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.
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 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, the 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 life cycle of multicellular organisms is a complex individual development, during which an adult organism is formed from a fertilized egg. The fertilized egg is crushed, and the resulting cells differentiate into germ layers and rudiments of organs.
There are two groups of multicellular organisms: radiant (radially symmetrical), or two-layered, and bilaterally symmetrical, or three-layered.
Radiant are characterized by several planes of symmetry and a radial arrangement of organs around main axis body. In the process of individual development, they form only two germ layers - ectoderm and endoderm. The radiant type is intestinal.
Most animals are bilaterally symmetrical. They have one plane of symmetry, which divides their body into two mirror identical halves - left and right. There are three germ layers - endoderm, mesoderm and ectoderm.
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.
Ticket number 22
1. A population is a structural unit of a species. (Textbook of biology, grade 9, section 1, chapter 5, § 10;)
Areas completely inhabited by one or another species do not exist in nature. Within the range, individuals of this species develop only habitats suitable for their life. The degree of filling of the occupied space in different species is different. But there are always “voids” and accumulations in it. In other words, the range consists of more or less numerous areas where a certain species is found. For example, colonies of the European mole, clearly visible on the mounds of the earth, are located on forest edges and meadows, common spruce grows mainly in lowlands with significantly moistened soil.
Accumulations of individuals of the same species in terms of numbers can be large or small, exist for a long time (centuries or more) or throughout the life of only two or three generations, after which they, as a rule, die from any accidents, for example, diseases, a sharp deterioration in weather conditions and etc. For the fate of the species is much more important role play those groups of individuals that are stably preserved throughout the life of many generations. The number of individuals in such groups can significantly increase under favorable conditions and decrease under unfavorable ones, however, they have a chance of long-term existence in the given territory. Such groupings (aggregations) of individuals of the same species, inhabiting a certain part of the range for a long time, freely interbreeding with each other and producing fertile offspring, relatively isolated from other aggregates of the same species, are called a population (from Latin populus - people, population). Due to the spatial dissociation of populations, the species is adapted to exist in a variety of environmental conditions. Thus, the population is an intraspecific grouping and, consequently, a specific form of existence of the species, and the species itself is a complex biological system.
Characteristics of populations. Each population of any species as a biological system has a certain structure.
The structure of a population is understood as a certain quantitative ratio of individuals that differ in morphological and physiological characteristics, age, sex, the nature of distribution in space and other properties.
The main parameters of a population are, first of all, its abundance and density.
Number - the total number of individuals in the population. It is not constant, as the conditions of the habitat of the population are changeable. The population size depends on the ratio of the intensity of reproduction (fertility) and mortality. In the process of reproduction, the population grows, while mortality leads to a decrease in its number. For each population there are upper and lower limits of abundance, which can be measured by studying its seasonal and interannual changes.
Population density is the number of individuals or their biomass per unit area or volume (for example, 150 pine plants per 1 ha; 0.5 cyclops per 1 m 3 of water). Population density is also variable and depends on abundance. With an increase in numbers, the density does not increase only if it is possible to resettle the population and expand its range.
Spatial distribution is the features of the distribution of individuals of the population in the occupied territory. It is determined by the degree of homogeneity of the habitat, the availability of habitable sites, as well as biological features species, the behavior of its individuals. Knowing the type of distribution of organisms allows you to correctly estimate the density by sampling.
Natural populations are characterized by three types of distribution of individuals: random, uniform (regular) and group (aggregated) (Fig. 1.3).
random distribution of individuals is observed in a homogeneous habitat, with a low population size and the absence of individuals' desire to form groups (for example, in planarians, hydras). In nature, this type of distribution is rare.
Uniform distribution characteristic of species characterized by fierce competition between individuals for the same resources and a strong territorial instinct (predatory fish, mammals, birds, spiders).
Aggregated (group) distribution occurs in nature most often. It is expressed in the formation of groups of individuals, between which there are significant uninhabited territories. The reasons for the aggregation of individuals may be the heterogeneity of the environment and the limited habitats suitable for life, the features of reproduction, the desire to live in a group.
