Invertebrate animals are characterized by the presence of several sources of origin of nerve cells. In the same type of animal, nerve cells can simultaneously and independently originate from three different germ layers.
Polygenesis of invertebrate nerve cells is the basis for the diversity of mediator mechanisms in their nervous system.
The nervous system first appeared in coelenterates.
Specialized nerve cells appear in hydra and other coelenterates. The nerve cells of coelenterates are not separated from each other by synapses and are not united into the nervous system, but either represent individual branched cells or form a nervous network consisting of cells interconnected by branched processes. An impulse originating in one part of the body can spread in all directions to all other parts of the body.
Hydra nerve cells are not differentiated into sensory, intercalary and motor neurons, but simply some branches of the nervous network are directed to receptor cells, and others to contractile cells. However, jellyfish and sea anemones tend to group neurons into nerve chains. Neurons are usually connected by synapses, and differentiation of nerve cells into non-sensory, ganglion and motor neurons is observed. In the further evolutionary series, neurons, synapses and neuromuscular junctions changed little.
Nervous system provides a connection between the body and environment, coordinated work of organs, their systems and the whole organism, a more active lifestyle of the animal.
The nervous system of the polyp is primitive, diffuse type, consisting of nerve cells scattered throughout the body. Thanks to the contraction of fibers (outgrowths of integumentary muscle cells - biofile.ru note), the polyp is able to move.
The nervous system of the hydra consists of stellate nerve cells connected by their processes.
The nervous system of jellyfish is much more complex than that of polyps. In jellyfish, in addition to the general subcutaneous nerve plexus Along the edge of the umbrella there is an accumulation of ganglion cells, which, together with their processes, form a continuous nerve ring. It innervates the muscle fibers of the sail, as well as special sensory organs located along the edge of the umbrella.
In some jellyfish these organs look like eyes, while in others they look like statocytes, which are not only organs of balance, but also devices that stimulate contractile movements of the edges of the umbrella: if you cut out all the statocytes from a jellyfish, it will stop moving.
The simplicity of the nervous system of these animals gives them a great advantage in life - they can regenerate both individual lost parts of the body and the entire body from one tenth of it. The disadvantage is that they do not have a structured nervous system, which only perceives information about changes in the environment, but does not provide the ability to quickly and correctly respond to these changes.
Basic terms and concepts tested in the examination paper: bilayer animals, hydroid, glandular cells, ectoderm cells, endoderm cells, coral polyps, jellyfish, nerve cells, stinging cells, scyphoid cells, development cycle of coelenterates.
Coelenterates- one of oldest groups multicellular animals, numbering 9000 thousand species. These animals lead an aquatic lifestyle and are common in all seas and freshwater bodies. Descended from colonial protozoa - flagellates. Coelenterates lead a free or sedentary lifestyle. The phylum Coelenterata is divided into three classes: Hydroid, Scyphoid and Coral polyps.
The most important common feature Coelenterates are considered to have a two-layer body structure. It consists of ectoderm And endoderm , between which there is a non-cellular structure - mesoglea. These animals got their name because they have intestinal cavity in which food is digested.
Basic aromorphoses, which contributed to the emergence of coelenterates, are the following:
– the emergence of multicellularity as a result of specialization and association;
– cells interacting with each other;
– the appearance of a two-layer structure;
– the occurrence of cavity digestion;
– the appearance of body parts differentiated by function; the appearance of radial or radial symmetry.
Hydroid class. Representative - freshwater hydra.
Hydra is a polyp about 1 cm in size.
Lives in freshwater bodies. It is attached to the substrate by the sole. The front end of the body forms a mouth surrounded by tentacles. Outer layer of the body - ectoderm consists of several types of cells differentiated by their functions:
– epithelial-muscular, ensuring the movement of the animal;
– intermediate, giving rise to all cells;
– stinging insects that perform a protective function;
– sexual, ensuring the process of reproduction;
– nerves, united into a single network and forming the first in organic world nervous system.
Endoderm consists of: epithelial-muscular, digestive cells and glandular cells that secrete digestive juice.
Hydra, like other coelenterates, has both intracellular and intracellular digestion. Hydras are predators that feed on small crustaceans and fish fry. Breathing and excretion in hydras is carried out over the entire surface of the body.
Irritability manifests itself in the form of motor reflexes. The tentacles react most clearly to irritation, because Nerve and epithelial-muscle cells are most densely concentrated in them.
Reproduction occurs budding And sexually. The sexual process occurs in the fall. Some intermediate cells ectoderms turn into germ cells. Fertilization occurs in water. In the spring, new hydras appear. Among the coelenterates there are hermaphrodites and dioecious animals.
