In the first approximation, multicellular (Metazoa) can be defined as animals whose body is composed of many cells and intercellular substance. However, this feature alone is not sufficient to establish that an animal belongs to multicellular organisms. Thus, colonies of protozoa can be formed from a large number of cells, but no one has ever attributed them to Metazoa. The most essential feature of a multicellular animal is structural differentiation of cells and them function specialization. Unlike the Metazoa, the cells that make up protozoan colonies are more or less the same. The only exceptions are germ cells, as well as relatively infrequent cases of a morphological and anatomical gradient, when the size of cells in a colony and the level of development of their individual structures gradually change in a certain direction.
Metazoa cells are parts a more complex organism, or an organism of a higher order. Being parts of the whole, they have largely lost their independence (individuality) and cannot realize the full range of life functions. Therefore, each cell of a multicellular animal in its existence needs to supplement its functions with other cells that are different from it. But, on the other hand, each cell of a multicellular animal is obliged to ensure the existence of those cells on which it depends, that is, in turn, to compensate for the incompleteness of their functions. Thus, the essence of a multicellular organism can be expressed in two words: specialization and cooperation.
The same essence at one time (1855) was extremely aptly expressed by the German scientist Rudolf Virchow, defining a multicellular organism as cell state. Yes, and in the scientific name of multicellular animals - Metazoa - the same theme sounds. The lexical meaning of the prefix Meta- in Latin can be transmitted by a Russian prefix above-, but Metazoa, in a somewhat loose translation, by the Russian expression "supracellular organism". In other words, Metazoa is an organism of a higher order, the level of which colonial protozoa do not reach.
It should be noted that in terms of the degree of integration of cells into a single whole, Metazoa are far from equivalent. On this basis, all multicellular organisms are usually divided into two unequal groups, each of which is appropriate to give the rank of a subkingdom. The first group - primary multicellular, or Prometazoa - stand at the pre-tissue level of organization. Their body, as befits a multicellular organism, is composed of many specialized cells, but these cells are not integrated into tissues. Due to this circumstance, the integrity of Prometazoa organisms is relatively small, and the cells that compose them retain a certain degree of independence. So, if you wipe the body of a sponge through a sieve, then the resulting slurry - that is, a cell suspension, quickly organizes itself into a new sponge, and small pieces of the sponge give rise to a new organism.
The second group - animals of the sub-kingdom Eumetazoa (true multicellular) - are characterized tissue structure. This circumstance served as some scientific reason to call these animals not so much multicellular as multi-tissue(term multitissue animal suggested by J. Corliss in 1983), which, from a formal standpoint, is hardly true, because among them there are creatures that have only one single tissue - the ectoderm (which, you see, is not very much). Eumetazoa cells are firmly connected to each other through special adhesion molecules (molecular cross-linking), plasmodesmata (cytoplasmic bridges that look like dense protein strands) and desmodesmata (cell outgrowths of a special configuration that form compounds such as curly paving slabs or children's puzzles). As a result, Eumetazoa cells have a strictly defined (fixed) position, which they cannot change at will.
It should be said that there are well-known reasons to single out a third group of Metazoa, namely multicellular animals with polyfunctional tissues. These include coelenterates and ctenophores, whose bodies are composed of peculiar "tissues" that do not satisfy the classical definition of tissue. If we recall the definition of the concept of “tissue” from a school textbook on general biology, then an expression in the spirit comes to mind: “tissue is a collection of cells that are similar in structure and perform the same functions.” The tissues of the coelenterates and ctenophores do not satisfy this definition in principle: they consist of heterogeneous cells(epithelial-muscular, stinging, nervous, etc.) performing various functions. In contrast to animals mixed, or polyfunctional tissues, all other Eumetazoa have not so much tissue as organ structure, i.e. are made up of a set bodies, consisting of tissues in their classical sense.
In addition to the specialization of cells and their cooperation within an organism of a higher order, multicellular organisms are characterized by a specific course of individual development (ontogenesis). Ontogeny of multicellular organisms includes crushing of the egg (homolog of protozoan palintomy), subsequent differentiation of cells into primary cell layers (germ layers) and rudiments of organs (Eumetazoas.str.), accompanied by a complex movement of cell masses. In protozoa, as already mentioned, ontogenesis also takes place, but, of course, does not go beyond the unicellular organization.