The age structure reflects the ratio of different age groups in the population (Fig. 1.4), as well as the seasonal and interannual dynamics of this ratio. Three ecological ages are usually distinguished in a population: pre-reproductive (before breeding), reproductive (during the breeding season), and post-reproductive (after breeding). Under favorable conditions, all age groups are present in the population and a more or less stable level of abundance is maintained. Decreasing populations are dominated by old individuals that are no longer able to reproduce intensively. Such an age structure indicates unfavorable living conditions. The study of the distribution of organisms by age has great importance in predicting the number of populations over the life of a number of next generations. Such studies make it possible to plan, for example, the fishing of fish or fur-bearing animals for a number of years ahead.
The sex structure is formed by the sex ratio in populations with dioecious individuals (see Fig. 1.4). These include most animals and all dioecious plants. The change in the sexual structure of a population is reflected in its role in the ecosystem, since males and females of many species have differences in the nature of nutrition, rhythm of life, and behavior. So, females of some species of mosquitoes, ticks and midges are blood-sucking, while males feed on plant sap or nectar. Fertility characterizes the frequency of the appearance of new individuals in the population due to reproduction.
Mortality (absolute and specific) is the opposite of fertility.
The ratio between the birth and death rates determines the population dynamics. So, if the birth rate is higher than the death rate, then the population will increase, and vice versa, it will decrease if the death rate exceeds the birth rate. In the case of equality of birth and death rates, the population will be maintained at a constant level.
The form of existence of a species is a population - a self-sustaining collection of individuals of the same species, which has its own gene pool. The ability of a population to long-term existence in a particular area of the species range is ensured by its characteristic structure and group properties: abundance, density, sex and age structure, fertility and mortality. The values of these indicators are not constant, which makes it possible for the population to adapt to changing environmental conditions.
2. The concept of systematics. Significance of the works of K. Linnaeus. binary nomenclature. ( Biology textbook, grade 9, section 1, chapter 5, § 10;)
Systematics is that part of zoology and botany that deals with the description and study of organic forms that now live on earth's surface. Systematics as a science pursues tasks of two kinds: practical and theoretical. The practical task of S. is to distinguish all the breeds (species) of animals and plants that exist on earth, to give each of them a special name and, if possible, an accurate and clear description (diagnosis) that would not allow mixing different kinds one with the other. But this practical side does not exhaust the task of S.
Its theoretical task is to 1) observing organic forms in terms of their constancy or variability, depending on external conditions, geographical distribution, etc. to determine the conditions for changing organisms, that is, the transition of one form to another; 2) in order, by studying organisms from the point of view of their similarity or difference, to notice between them related features that indicate a common origin, and thus restore their genealogy. The ultimate goal of S. is an explanation of the process of origin of the entire variety of organic forms. The theory of S. is, after all, the theory of evolution. Therefore, S. is often unfairly called a descriptive science. It deserves this name as much as any other science based on positive facts. Method C. To achieve these goals, naturalists arrange the forms of animals and plants in a system, that is, they distribute them according to the degree of similarity into groups, and these latter, in one way or another, arrange them in classes or groups of a higher order.
In practical terms, it is required of a system that every organism occupies a completely definite position in it, in accordance with its characteristics, so that, having met any organism unknown to us, it would be easy to determine its place in the system, thus finding out its name, if it has already been described, or make sure that this form has not yet been described by anyone and does not yet have a name. Theoretically, the system should clearly express the degrees of relatedness of organisms and outline, as far as possible, their genealogy. Both in zoology and botany, many systems have been proposed by various scientists. Judging by the extent to which these latter satisfy more practical or theoretical requirements, they are called artificial or natural. The artificial system is not consistent with the natural relationship of organisms; it distributes them simply on the basis of purely arbitrary, but as clear and constant as possible features. Artificial systems used to play an important role in botany, especially the sexual system of Linnaeus, established by him in 1735 and dominating science for almost 100 years. In zoology purely artificial systems, as a matter of fact, never existed, because here the natural similarity of organisms and groups is expressed comparatively much more sharply. As far as the natural system is concerned, its main goal is to express general similarity, i.e., kinship.