Many coelenterates are characterized by alternating generations. For example, jellyfish are formed from polyps. Larvae develop from fertilized jellyfish eggs - planulae. The larvae develop into polyps again.
Hydras are able to restore lost body parts due to the reproduction and differentiation of nonspecific cells. This phenomenon is called regeneration .
Class Scyphoid. Unites jellyfish large sizes. Representatives: Kornerot, Aurelia, Cyanea.
Jellyfish live in the seas. The body resembles an umbrella in shape and consists mainly of gelatinous mesoglea, covered on the outside with a layer of ectoderm, and on the inside with a layer of endoderm. Along the edges of the umbrella there are tentacles surrounding the mouth, located on the underside. The mouth leads into the gastric cavity, from which radial canals extend. The channels are connected to each other by a ring channel. As a result, gastric system .
The nervous system of jellyfish is more complex than that of hydras. In addition to the general network of nerve cells, along the edge of the umbrella there are clusters of nerve ganglia, forming a continuous nerve ring and special balance organs - statocysts. Some jellyfish develop light-sensitive eyes and sensory and pigment cells corresponding to the retina of higher animals.
IN life cycle Jellyfish naturally alternate between sexual and asexual generations. They are dioecious. The gonads are located in the endoderm under the radial canals or on the oral stalk. Reproductive products exit through the mouth into the sea. A free-living larva develops from the zygote. planula. The planula turns into a small polyp in the spring. Polyps form groups similar to colonies. Gradually they disperse and turn into adult jellyfish.
Class Coral polyps. Includes solitary (anemones, brain sea anemones) or colonial forms (red coral). They have a calcareous or silicon skeleton formed by needle-shaped crystals. They live in tropical seas. Clusters of coral polyps form coral reefs. They reproduce asexually and sexually. Coral polyps do not have a jellyfish stage of development.
EXAMPLES OF TASKS
Part A
A1. One of the major aromorphoses in coelenterates was the emergence
1) stinging cells
2) multicellularity
3) intracellular digestion
4) budding abilities
A2. Polyp is the name
1) type of animal
2) class of animals
3) animal subkingdoms
4) stages of animal development
A3. The cells from which all other hydra cells are formed are called
1) glandular 3) stinging
2) intermediate 4) epithelial-muscular
A4. The endoderm of Hydra contains cells
1) intermediate 3) glandular
2) sexual 4) nervous
A5. From a zygote, jellyfish first develop
1) planula 3) adult form
2) polyp 4) colony of polyps
A6. The nervous system is the most complex structure
1) hydra 3) cornerrota
2) brain sea anemone 4) sea anemone
A7. The gonads of jellyfish develop into
1) ectoderm 3) mesoglea
2) stomach pockets 4) throat
A8. Internal skeleton have
1) aurelia 3) sea anemone
2) hydra 4) cornerrota
A9. The nervous system of coelenterates consists of
1) single cells
2) individual nerve nodes
3) one nerve
4) interconnected nerve cells
Part B
IN 1. Select the cells found in the ectoderm of Hydra
1) glandular 4) digestive
2) intermediate 5) stinging
3) nervous 6) sexual
Part C
C1. Why do reef-building corals live at depths not exceeding 50 m?
Hydra stinging cells
The endoderm of Hydra contains cells
Contraction of the hydra body is carried out by cells
Refers to coelenterates
Refers to coelenterates
Refers to coelenterates
Refers to coelenterates
Body wall of coelenterates
According to the method of feeding the sponge
They live only in warm, salty tropical seas.
Lives in fresh water bodies
Coral polyps have no stage in their life cycle
Asexual reproduction in hydra is carried out by
Regeneration in hydra is ensured by cells
Digestion of food in hydra is ensured by cells
Refers to coelenterates
Organs and tissues are not expressed in representatives of the type
-: Coelenterates
-: Flatworms
-: Roundworms
-: Shellfish
-: roundworm
-: slug
-: stinging
-: epithelial
-: muscular
-: glandular
-: intermediate
-: stinging
-: nervous
-: intermediate
-: dermal-muscular
-: glandular
-: amitosis
-: sporulation
-: schizogony
-: budding
-: budding
-: crushing
-: gastrulation
-: blastulation
-: sea anemone
-: aurelia
-: corner mouth
-: hydroid polyps
-: scyphoid jellyfish
-: hydroid jellyfish
-: coral polyps
-: Mechnikov
-: Mechnikov
-: Haeckel
-: Zakhvatkin
The skeleton of sponges is formed in
-: ectoderm
-: endoderm
-: mesoderm
-: mesoglea
-: cuticle
Participates in the formation of the skeleton in sponges
-: archaeocytes
-: collencites
-: scleroblasts
-: amebocytes
-: choanocytes
Based chemical composition skeletal based classification
-: coelenterates
-: double-layer
-: larval chordates
-: arthropods
-: Coelenterates
-: Flatworms
-: Roundworms
-: Shellfish
-: filterers
-: predators
-: detritivores
-: saprophages
-: single layer
-: double layer
-: three-layer
-: four-layer
-: five-layer
-: roundworm
-: sea anemone
-: slug
-: roundworm
-: slug
-: planaria
-: corner mouth
-: slug
-: roundworm
-: sea anemone
-: slug
-: epithelial
-: muscular
-: intermediate
-: nervous
-: stinging
-: stinging
-: nervous
-: genitals
-: glandular
-: sensitive
-: have excitability
-: serve for protection
-: provide movement
-: participate in digestion
-: promote fertilization
-: cylindrical
-: staircase
-: diffuse
-: nerve cord
-: ring-shaped
Read also:
Type Coelenterates.