Multicellular animals form the largest group of living organisms on the planet, numbering more than 1.5 million species. Leading their origin from the simplest, they have undergone significant transformations in the process of evolution associated with the complication of organization.
Coelenterates: There are more than 9 thousand species of coelenterates. These are lower, predominantly marine, multicellular animals attached to the substrate or floating in the water column. The body is saccular, formed by two layers of cells: the outer - ectoderm, and the inner - endoderm, between which there is a structureless substance - mesoglea.
Reproduction occurs both asexually and sexually. Incomplete to the end asexual reproduction - budding - leads in a number of species to the formation of colonies.
Sponges are multicellular animals:
Sponges are characterized by a modular structure, often associated with the formation of colonies, as well as the absence of true tissues and germ layers. Unlike true multicellular animals, sponges lack the muscular, nervous, and digestive systems. The body is composed of an integumentary layer of cells, subdivided into pinacoderm and choanoderm, and a jelly-like mesochyl permeated by the channels of the aquifer system and containing skeletal structures and cellular elements. The skeleton in different groups of sponges is represented by various protein and mineral (calcareous or silicic) structures. Reproduction is carried out both sexually and asexually.
Multicellular:
One of the most important features of the organization of multicellular organisms is the morphological and functional difference between the cells of their body. Over the course of evolution, similar cells in the body of multicellular animals specialized in performing certain functions, which led to the formation of tissues.
Different tissues united into organs, and organs - and organ systems. To implement the relationship between them and coordinate their work, regulatory systems were formed - 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 the anatomical structure and physiological functions. Thus, an increase in body size led to the development of the alimentary canal, which allowed them to eat large food material, which supplies a large amount of energy for the implementation of all life processes. The developed muscular and skeletal systems ensured the movement of organisms, the maintenance of a certain body shape, protection and support for organs. The ability to actively move allowed animals to search for food, find shelter and settle.
With an increase in the size of the body of animals, a need arose for the appearance of intratransport circulatory systems that deliver life-support means - nutrients, oxygen, and also remove end products of metabolism to tissues remote from the surface of the body and organs.
Liquid tissue - blood - became such a circulatory transport system.
The intensification of respiratory activity went in parallel with the progressive development of the nervous system and sensory organs. The central sections of the nervous system moved to the anterior end of the animal's body, as a result of which the head section became isolated. Such a structure of the anterior part of the animal's body allowed it to receive information about changes in the environment and adequately respond to them.
According to the presence or absence of an internal skeleton, animals are divided into two groups - invertebrates (all types except Chordates) and vertebrates (Chordates).
Depending on the origin of the mouth opening in an adult organism, two groups of animals are distinguished: primary and secondary-stomes. Protostomes unite animals in which the primary mouth of the embryo at the gastrula stage - the blastopore - remains the mouth of an adult organism. These include animals of all types except echinoderms and chordates. In the latter, the primary mouth of the embryo turns into an anus, and the true mouth is formed a second time in the form of an ectodermal pocket. For this reason, they are called deuterostomes.
According to the type of body symmetry, a group of radiant, or radially symmetrical, animals (types of Sponge, Coelenterates and Echinoderms) and a group of bilaterally symmetrical (all other types of animals) are distinguished. Radial symmetry is formed under the influence of the sedentary lifestyle of animals, in which the entire organism is placed in relation to environmental factors in 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 to it.
Bilaterally symmetrical animals are mobile, have one plane of symmetry, on both sides of which there are various paired organs. They distinguish between left and right, dorsal and ventral sides, anterior and posterior ends of the body.
Multicellular animals are extremely diverse in structure, life characteristics, different in size, body weight, etc. Based on the most significant common structural features, they are divided into 14 types, some of which are discussed in this manual.
In multicellular organisms, ontogenesis usually begins with the formation of a zygote and ends with death. At the same time, the organism not only grows, increasing in size, but also goes through a number of different life phases, each of which has a special structure, functions differently, and in some cases radically different way of life. The process of embryonic development of multicellular animals includes three main stages: cleavage, gastrulation, and primary organogenesis. Embryogenesis begins with the formation of a zygote.