Karl Linnaeus (1707-1778), Swedish naturalist, creator of the system of flora and fauna, the first president of the Swedish Academy of Sciences (from 1739), a foreign honorary member of the St. Petersburg Academy of Sciences (1754). For the first time he consistently applied binary nomenclature and built the most successful artificial classification of plants and animals, described approx. 1500 plant species. He advocated the permanence of species and creationism. Author of "The System of Nature" (1735), "Philosophy of Botany" (1751), etc.
binary, or binomial nomenclature - a method of designating species adopted in biological systematics using a two-word name (binomen), consisting of a combination of two names (names): the name of the genus and the name of the species (according to the terminology adopted in zoological nomenclature) or the name of the genus and the specific epithet (according to botanical terminology).
The genus name is always written with capital letter, the name of the species (specific epithet) - always with a small one (even if it comes from a proper name). In the text, the binomen is usually written in italics. The species name (specific epithet) should not be given separately from the genus name, since without the genus name it is meaningless. In some cases, the genus name can be shortened to a single letter or a standard abbreviation.
According to the tradition established in Russia, the phrase binomial nomenclature (from the English binomial) has become widespread in the zoological literature, and in the botanical literature - binary, or binomial nomenclature (from the Latin binominalis).
Rosacanina L. - dog rose (rose hip) (Linnaeus)
Ticket number 23
1. Driving forces of evolution(Textbook of biology, grade 9, section 1, chapter 3, §5)
AT evolutionary theory Darwin, the prerequisite for evolution is hereditary variability, and the driving forces of evolution are the struggle for existence and natural selection. When creating the evolutionary theory, Ch. Darwin repeatedly refers to the results of breeding practice. He showed that the diversity of varieties and breeds is based on variability. Variability is the process of the emergence of differences in descendants compared to ancestors, which determine the diversity of individuals within a variety or breed. Darwin believes that the causes of variability are the impact on organisms of environmental factors (direct and indirect), as well as the nature of the organisms themselves (since each of them reacts specifically to the impact of the external environment). Darwin, analyzing the forms of variability, singled out three among them: definite, indefinite and correlative.
A certain, or group, variability is a variability that occurs under the influence of some environmental factor that acts equally on all individuals of a variety or breed and changes in a certain direction. Examples of such variability are an increase in body weight in animal individuals with good feeding, a change in the hairline under the influence of climate, etc. A certain variability is massive, covers the entire generation and is expressed in each individual in a similar way. It is non-hereditary, that is, in the descendants of the modified group, under other conditions, the traits acquired by the parents are not inherited.
Indefinite, or individual, variability manifests itself specifically in each individual, that is, it is single, individual in nature. It is associated with differences in individuals of the same variety or breed under similar conditions. This form of variability is indefinite, i.e., a trait under the same conditions can change in different directions. For example, in one variety of plants, specimens appear with different colors of flowers, different intensity of color of petals, etc. The reason for this phenomenon was unknown to Darwin. Indefinite variability is hereditary, that is, it is stably transmitted to offspring. This is its importance for evolution. Darwin comes to the conclusion that only heritable changes are important for the evolutionary process, since only they can accumulate from generation to generation. According to Darwin, the main factors in the evolution of cultural forms are hereditary variability and human selection (Darwin called such selection artificial). Variability is a necessary prerequisite for artificial selection, but it does not determine the formation of new breeds and varieties.
Darwin considered the explanation of the historical variability of species possible only through the disclosure of the causes of adaptability to certain conditions. He came to the conclusion that the adaptability of natural species, as well as cultural forms, is the result of selection, which was carried out not by man, but by environmental conditions.
How is natural selection carried out? Darwin considers one of its most important conditions in the natural environment to be the overpopulation of species, which arises as a result of a geometric progression of reproduction. Darwin drew attention to the fact that individuals of species that give even relatively small real offspring eventually reproduce quite intensively. For example, ascaris produces up to 200 thousand eggs per day, the female perch spawns 200-300 thousand eggs, and cod - up to 10 million eggs.
Overpopulation is the main (although not the only) cause of the struggle for existence between organisms. In the concept of "struggle for existence" he puts a broad and metaphorical meaning.
The struggle of organisms occurs both among themselves and with the physico-chemical conditions of the environment. It has the character of direct collisions between organisms or, more often, indirect conflicts. Competing organisms may not even come into contact with each other and still be in a state of fierce struggle (for example, spruce and acid growing under it).
