D model of the EFF1 structure and showed that it is what binds the particles together. This experience confirms the fact that all known fusions of tiny particles into molecules are of viral origin. This also suggests that viruses were vital for the communication of internal structures, and without them the emergence of colonies in the subkingdom of multicellular sponges would have been impossible.

All these theories, as well as many others that famous scientists continue to propose, are very interesting. However, none of them can clearly and unambiguously answer the question: how could such a huge variety of species arise from a single cell that originated on Earth? Or: why did single individuals decide to unite and begin to exist together?

Maybe in a few years, new discoveries will be able to give us answers to each of these questions.

Organs and tissues

Complex organisms have biological functions such as defense, circulation, digestion, respiration, and sexual reproduction. They are performed by specific organs such as the skin, heart, stomach, lungs and reproductive system. They are made up of many different types of cells that work together to perform specific tasks.

For example, heart muscle has a large number of mitochondria. They produce adenosine triphosphate, which keeps blood moving continuously through the circulatory system. Skin cells, on the contrary, have fewer mitochondria. Instead, they have dense proteins and produce keratin, which protects the soft internal tissues from damage and external factors.

Reproduction

While all simple organisms, without exception, reproduce asexually, many of the subkingdom metazoans prefer sexual reproduction. Humans, for example, are highly complex structures created by the fusion of two single cells called an egg and a sperm. The fusion of one egg with a gamete (gametes are special sex cells containing one set of chromosomes) of a sperm leads to the formation of a zygote.

The zygote contains the genetic material of both the sperm and the egg. Its division leads to the development of a completely new, separate organism. During development and division, cells, according to the program laid down in the genes, begin to differentiate into groups. This will further allow them to perform completely different functions, despite the fact that they are genetically identical to each other.

Thus, all the organs and tissues of the body that form nerves, bones, muscles, tendons, blood - they all arose from one zygote, which appeared due to the fusion of two single gametes.

Multicellular advantage

There are several main advantages of the sub-kingdom of multicellular organisms, due to which they dominate our planet.

As the complex internal structure allows for increased size, it also helps develop higher order structures and tissues with multiple functions.

Large organisms have better protection from predators. They also have greater mobility, which allows them to migrate to more favorable places to live.

There is another undeniable advantage of the multicellular subkingdom. A common characteristic of all its species is a fairly long life expectancy. The cell body is exposed to the environment from all sides, and any damage to it can lead to the death of the individual. A multicellular organism will continue to exist even if one cell dies or is damaged. DNA duplication is also an advantage. The division of particles within the body allows damaged tissue to grow and repair faster.

During its division, a new cell copies the old one, which makes it possible to preserve favorable features in subsequent generations, as well as improve them over time. In other words, duplication allows for the retention and adaptation of traits that will improve the survival or fitness of an organism, especially in the animal kingdom, a subkingdom of metazoans.

Disadvantages of multicellular

Complex organisms also have disadvantages. For example, they are susceptible to various diseases arising from their complex biological composition and functions. Protozoa, on the contrary, lack developed organ systems. This means that their risks of dangerous diseases are minimized.

It is important to note that, unlike multicellular organisms, primitive individuals have the ability to reproduce asexually. This helps them not waste resources and energy on finding a partner and sexual activity.

They also have the ability to accept energy through diffusion or osmosis. This frees them from the need to move around to find food. Almost anything can be a potential food source for a single-celled creature.

Vertebrates and invertebrates

The classification divides all multicellular creatures without exception into the subkingdom into two species: vertebrates (chordates) and invertebrates.

Invertebrates do not have a hard frame, while chordates have a well-developed internal skeleton of cartilage, bones and a highly developed brain, which is protected by the skull. Vertebrates have well-developed sensory organs, a respiratory system with gills or lungs, and a developed nervous system, which further distinguishes them from their more primitive counterparts.

Both types of animals live in different habitats, but chordates, thanks to their developed nervous system, can adapt to land, sea and air. However, invertebrates also occur in a wide range, from forests and deserts to caves and the mud of the seafloor.

To date, almost two million species of the subkingdom of multicellular invertebrates have been identified. These two million make up about 98% of all living beings, that is, 98 out of 100 species of organisms living in the world are invertebrates. Humans belong to the chordate family.

Vertebrates are divided into fish, amphibians, reptiles, birds and mammals. Those that do not are represented by such types as arthropods, echinoderms, worms, coelenterates and mollusks.

One of the biggest differences between these species is their size. Invertebrates, such as insects or coelenterates, are small and slow because they cannot develop large bodies and strong muscles. There are a few exceptions, such as the squid, which can reach 15 meters in length. Vertebrates have a universal support system, and therefore can develop faster and become larger than invertebrates.

Chordates also have a highly developed nervous system. With the help of specialized connections between nerve fibers, they can respond very quickly to changes in the environment, which gives them a distinct advantage.

Compared to vertebrates, most spineless animals use a simple nervous system and behave almost entirely instinctively. Such a system works well most of the time, although these creatures are often unable to learn from their mistakes. The exceptions are octopuses and their close relatives, which are considered among the most intelligent animals in the invertebrate world.

All chordates, as we know, have a backbone. However, a feature of the subkingdom of multicellular invertebrate animals is their similarity to their relatives. It lies in the fact that at a certain stage of life, vertebrates also have a flexible supporting rod, a notochord, which subsequently becomes the spine. The first life developed as single cells in water. Invertebrates were the initial link in the evolution of other organisms. Their gradual changes led to the emergence of complex creatures with well-developed skeletons.

Coelenterates

Today there are about eleven thousand species of coelenterates. These are some of the oldest complex animals to appear on earth. The smallest of the coelenterates cannot be seen without a microscope, and the largest known jellyfish is 2.5 meters in diameter.