Consider the stages of embryonic development of a multicellular animal using the example of a lake frog. Within a few hours (in other species of vertebrates, even after a few minutes) after the introduction of the sperm into the egg, the first stage of embryogenesis begins - crushing, which is a series of successive mitotic divisions of the zygote. At the same time, with each division, smaller and smaller cells are formed, which are called blastomeres (from the Greek blastos - sprout, meros - part). Crushing of cells occurs due to a decrease in the volume of the cytoplasm. Moreover, the process of cell division continues until the size of the resulting cells is equal to the size of other somatic cells of organisms of this species. As a result, the mass of the embryo in the final period and its volume remain constant and approximately equal to the zygote.
Multicellular organisms (Metazoa) - these are organisms consisting of a set of cells, groups of which specialize in performing certain functions, creating qualitatively new structures: tissues, organs, organ systems. In most cases, due to this specialization, individual cells cannot exist outside the body. The subkingdom Multicellular has about 30 types. The organization of the structure and life of multicellular animals differs in many ways from the organization of unicellular ones.
■ In connection with the appearance of organs, formed body cavity- the space between the organs, which ensures their relationship. The cavity can be primary secondary and mixed.
■ Due to the complication of lifestyle, radial (radial) or bilateral (bilateral) symmetry, which gives grounds to separate multicellular animals as radially symmetric and binary symmetric.
■ As food needs grow, efficient means of transportation emerge that allow active foraging, resulting in musculoskeletal system.
■ multicellular animals require much more food than unicellular animals, and therefore most animals switch to eating solid organic food, which leads to digestive system.
■ In most organisms, the outer covers are impermeable, so the exchange of substances between the organism and the environment occurs through limited areas of its surface, which leads to the appearance respiratory system.
■ As the size increases, circulatory system, which carries blood due to the work of the heart or pulsating vessels.
■ Formed excretory systems for the withdrawal of exchange products
■ Regulatory systems emerge - nervous And endocrine, which coordinate the work of the whole organism.
■ In connection with the emergence of the nervous system, new forms of irritability appear - reflexes.
■ The development of multicellular organisms from a single cell is a long and complex process, and therefore life cycles become more complicated, which will certainly include a number of stages: zygote - embryo - larva (baby) - young animal - adult animal - sexually mature animal - an aging animal - an animal has died.
General signs of the structure and life of representatives of the Sponge type
Sponges - multicellular bilayered radially or asymmetric animals, the body of which is riddled with pores. About 5000 species of freshwater and sea sponges belong to the type. The vast majority of these species inhabit tropical and subtropical seas, where they are found at depths up to 500 m. However, among the sponges, there are also deep-sea forms that were found at a depth of 10,000 - 11,000 m (for example, sea brushes). There are 29 species in the Black Sea, 10 species in fresh waters of Ukraine. Sponges belong to the most primitive multicellular organisms, since tissues and organs are not clearly expressed in them, although cells perform various functions. The main reason preventing the mass distribution of sponges is the lack of an appropriate substrate. Most sponges cannot live on muddy bottoms because the mud particles clog their pores, leading to the death of the animal. Salinity and mobility of water, temperature have a great influence on distribution. The most common features of sponges are: 1 ) the presence of pores in the walls of the body 2) absence of tissues and organs; 3) the presence of a skeleton in the form of needles or fibers; 4) well developed regeneration and etc.
Widespread from freshwater forms body sponge(Spongilla lacustris), which lives on the stony soils of water bodies. The green color is due to the presence of algae cells in their protoplasm.
structural features
Body multicellular, stalked, bushy, cylindrical, funnel-shaped, but most often in the form of a bag or glass. Sponges lead an attached lifestyle, so in their body there is the foundation for attaching to the substrate, and on top - a hole ( mouth), which leads to a Triolny (paragastric) cavities. The walls of the body are permeated with many pores through which water enters this body cavity. The walls of the body are formed from two layers of cells: the outer - pinacoderm and internal - choanoderma. Between these layers there is a structureless gelatinous substance - mesoglea that contains cells. Sponge body sizes - from a few millimeters to 1.5 m (sponge cup of neptune).