The natural result of contradictions between organisms and external environment is the extermination of a part of the individuals of the species (elimination). The struggle for existence is thus the eliminating factor.
The scheme of action of natural selection in the species system according to Darwin is as follows:
Variability is inherent in any group of animals and plants, and organisms differ from each other in many ways.
The number of organisms of each species that are born into the world exceeds the number of those that can find food and survive. However, since the abundance of each species is constant under natural conditions, it should be assumed that most of the offspring perish. If all the descendants of any one species survived and multiplied, they would very soon outcompete all other species on the globe.
Since more individuals are born than can survive, there is a struggle for existence, competition for food and habitat. This may be an active life-and-death struggle, or less obvious, but no less effective competition, as, for example, for plants during a period of drought or cold.
Among the many changes observed in living beings, some make it easier to survive in the struggle for existence, while others lead to the fact that their owners die. The concept of "survival of the fittest" is the core of the theory of natural selection.
Surviving individuals give rise to the next generation, and in this way "fortunate" changes are transmitted to subsequent generations. As a result, each next generation is more adapted to the environment; as the environment changes, further adaptations occur. If natural selection has been operating for many years, then the last offspring may turn out to be so dissimilar to their ancestors that it would be advisable to single them out as an independent species.
It may also happen that some members of a given group of individuals will acquire some changes and be adapted to environment in one way, while its other members, having a different set of changes, will be adapted in a different way; in this way, two or more species may arise from one ancestral species, provided that such groups are isolated.
Morphology of multicellular animals
The body of multicellular organisms consists of a collection of many cells, groups of which are specialized in the implementation of certain functions, forming tissues. Complexes of tissues form the highest category - organs. The functional activity of the organs constitutes the organ system, for example, 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 organism.
An idea about the features of the structure 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, get food for itself, or perform other functions, i.e. they are interconnected in effectively interacting systems.
The number of cells in different multicellular organisms is not the same. So, for example, in primitive invertebrates $10^2 -10^4$, in highly organized vertebrates the number is represented from $10^(15)$ to $10^(17)$. The average mass of a cell weighs about $10^(-8)-10^(-9)$ g.
Cells are characterized by two vital systems:
- The system associated with the reproduction, development, and growth of the cell. Such a cell includes structures that will ensure DNA replication, RNA and protein synthesis.
- Energy supply system for the synthesis of substances and other types of physiological work of the cell.
Both systems are closely related. In addition, cell elements of different origin are characterized by similarities at different levels: atomic - carbon, hydrogen, oxygen, etc., molecular - nucleic acids, proteins, carbohydrates, etc., supramolecular - membrane structures and cell organelles.
Cells also have chemical processes Key words: respiration, consumption and transformation of energy, synthesis of macromolecules. All chemical reactions cells are well ordered and inextricably linked to molecular structures.
Evolutionary features in the morphology of a multicellular organism
Multicellular represent a leap in evolution, since in relation to unicellular they have more advantages in organization.
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, directly, to the complication of the structure and physiological functions. As a result, the increase in the size of the body 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 size of the body of animals led to the emergence of intratransport circulating systems. Such systems supply nutrients removed from the surface of the body, and also remove the end products of metabolism from the body. Blood became the main transport system.
body symmetry
According to the type of symmetry of the body, the following groups are distinguished:
- Radiant, or radially symmetrical;
- Bilaterally symmetrical.
Radiation symmetry is characteristic of animals with a sedentary lifestyle. The organs of such animals are arranged around the main axis, and pass through the mouth to the oppositely attached pole. Such animals include the Sponge type, the Coelenterates type and the Echinoderm type.
Bilaterally symmetrical animals are mobile. The body is on the same plane, on both sides of which are paired organs. The body is divided into left and right sides, dorsal and ventral sides, as well as 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 cavity. The primary body cavity is the presence of a remnant of the blastula, in which mesoderm derivatives develop. Such a cavity is typical for three-layer low-organized animals such as Roundworms.
Secondary body cavity, or coelom lined with epithelium from the mesoderm. Such a cavity is characteristic of the types Annelids, Mollusks and Chordates.