Question 1. What are the features external structure hydra?
Hydra is an elongated sac-shaped polyp, reaching 1.5 cm in length. It is attached to the substrate by a sole located at one end of the body. At the other end there is a mouth opening surrounded by a corolla of tentacles. The hydra body wall is formed by two layers of cells: the outer - ectoderm and the inner - endoderm.
Question 2. How is the ectoderm of coelenterates structured?
Several types of cells can be distinguished in the ectoderm. The bulk is represented by epithelial-muscle cells that have processes in which contractile elements are concentrated. Also in the ectoderm are sensory, nervous, glandular, stinging and intermediate cells. Sensitive cells are located in the same way as epithelial-muscular cells, i.e., one end faces outward and the other is adjacent to the basement membrane. Nerve cells lie between the contractile processes on the basement membrane. Intermediate cells are undifferentiated cells from which specialized cells subsequently develop; in addition, they participate in regeneration. Sex cells are formed in the ectoderm.
Question 3. What type of nervous system do coelenterates have?
Coelenterates have a diffuse type of nervous system. Sensitive cells are located in the same way as epithelial-muscular cells, i.e., one end faces outward and the other is adjacent to the basement membrane. Nerve cells lie between the contractile processes on the basement membrane. If you touch the hydra, the excitation that arises in the primary cells quickly spreads throughout the entire nervous network and the animal responds to the irritation by contracting the processes of the epithelial-muscle cells.
Question 4.
How does a hydra stinging cell work?
The largest number of stinging cells are located in the tentacles. Inside the cell there is a stinging capsule with a poisonous liquid and a spirally coiled hollow thread. On the surface of the cell there is a sensitive spine that perceives external influences. In response to irritation, the stinging capsule throws out the thread it contains, which turns out like the finger of a glove. Burning or poisonous contents are released along with the thread. Thus, hydroids can immobilize and paralyze fairly large prey, such as cyclops or daphnia. Stinging cells are replaced with new ones after use.
Question 5. What cells form the inner layer of the hydra?
The cellular elements of the endoderm are represented by epithelial-muscular and glandular cells. Epithelial muscle cells often have flagella and processes resembling pseudopodia. Glandular cells secrete digestive enzymes into the digestive cavity: greatest number such cells are located near the mouth.
Question 6. Tell us about the nutrition of hydra.
Hydra is a predator. It feeds on plankton - ciliates, small crustaceans (cyclops and daphnia). The stinging threads entangle the prey and paralyze it. Then the hydra grabs it with its tentacles and directs it into the mouth opening.
Question 7. How is the hydra’s digestion process carried out?
Digestion in hydras is combined (intracavitary and intracellular). Swallowed food enters the digestive cavity. First, the food is processed with enzymes and crushed in the digestive cavity. The food particles are then phagocytosed by epithelial muscle cells and digested in them. Nutrients are diffusely distributed among all cells of the body. From the cells, metabolic products are released into the digestive cavity, from where, along with undigested food debris, they are released into the environment through the mouth.
Question 8. What are intermediate cells, what are their functions?
Intermediate cells are undifferentiated cells that give rise to all other types of ecto- and endoderm cells. These cells ensure the restoration of body parts when damaged - regeneration.
Question 9. What is hermaphroditism?
Hermaphroditism is the simultaneous presence of both male and female organs in one organism.
Question 10. How does hydra reproduce and develop?