Sponge structure: 1 - mouth; 2 - pinacoderma; 3 - choanoderma; 4 - time; 5 - mesoglea; 6 - archeocyte; 7 - base; 8 - triaxial branch; 9 - atrial cavity; 10 - spicules; 11 - amebocytes; 12 - colencite; 13 - porocyte; fourteen - pinacocyte
Diversity of sponge cells and their functions
|
cells |
Location |
functions |
|
Pinacocytes |
Pinacoderma |
squamous cells that form the surface epithelium |
|
Porocytes |
Pinacoderma |
Cells with an intracellular time-channel capable of contracting and opening or closing it |
|
choanocytes |
Choanoderma |
Cylindrical cells with a long flagellum that create a flow of water and are able to absorb nutrient particles and transfer them to the mesoglea |
|
Colencites |
mesoglea |
Fixed stellate cells, which are connective tissue supporting elements |
|
Sclerocytes |
mesoglea |
Cells from which the skeletal formations of sponges develop - spicules |
|
mesoglea |
Cells that are interconnected with the help of processes and provide some reduction in the body of sponges |
|
|
amebocytes |
mesoglea |
Mobile cells that carry out the digestion of food and the spread of nutrients throughout the body of the sponge |
|
archeocytes |
mesoglea |
Reserve cells that are able to transform into all other cells and give rise to germ cells |
Features of the organization of sponges are reduced to three main types:
ASCON - body with a paragastric cavity lined with choanocytes (in limestone sponges)
Seacon- a body with thickened walls, into which sections of the paragastric cavity protrude, forming flagellar pockets (in glass sponges)
leukone- a body with thick walls, in which small flagellar chambers are distinguished (in ordinary sponges).
Covers. The body is covered with a squamous epithelium formed by pinacocytes.
Cavity body is called paragastric and is lined with choanocytes.
Features of life processes
Support provided by the skeleton, it can be limestone (spicule with CaCO3), silicon (spicule with SiO2) or horny (from collagen fibers and spongin substance, which contains a significant amount of iodine).
Motion. Adult sponges are not capable of active movement and lead an attached lifestyle. Some minor contractions of the body are carried out thanks to myocytes, which can thus respond to irritation. Amebocytes are capable of moving inside the body due to pseudopodia. Sponge larvae, unlike adults, are able to move vigorously in water due to the coordinated work of flagella, which in most cases almost completely cover the surface of the body.
Nutrition in sponges is passive and is carried out by a continuous flow of water through the body. Due to the rhythmic work of the flagella hoonocyte water enters the pores, enters the paragastric cavity and is brought out through the mouths. Dead remains of animals and plants suspended in water, as well as microorganisms, are carried away by choanocytes, transferred to amoebocytes, where they are digested and carried by them throughout the body.
Digestion in sponges, it is intracellular. Amebocytes' interest in nutrient particles occurs by phagocytosis. Undigested residues are thrown into the body cavity and excreted.
Transportation of substances inside the body is carried out by amoebocytes.
Breath occurs throughout the body. For respiration, oxygen dissolved in water is used, which is absorbed by all cells. Carbon dioxide is also removed in a dissolved state.
Selection undigested residues and metabolic products occurs along with water through the mouth.
Process regulation is carried out with the participation of cells that are able to contract or make movements - porocyte, myocytes, choanocytes. The integration of processes at the level of the organism is almost not developed.
Irritability. Sponges react very weakly even to the strongest irritations, and their transmission from one area to another is almost imperceptible. This indicates the absence of a nervous system in sponges.
reproduction asexual and sexual. Asexual reproduction is carried out by external and internal budding, fragmentation, longitudinal division, etc. In the case of external budding, the daughter individual is formed on the mother and, as a rule, contains all types of cells. In rare forms, the kidney separates (for example, in sea orange), and in the colonial - retains a connection with the mother's organism. IN body sponges and in other freshwater sponges, in addition to external, internal budding is also observed. In the second half of summer, when the water temperature drops from archaeocytes, internal buds are formed - gemmules. For the winter, the bodyagi's body dies off, and the gemmules sink to the bottom and, protected by a shell, hibernate. In the spring, a new sponge develops from it. As a result of fragmentation, the body of the sponge breaks up into parts, each of which, under favorable conditions, gives rise to a new organism. Sexual reproduction occurs with the participation of gametes, which are formed from archaeocytes in mesoglea. Most sponges are hermaphrodites (sometimes dioecious). In the case of sexual reproduction, the mature spermatozoon of one sponge leaves the mesoglea through the mouth and enters the cavity of the other with the flow of water, where it is delivered to the mature egg cell with the help of amoebocytes.
Development indirect(with transformation). Cleavage of the zygote and the formation of the larva occurs mainly inside the maternal organism. The larva, which has flagella, exits through the mouth into the environment, attaches to the substrate and turns into an adult sponge.