With a mixed body cavity, the rudiments of a secondary cavity develop, but this process does not go 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 phylum Arthropoda.
Remark 1
Flatworms do not have a body cavity at all, they have a muscular sac filled with parenchyma cells.
Body cavity functions:
- Free arrangement of organs;
- reference;
- transport of nutrients;
- Sexual.
The animal world is large and diverse. Animals are animals, but adults decided to divide them all into groups according to some characteristics. The science of classifying animals is called systematics or taxonomy. This science determines the relationship between organisms. The degree of relationship is not always determined by external similarity. For example, marsupial mice are very similar to ordinary mice, and tupai are very similar to squirrels. However, these animals belong to different orders. But armadillos, anteaters and sloths, completely different from each other, are united in one squad. The fact is that family ties between animals are determined by their origin. By studying the structure of the skeleton and the dental system of animals, scientists determine which animals are closest to each other, and paleontological finds of ancient extinct animal species help to establish more accurately the relationship between their descendants.
Types of multicellular animals: sponges, bryozoans, flat, round and annelids(worms), coelenterates, arthropods, mollusks, echinoderms and chordates. Chordates are the most progressive type of animal. They are united by the presence of a chord - the primary skeletal axis. The most highly developed chordates are grouped into the vertebrate subphylum. Their notochord is transformed into a spine. The rest are called invertebrates.
Types are divided into classes. In total there are 5 classes of vertebrates: fish, amphibians, birds, reptiles (reptiles) and mammals (animals). Mammals are the most highly organized animals of all vertebrates.
Classes can be divided into subclasses. For example, in mammals, subclasses are distinguished: viviparous and oviparous. Subclasses are divided into infraclasses, and then into detachments. Each squad is divided into families, families - on childbirth, childbirth - on kinds. Species is the specific name of an animal, such as a white hare.
Classifications are approximate and change all the time. For example, now lagomorphs have been taken out of rodents into an independent detachment.
In fact, those groups of animals that are studied in primary school- these are types and classes of animals, given mixed up.
The first mammals appeared on Earth about 200 million years ago, having separated from the animal-like reptiles.
The emergence of multicellularity was the most important stage in the evolution of the entire animal kingdom. The dimensions of the body of animals, previously limited to one cell, in multicellular organisms increase significantly due to an increase in the number of cells. The body of multicellular organisms consists of several layers of cells, at least two. Among the cells that form the body of multicellular animals, there is a separation of functions. Cells differentiate into integumentary, muscular, nervous, glandular, sex, etc. In most multicellular complexes of cells that perform the same functions, they form the corresponding tissues: epithelial, connective, muscle, nerve, and blood. The tissues, in turn, form complex organs and organ systems that provide the vital functions of the animal.
Multicellularity greatly expanded the possibilities of the evolutionary development of animals and contributed to the conquest of all possible habitats by them.
All multicellular animals reproduce sexually. Sex cells - gametes - are formed in them very similarly, by cell division - meiosis - which leads to a reduction, or reduction, in the number of chromosomes.
All multicellular organisms have a certain life cycle: a fertilized diploid egg - a zygote - begins to split up and gives rise to a multicellular organism. When the latter matures, sex haploid cells - gametes are formed in it: female - large eggs or male - very small spermatozoa. The fusion of an egg with a sperm cell is fertilization, as a result of which a diploid zygote, or a fertilized egg, is formed again.
Modifications of this main cycle in some groups of multicellular organisms can occur a second time in the form of alternation of generations (sexual and asexual), or replacement of the sexual process by parthenogenesis, i.e., sexual reproduction, but without fertilization.
Asexual reproduction, so characteristic of the vast majority of unicellular organisms, is also characteristic of the lower groups of multicellular organisms (sponges, coelenterates, flat and annelids, and partly echinoderms). Very close to asexual reproduction is the ability to restore lost parts, called regeneration. It is inherent to one degree or another in many groups of both lower and higher multicellular animals that are not capable of asexual reproduction.
Sexual reproduction of multicellular animals
All cells of the body of multicellular animals are divided into somatic and sexual. Somatic cells (all body cells, except sex cells) are diploid, that is, all chromosomes are represented in them by pairs of similar homologous chromosomes. Sex cells have only a single, or haploid, set of chromosomes.