So, let's take a closer look at the subkingdom of multicellular organisms, such as the coelenterates. The description of the main characteristics of habitats can be determined by the presence of an aquatic or marine environment. They live alone or in colonies that can move freely or live in one place.

The body shape of coelenterates is called a “bag”. The mouth connects to a blind sac called the gastrovascular cavity. This sac functions in the process of digestion, gas exchange and acts as a hydrostatic skeleton. The single opening serves as both the mouth and anus. Tentacles are long, hollow structures used to move and capture food. All coelenterates have tentacles covered with suckers. They are equipped with special cells - nemocysts, which can inject toxins into their prey. The suction cups also allow them to capture large prey, which the animals place in their mouths by retracting their tentacles. Nematocysts are responsible for the burns that some jellyfish cause to humans.

Animals of the subkingdom are multicellular, such as coelenterates, and have both intracellular and extracellular digestion. Respiration occurs by simple diffusion. They have a network of nerves that spread throughout the body.

Many forms exhibit polymorphism, which is a variety of genes in which different types of creatures are present in the colony for different functions. These individuals are called zooids. Reproduction can be called random (external budding) or sexual (formation of gametes).

Jellyfish, for example, produce eggs and sperm and then release them into the water. When the egg is fertilized, it develops into a free-swimming, ciliated larva called a planla.

Typical examples of the subkingdom Multicellular are hydras, obelia, man of war, sailfish, sea anemones, corals, sea pens, gorgonians, etc.

Plants

In the subkingdom Multicellular plants are eukaryotic organisms that are able to feed themselves through the process of photosynthesis. Algae were originally considered plants, but they are now classified as protists, a special group that is excluded from all known species. The modern definition of plants refers to organisms that live primarily on land (and sometimes in water).

Another distinctive feature of plants is the green pigment - chlorophyll. It is used to absorb solar energy during the process of photosynthesis.

Every plant has haploid and diploid phases that characterize its life cycle. It is called alternation of generations because all phases in it are multicellular.

The alternating generations are the sporophyte generation and the gametophyte generation. During the gametophyte phase, gametes are formed. The haploid gametes fuse to form a zygote, called a diploid cell because it has a complete set of chromosomes. From there, diploid individuals of the sporophyte generation grow.

Sporophytes go through a phase of meiosis (division) and form haploid spores.

So, the subkingdom of multicellular organisms can be briefly described as the main group of living beings that inhabit the Earth. These include everyone who has a number of cells, different in their structure and functions and united into a single organism. The simplest multicellular organisms are the coelenterates, and the most complex and developed animal on the planet is man.

General characteristics of multicellular organisms

Multicellular animals form the largest group of living organisms on the planet, numbering more than 1.5 million species. Tracing their origins from protozoa, they underwent significant transformations in the process of evolution associated with the complication of organization.

One of the most important features of the organization of multicellular organisms is the morphological and functional differences in the cells of their body. During evolution, similar cells in the body of multicellular animals specialized to perform certain functions, which led to the formation fabrics.

Different fabrics united organs, and organs - organ systems. To implement the relationship between them and coordinate their work, regulatory systems- nervous and endocrine. Thanks to the nervous and humoral regulation of the activity of all systems, a multicellular organism functions as an integral biological system.

The prosperity of a group of multicellular animals is associated with the complication of their anatomical structure and physiological functions. Thus, an increase in body size led to the development of the digestive canal, which allowed them to feed on large food material, supplying a large amount of energy for all life processes. The developed muscular and skeletal systems provided the movement of organisms, maintaining a certain body shape, protection and support for organs. The ability for active movement allowed animals to search for food, find shelter and settle.

With the increase in the body size of animals, the need arose for the appearance intratransport circulatory systems, delivering life support to tissues and organs remote from the surface of the body - nutrients, oxygen, and also removing the end products of metabolism.

Liquid tissue—blood—became such a circulatory transport system.

The intensification of respiratory activity went in parallel with the progressive development nervous system And sense organs. The central sections of the nervous system moved to the anterior end of the animal’s body, resulting in the separation of the head section. This structure of the front part of the animal’s body allowed it to receive information about changes in the environment and respond adequately to them.

Based on the presence or absence of an internal skeleton, animals are divided into two groups - invertebrates(all types except Chordata) and vertebrates(phylum Chordata).

Depending on the origin of the oral opening in an adult organism, two groups of animals are distinguished: primary and deuterostomes. Protostomes combine animals in which the primary mouth of the embryo at the gastrula stage - the blastopore - remains the mouth of the adult organism. These include animals of all types, except Echinoderms and Chordata. In the latter, the primary mouth of the embryo turns into the anus, and the true mouth is formed secondarily in the form of an ectodermal pouch. For this reason they are called deuterostomes animals.

Based on the type of body symmetry, a group is distinguished radiant, or radially symmetrical, animals (types Sponges, Coelenterates and Echinoderms) and a group bilaterally symmetrical(all other types of animals). Radial symmetry is formed under the influence of the sedentary lifestyle of animals, in which the entire organism is positioned in relation to environmental factors under exactly the same conditions. These conditions form the arrangement of identical organs around the main axis passing through the mouth to the attached pole opposite it.

Bilaterally symmetrical animals are mobile, have one plane of symmetry, on both sides of which are located various paired organs. They are distinguished between left and right, dorsal and ventral sides, anterior and posterior ends of the body.

Multicellular animals are extremely diverse in structure, features of life activity, different in size, body weight, etc. Based on the most significant general structural features, they are divided into 14 types, some of which are discussed in this manual.