Regeneration well developed. Sponges have a very high level of regeneration, which ensures the reproduction of a whole independent organism, even from the very piece of the sponge's body. For sponges inherent and somatic embryogenesis - formation, development of a new individual from cells of the body that are not adapted for reproduction. If you pass the sponge through a sieve, you can get a filtrate containing living individual cells. These cells remain viable for several days and, with the help of pseudopodia, actively move and gather in groups. These groups turn into small sponges after 6-7 days.
Animals - a group of living organisms that includes more than one million identified and millions of species that are not yet known to science. Scientists have calculated that the number of all animal species that have already been discovered and yet to be discovered is from 3 to 30 million.
Animals are divided into more than thirty groups (the number of groups varies depending on different opinions and recent phylogenetic studies).
For purposes of this article, we will focus on the six most well-known groups of animals: amphibians, birds, fish, mammals, and reptiles. We'll also take a look at many lesser known bands, some of which are featured below.
First, let's look at what animals are and also list some of the characteristics that distinguish them from other organisms such as plants, fungi, protozoa, bacteria, and archaea.
Who are the animals?
Animals are a diverse group of living organisms that include many sub-groups such as arthropods, chordates, coelenterates, echinoderms, molluscs, sponges, etc. They also include a wide variety of lesser known creatures such as flatworms, rotifers, lamellae and tardigrades. These groups of animals may seem rather strange to those who have not taken a course in zoology, but the animals with which we are most familiar belong to other groups. For example, insects, crustaceans, horseshoe crabs, all members of arthropods, amphibians, birds, reptiles, mammals, fish, and all members of chordates. Also not worth mentioning are jellyfish, corals, anemones and all members of the cnidarians.
The overwhelming diversity of living organisms that are classified as animals makes it impossible to generalize into separate groups. But there are a few general characteristics of animals, the proportion of which describes the majority of the members of a particular group. These common characteristics include multicellularity, tissue specialization, locomotion, heterotrophy, and sexual reproduction.
Multicellular animals are united by the fact that their body consists of more than one cell. With the exception of sponges, animals have organs that have differentiated into tissues and perform specific functions. These tissues are in turn organized into organ systems. Animals do not have the rigid cell walls characteristic of plants.
Animals are also mobile (they are able to move in space). The body of most animals is arranged in such a way that the head is located in the direction of movement, and the rest of the body follows it. Of course, the wide variety of animal body structures means that there are exceptions to this rule.
Heterotrophic animals obtain energy by consuming other living organisms. Most animals reproduce sexually by differentiated eggs and sperm. In addition, many animals (adult cells contain two copies of their genetic material). Multicellular animals go through various stages of embryonic development: zygote, blastula, gastrula, neurula, primary organogenesis and prenatal development).
Animals come in a wide variety of sizes, from microscopic, such as plankton, to gigantic, such as the blue whale. They inhabit virtually every habitat on the planet from the poles to the tropics and from mountain peaks to deep and dark ocean waters.
Animals are believed to have evolved from flagellar protozoa, and the oldest animal remains date back to about 600 million years. During the Cambrian period (about 570 million years ago), most animal groups evolved.
Main characteristics
Key characteristics of multicellular animals include:
- multicellularity;
- eukaryotic cells;
- sexual reproduction;
- tissue specialization;
- motion;
- heterotrophy.
Classification of multicellular animals
The best-known groups of animals include:
(Arthropoda)- There are at least one million arthropods known to science and many millions yet to be discovered. Scientists have calculated that the arthropod group could have up to 30 million species (most of which are insects). This group includes the following members: centipedes, spiders, ticks, horseshoe crabs, scorpions, and insects. Arthropods are symmetrical and have a segmented body, exoskeleton, articulated appendages, and numerous pairs of legs and specialized limbs.
(Chordata)- about 75,000 known species of chordates live on earth. Members of this group include vertebrates, tunicates, and non-craniates. Chordates have a notochord that is present throughout the entire or at least a certain period of the animal's life cycle.
(Cnidaria)- Science knows about 11,000 species of cnidarians. Members of this group include jellyfish, corals, sea anemones, and hydras. Cnidarians are radially symmetrical and have a gastrovascular cavity with a single opening that is surrounded by tentacles.
(Echinodermata)- open about 6000 species of echinoderms inhabiting our planet today. Members of this group include starfish, sea lilies, sea urchins, brittle stars and sea cucumbers. Echinoderms are radially symmetrical and have an endoskeleton composed of calcareous plates.