Sexual reproduction of multicellular organisms occurs with the help of germ cells: the female egg, or egg, and the male germ cell, the sperm. The process of fusion of the egg and sperm is called fertilization, resulting in a diploid zygote. A fertilized egg receives from each parent a single set of chromosomes, which again form homologous pairs.
From a fertilized egg, by its repeated division, a new organism develops. All cells of this organism, except for the sex cells, contain the initial diploid number of chromosomes, the same as those of its parents. The preservation of the number and individuality of chromosomes (karyotype) characteristic of each type is ensured by the process of cell division - mitosis.
Sex cells are formed as a result of a special modified cell division called meiosis. Meiosis results in the reduction, or halving, of the number of chromosomes through two successive cell divisions. Meiosis, like mitosis, proceeds very similarly in all multicellular organisms, in contrast to unicellular organisms, in which these processes vary greatly.
In meiosis, as in mitosis, the main stages of division are distinguished: prophase, metaphase, anaphase and telophase. The prophase of the first division of meiosis (prophase I) is very complex and the longest. It is divided into five stages. In this case, paired homologous chromosomes, obtained one from the maternal and the other from the paternal organism, are closely connected or conjugated with each other. The conjugating chromosomes thicken, and at the same time it becomes noticeable that each of them consists of two sister chromatids connected by a centromere, and together they form a quadruple of chromatids, or a tetrad. During conjugation, chromatid breaks and the exchange of identical sections of homologous, but not sister chromatids from the same tetrad (from a pair of homologous chromosomes) can occur. This process is called chromosome crossing or crossing over. It leads to the formation of compound (mixed) chromatids containing segments obtained from both homologues, and therefore from both parents. At the end of prophase I, homologous chromosomes line up in the plane of the cell equator, and achromatin spindle filaments are attached to their centromeres (metaphase I). The centromeres of both homologous chromosomes repel each other and move to different poles of the cell (anaphase I, telophase I), which leads to a reduction in the number of chromosomes. Thus, only one chromosome from each pair of homologues enters each cell. The resulting cells contain half, or haploid, the number of chromosomes.
After the first division of meiosis, the second usually follows almost immediately. The phase between these two divisions is called interkinesis. The second division of meiosis (II) is very similar to mitosis, with a greatly shortened prophase. Each chromosome consists of two chromatids held together by a centromere. In metaphase II, the chromosomes line up in the equatorial plane. In anaphase II, the division of the centromeres occurs, after which the spindle threads pull them apart to the division poles, and each chromatid becomes a chromosome. Thus, four haploid cells are formed from one diploid cell during meiosis. In the male body, spermatozoa are formed from all cells; in the female, only one of the four cells turns into an egg, and three (small polar bodies) degenerate. The complex processes of gametogenesis (spermato- and oogenesis) in all multicellular organisms are very similar.
sex cells
In all multicellular animals, germ cells are differentiated into large, usually immobile female cells - eggs - and very small, more often mobile male cells - spermatozoa.
The female sex cell - an egg - is most often spherical, and sometimes more or less elongated. The egg cell is characterized by the presence of a significant amount of cytoplasm, in which a large bubble-shaped nucleus is placed. Outside, the egg is dressed in more or less shells. The egg cells in most animals are the largest cells in the body. However, their sizes are not the same in different animals, which depends on the amount of nutritious yolk. There are four main types of egg structure: alecithal, homolecital, telolecital and centrolecital eggs.
Alecithal eggs are almost devoid of yolk or contain very little of it. Alecithal eggs are very small, they are characteristic of some flatworms and mammals.
Homolecithal, or isolecithal, eggs contain relatively little yolk, which is distributed more or less evenly in the cytoplasm of the egg. The nucleus occupies an almost central position in them. Such are the eggs of many mollusks, echinoderms, etc. However, some homolecithal eggs have a large amount of yolk (hydra eggs, etc.).
Telolecital eggs always contain a large amount of yolk, which is very unevenly distributed in the cytoplasm of the egg. Most of the yolk is concentrated at one pole of the egg, called the vegetative pole, while the nucleus is shifted to a greater or lesser extent towards the opposite pole, called the animal pole. Such eggs are characteristic of various groups of animals. Telolecithal eggs reach the most large sizes, and depending on the degree of yolk loading, their polarity is expressed to varying degrees. Typical examples of telolecithal eggs are the eggs of frogs, fish, reptiles and birds, and of invertebrates, the eggs of cephalopods.