(Mollusca)- today we know more than 100,000 species of molluscs. The group includes the following classes: bivalves, gastropods, cephalopods, spadefoots, pit-tails, furrow-bellied, monoplacophores and shellfish. Mollusks have a soft body, which consists of three main parts: the legs, the visceral mass, and the mantle with the organ system.
(Annelida)- the type has about 12,000 described species living on our planet. This group includes polychaete and oligochaete worms, leeches and misostomids. Annelids are symmetrical, and the body consists of a head and tail region, as well as a middle region of many repeating segments.
(Porifera)- Today on Earth, at least, there are about 10,000 species of sponges. Members of this group include calcareous sponges, common sponges, six-beam sponges. Sponges are primitive multicellular animals that do not have a digestive, circulatory or nervous system.
Other groups of animals
Some of the lesser known animal groups include:
Bristles, or marine arrows (Chaetognatha)- a group of marine animals from 120 species known to science. Members of this group include predatory marine worms. Chaetognaths live in various marine waters, including shallow coastal areas. They are found in all climatic regions, from the tropics to the polar regions.
bryozoans (Ectoprocta, or Bryozoa)- about 5000 species of bryozoans are known. The group includes tiny (about 1-3 mm) aquatic invertebrates that feed on microorganisms by filtering water.
ctenophores (Ctenophora)- a type of marine animal, which has about 100 known species. Members of this group have ciliated combs used for swimming. Most ctenophores are predators and feed on plankton.
flatworms (Plathelminthes, or Platyhelminthes)- a type of invertebrates, numbering about 20,000 species. The members of this group are divided into the following classes: monogeneans, tapeworms, amphilinids, gyrocotylids, trematodes, aspidogasters. Flatworms are soft-bodied invertebrates that do not have a body cavity, circulatory or respiratory systems. Oxygen and nutrients pass through their body walls by diffusion. This affects the body structure of flatworms and for this reason they are flat.
Gastrotrichous worms or gastrotrichs (Gastrotricha)- a phylum of invertebrates, which has about 500 known species. Most species of gastro worms are freshwater, although there are a small number of marine and terrestrial species. Gastrotrichous are microscopic animals with transparent organs and cilia on the abdomen.
Hemishordates (Hemichordata)- a phylum of invertebrates with about 100 known species. Hemichordates are divided into the following classes: intestinal-pneumatic and pinnatibranch.
Phoronids (Phoronida)- a type of marine invertebrates, which includes about 20 known species. They stick to a hard surface at the bottom and feed on microorganisms that stick to their tentacles.
Brachiopods, or brachiopods (Brachiopoda)- a type of marine invertebrates, which unites about 350 species. Brachiopods look like mollusks, although the anatomical structure has nothing to do with mollusks. Brachiopods live in the cold waters of the polar regions and the depths of the ocean.
loricifera (Loricifera)- a group of marine invertebrates, which consists of approximately 10 species. Members of this group are tiny (in many cases microscopic) animals that live in marine sediments.
kinorhynchus (Kinorhyncha)- a class of invertebrates, uniting about 150 species of animals. Like loricifers, kinorhynchus live in marine sediments.
Gnastomulides (Gnathostomulida)- a type of invertebrates, which has about 100 species known to science. These are small marine animals that live in shallow coastal waters. Gnastomulides are able to survive in low oxygen conditions.
orthonectids (Orthonectida)- a type of marine invertebrates, which includes more than 20 living species.
priapulides (Priapulida)- a group of marine animals, uniting 18 living species. Members of this group are marine worms that live in silt deposits in shallow water.
Nemertines (Nemertea)- a type of invertebrates, which has about 1150 known species. Most representatives of nemerteans live in bottom sediments or attach themselves to hard surfaces, such as rocks and shells. Nemerteans are predators, feeding on invertebrates such as annelids, mollusks and crustaceans.
Rotifers (Rotifera)- a type of tiny invertebrates, which includes about 2000 species. Most members of this group live in freshwater, although a few species can be found in marine environments.
sipunculids (Sipuncula or Sipunculida)- a type of marine invertebrates, uniting about 150 described species. Members of this group of marine worms inhabit the shallow waters of the intertidal zone.
Onychophora, or primary tracheal, or velvet worms (Onychophora)- a type of invertebrates, which has about 110 species. Velvet worms have a long, segmented body and numerous pairs of limbs.
tardigrades (Tardigrada)- a type of aquatic microscopic animals, uniting more than 1000 described species.
On a note
Not all living organisms are animals. In fact, animals are only one of several major groups of living organisms. In addition to animals, other groups of organisms include plants, fungi, protists, bacteria, and archaea. To understand what is an animal it is necessary to be able to determine the belonging of living organisms to other groups that are not animals.
All living organisms are divided into sub-kingdoms of multicellular and unicellular creatures. The latter represent a single 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 about 3.5 billion years ago.
In our time, all the processes that occur with living organisms are studied by biology. It is this science that deals with the sub-kingdom of multicellular and unicellular.
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 the ciliate shoe are primitive and, at the same time, the oldest life forms that are representatives of this species. They were the first living beings that lived on Earth. This also includes groups such as sporozoans, sarcodes 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 fungi of some species. Some of them live in colonies, where all individuals are the same. The whole 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 cell organelles. These are usually bacteria and cyanobacteria, such as E. coli, salmonella, nostocs, etc.
All representatives of these groups differ in size. The smallest bacterium is only 300 nanometers long. Unicellular organisms usually have special flagella or cilia that are involved in their locomotion. They have a simple body with pronounced basic features. Nutrition, as a rule, occurs in the process of absorption (phagocytosis) of food and is stored in special cell organelles.
Single-celled animals have dominated the life form on Earth for billions of years. However, evolution from the simplest to more complex individuals has changed the entire landscape as it has led to the emergence of biologically advanced relationships. In addition, the emergence of new species has led to the formation of a new environment with diverse ecological interactions.
Multicellular organisms
The main characteristic of the multicellular 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 derived parts. Most of them can be seen without any special instruments. Plants, fish, birds and animals come out of a single cage. All creatures included in the multicellular sub-kingdom 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 sub-kingdom of multicellular, the classification clearly separates the functions in which each of the individual particles performs its task. They are engaged in vital processes, thus supporting the existence of the whole organism.
Subkingdom Multicellular in Latin sounds like Metazoa. To form a complex organism, cells must be identified and attached to others. Only about a dozen protozoa can be seen individually with the naked eye. The remaining nearly two million visible individuals are multicellular.
Pluricellular animals are created as a result of the association of individuals through the formation of colonies, filaments or aggregation. Pluricellular developed independently, like Volvox and some flagellated green algae.
A sign of the sub-kingdom of multicellular, that is, its early primitive species, was the absence of bones, shells and other solid parts of the body. Therefore, their traces have not survived to this day. The exceptions are sponges that still live in the seas and oceans. Perhaps their remains are found in some ancient rocks, such as Grypania spiralis, whose fossils are 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 unicellular organisms could have evolved.
Origin theories
To date, there are three main theories of the emergence of the subkingdom of multicellular organisms. A summary of the syncytial theory, so as not to go into details, can be described in a few words. Its essence lies in the fact 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 a mold fungus, as well as a ciliate shoe, which confirms this theory. However, having multiple nuclei is not enough for science. To confirm the theory of their multiplicity, a visual transformation into a well-developed animal of the simplest eukaryote is necessary.
Colony theory says that symbiosis, consisting of different organisms of the same species, led to their change and the emergence of more perfect creatures. Haeckel is the first scientist to present this theory in 1874. The complexity of organization arises because cells stay together, rather than being pulled apart during division. Examples of this theory can be seen in such protozoan metazoans as green algae called eudorina or volvax. They form colonies that number up to 50,000 cells depending on the species.
The colony theory proposes the fusion of different organisms of the same species. The advantage of this theory is that it has been observed that during food shortages, amoebas cluster into a colony that moves as a 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 not known how the DNA of different individuals can be included in a single genome.
For example, mitochondria and chloroplasts can be endosymbionts (organisms in an organism). 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 to form a single organism again.
Other theories that also consider the emergence of the sub-kingdom of multicellular organisms:
- GK-PID theory. About 800 million years ago, a slight genetic change in a single molecule called GK-PID may have allowed individuals to move from a single cell to a more complex body 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, in the fusion of egg and sperm. The first syncytin-1 protein was found, which was transmitted from a virus to a person. It is found in the intercellular membranes that separate the placenta and the brain. The second protein was identified in 2007 and named EFF1. It helps form the skin of nematode roundworms and is part of the entire FF protein family. Dr. Felix Rey at the Institut Pasteur in Paris built a 3D layout of the EFF1 structure and showed that it is what binds the particles together. This experience confirms the fact that all known fusions of the smallest particles into molecules are of viral origin. This also suggests that viruses were vital to the communication of internal structures, and without them it would not have been possible for a colony of the sub-kingdom of the multicellular sponge type to emerge.