However, not only telolecithal eggs, but also all other types of eggs, polarity is inherent, i.e. they also have differences in the structure of the animal and vegetative poles. In addition to the indicated increase in the amount of yolk at the vegetative pole, polarity can manifest itself in an uneven distribution of cytoplasmic inclusions, egg pigmentation, etc. There is evidence of differentiation of the cytoplasm at the animal and vegetative poles of the egg.
Centrolecithal eggs are also very rich in yolk, but it is evenly distributed throughout the egg. The nucleus is placed in the center of the egg, it is surrounded by a very thin layer of cytoplasm, the same layer of cytoplasm covers the entire egg near its surface. This peripheral layer of cytoplasm communicates with the perinuclear plasma using thin cytoplasmic filaments. Centrolecithal eggs are characteristic of many arthropods, in particular all insects.
All eggs are covered with the thinnest plasma membrane, or plasmalemma. In addition, almost all eggs are surrounded by another, the so-called yolk membrane. It is formed in the ovary, and it is called the primary membrane. Eggs can also be dressed with secondary and tertiary shells.
The secondary shell, or chorion, of the eggs is formed by the ovarian follicular cells surrounding the egg. best example the outer shell - the chorion - of insect eggs, consisting of solid chitin and equipped with a hole at the animal pole - a micropyle, through which spermatozoa penetrate, can serve.
Tertiary membranes, which usually have a protective value, develop from the secretions of the oviducts or accessory (shell) glands. Such are, for example, the shells of eggs flatworms, cephalopods, gelatinous shells of gastropods, frogs, etc.
Male germ cells - spermatozoa - unlike egg cells, are very small, their sizes range from 3 to 10 microns. Spermatozoa have a very small amount of cytoplasm, their main mass constitutes the core. Due to the cytoplasm, spermatozoa develop adaptations for movement. The shape and structure of spermatozoa of various animals are extremely diverse, but the most common is the form with a long flagella-like tail. Such a spermatozoon consists of four sections: the head, neck, connecting part and tail.
The head is almost entirely formed by the nucleus of the sperm, it carries a large body - the centrosome, which helps the penetration of the sperm into the egg. Centrioles are located on its border with the neck. From the neck, the axial thread of the spermatozoon originates, passing through its tail. According to electron microscopy, its structure turned out to be very close to that of flagella: two filaments in the center and nine along the periphery of the axial filament. In the central part, the axial filament is surrounded by mitochondria, which represent the main energy center sperm.
Fertilization
In many invertebrates, fertilization is external and occurs in water, while in others, internal fertilization takes place.
The process of fertilization consists in the penetration of spermatozoa into the egg and in the formation of one fertilized egg from two cells.
This process occurs differently in different animals, depending on the presence of micropyle, the nature of the membranes, etc.
In some animals, as a rule, one spermatozoon penetrates the egg, and at the same time, due to the yolk membrane of the egg, a fertilization membrane is formed that prevents the penetration of other spermatozoa.
In many animals, a larger number of spermatozoa penetrate the egg (many fish, reptiles, etc.), although only one takes part in fertilization (in fusion with the egg cell).
During fertilization, the hereditary characteristics of two individuals are combined, which ensures greater viability and greater variability of the offspring, and, consequently, the possibility of the appearance of useful adaptations to various living conditions.
Embryonic development of multicellular animals
The whole process, from the beginning of the development of a fertilized egg to the beginning of the independent existence of a new organism outside the mother's body (during live birth) or after it leaves the egg shells (during oviparity), is called embryonic development.
Gallery
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 choanoderma, and a jelly-like mesochil 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. In 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, it became extremely important in the appearance of intratransport circulatory systems, delivering life-support means - nutrients, oxygen, to tissues remote from the surface of the body - 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, due to 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).
Given the dependence 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 for 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 basic 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.
general characteristics multicellular - concept and types. Classification and features of the category "General characteristics of multicellular" 2017, 2018.