All these theories, as well as many others that famous scientists continue to offer, are very interesting. However, none of them can clearly and unambiguously answer the question: how could such a huge variety of species have appeared from a single cell that originated on Earth? Or: why did single individuals decide to unite and begin to exist together?
Maybe a few years will pass, and new discoveries will be able to give us answers to each of these questions.
Organs and tissues
Complex organisms have biological functions such as protection, 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, the heart muscle has a large number of mitochondria. They produce adenosine triphosphate, thanks to which blood moves continuously through the circulatory system. Skin cells, on the other hand, have fewer mitochondria. Instead, they have dense proteins and produce keratin, which protects soft internal tissues from damage and external factors.
reproduction
While without exception all protozoa reproduce asexually, many of the sub-kingdom of multicellular organisms prefer sexual reproduction. Humans, for example, are a complex structure 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 the development and division of cells, according to the program laid down in the genes, they 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 the 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, thanks to which they dominate our planet.
Since 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 against predators. They also have greater mobility, which allows them to migrate to more favorable places to live.
There is another indisputable advantage of the multicellular sub-kingdom. 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 tissues to grow and repair faster.
During its division, the new cell copies the old one, which allows you to save favorable features in the next generations, as well as improve them over time. In other words, duplication allows the preservation and adaptation of traits that will improve the survival or fitness of an organism, especially in the animal kingdom, the sub-kingdom of the multicellular organisms.
Disadvantages of multicellular
Complex organisms also have disadvantages. For example, they are susceptible to various diseases arising from their complex biological composition and functions. In protozoa, on the contrary, there are not enough developed organ systems. This means that the 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 to waste resources and energy on finding a partner and sexual activity.
They also have the ability to receive energy by diffusion or osmosis. This frees them from the need to move around to find food. Almost anything can become a potential source of food for a single-celled creature.
Vertebrates and invertebrates
Without exception, the classification divides all multicellular creatures included in the sub-kingdom into two types: vertebrates (chordates) and invertebrates.
Invertebrates do not have a hard frame, while chordates have a well-developed internal skeleton of cartilage, bone, and a highly developed brain that is protected by a skull. Vertebrates have well-developed sense 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 a developed nervous system, can adapt to land, sea and air. However, invertebrates are also found in a wide range, from forests and deserts to caves and seabed mud.
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 things, 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 represent phyla such 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 a specialized connection between nerve fibers, they can react very quickly to changes in the environment, which gives them an undeniable advantage.
Compared to vertebrates, most backboneless animals use a simple nervous system and behave almost entirely instinctively. This 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 invertebrates is the similarity with their relatives. It lies in the fact that at a certain stage of life, vertebrates also have a flexible support rod, the notochord, which later 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 a well-developed skeleton.
coelenterates
Today there are about eleven thousand species of coelenterates. These are one of the oldest complex animals that appeared 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 sub-kingdom of multicellular, the intestinal type. 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 are free to roam or live in one place.
The body shape of the 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 a mouth and an 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. Suckers also allow the capture of large prey, which animals place in their mouths by retracting their tentacles. Nematocysts are responsible for the burns some jellyfish inflict on humans.
Animals of the sub-kingdom are multicellular, such as coelenterates, have both intracellular and extracellular digestion. Respiration occurs by simple diffusion. They have a network of nerves that extend throughout the body.
Many forms exhibit polymorphism, i.e. gene diversity, 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 an egg is fertilized, it develops into a free-swimming, ciliated larva called a planla.
Typical examples of the sub-kingdom Multicellular are hydras, obelia, Portuguese man-of-war, sailboat, sea anemones, corals, sea pen, gorgonians, etc.
Plants
In the sub-kingdom Multicellular Plants are eukaryotic organisms that are able to feed themselves through the process of photosynthesis. Algae were originally thought to be plants, but now they are 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 photosynthesis.
Each 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. In 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 multicellular sub-kingdom 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 structure and function, and combined into a single organism. The simplest of multicellular organisms are intestinal, and the most complex and developed animal on the planet is man.
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