Hydra reproduces asexually and sexually. During asexual reproduction, which occurs during a period favorable for life, one or more buds are formed on the body of the mother’s body, which grow, their mouth breaks through and tentacles form. Daughter individuals are separated from the mother. Hydras do not form real colonies. Sexual reproduction occurs in the fall. Hydras are mostly dioecious, but there are also hermaphrodites. Sex cells are formed in the ectoderm. In these places, the ectoderm swells in the form of tubercles, in which either numerous spermatozoa or one amoeboid egg are formed. Spermatozoa, equipped with flagella, are released into the environment and delivered to the eggs by a current of water. After fertilization, the zygote forms a shell, turning into an egg. The maternal organism dies, and the shell-covered egg overwinters and begins development in the spring. The embryonic period includes two stages: cleavage and gastrulation. After this, the young hydra leaves the egg shells and goes outside.
Question 11. What are hydromedusae?
Hydromedusae are free-swimming sexual specimens of some representatives of the hydroid class; they are formed by budding.
Question 12. What is a planula?
Planula is a larva covered with cilia. Formed after fertilization in some hydroids. Attaches to underwater objects and gives rise to a new polyp.
Question 13. What is it like? internal structure coral polyp?
Coral polyps have all characteristic features coelenterates. The body of coral polyps has the shape of a cylinder. They have a mouth surrounded by tentacles leading into a throat. The digestive cavity is divided into a large number of chambers, thereby increasing its surface and, consequently, the efficiency of food digestion. There are muscle fibers in the ecto- and endoderm that allow the polyp to change its body shape. A characteristic feature of coral polyps is that most of them have a hard calcareous skeleton or a skeleton consisting of a horn-like substance.
Question 14. What role do coelenterates play in nature?
Coelenterates are predators and occupy a corresponding niche in the food chains of reservoirs, seas and oceans, regulating the number of single-celled organisms, small crustaceans, worms, etc. Some deep-sea species of jellyfish feed on dead organisms. Coral polyps, which live in shallow tropical seas, form the basis of reefs, atolls and islands. These corals play an important role in coastal communities including significant amount animals and plants.
As is known, the nervous system first appears in lower multicellular invertebrates. The emergence of the nervous system is a major milestone in the evolution of the animal world, and in this respect even primitive multicellular invertebrates are qualitatively different from protozoa. An important point here there is already a sharp acceleration of the conduction of excitation in nervous tissue: in uprotoplasm, the speed of excitation conduction does not exceed 1-2 microns per second, but even in the most primitive nervous system, consisting of nerve cells, it is 0.5 meters per second!
The nervous system exists in lower multicellular organisms in a very various forms: reticulate (for example, in hydra), annular (jellyfish), radial (starfish) and bilateral. The bilateral form is presented in the lower (intestinal) flatworms and primitive mollusks (chiton) are still only a network located near the surface of the body, but several longitudinal cords are distinguished by more powerful development. As the nervous system develops progressively, it sinks under the muscle tissue, and the longitudinal cords become more pronounced, especially on the ventral side of the body. Everything at the same time higher value acquires the anterior end of the body, a head appears (the process of cephalization), and with it the brain - an accumulation and compaction of nerve elements at the anterior end. Finally, in higher worms, the central nervous system already fully acquires the typical structure of the “nervous ladder”, in which the brain is located above the digestive tract and is connected by two symmetrical commissures (“periopharyngeal ring”) with the subpharyngeal ganglia located on the abdominal side and then with paired abdominal nerves trunks. The essential elements here are the ganglia, which is why they also speak of the ganglionic nervous system, or the “ganglionic staircase”. In some representatives of this group of animals (for example, leeches), the nerve trunks come together so close that a “nerve chain” is obtained.
Powerful conductive fibers depart from the ganglia, which make up the nerve trunks.
In giant fibers, nerve impulses are carried out much faster due to their large diameter and small number of synaptic connections (places of contact between the axons of some nerve cells and the dendrites and cell bodies of other cells). As for the cephalic ganglia, i.e. brain, then they are more developed in more active animals, which also have the most developed receptor systems.
The origin and evolution of the nervous system are determined by the need to coordinate different-quality functional units of a multicellular organism, harmonize the processes occurring in different parts of it when interacting with external environment, ensuring the activity of a complex organism as a single integrated system. Only a coordinating and organizing center, such as the central nervous system, can provide flexibility and variability in the body's response in a multicellular organization.
The process of cephalisapia was also of great importance in this regard, i.e. separation of the head end of the organism and the associated appearance of the brain. Only in the presence of a brain is truly centralized “coding” of signals coming from the periphery and the formation of integral “programs” of innate behavior possible, not to mention a high degree of coordination of all external activity of the animal.
Of course, the level mental development depends not only on the structure of the nervous system. For example, rotifers, closely related to annelids, also have, like them, a bilateral nervous system and brain, as well as specialized sensory and motor nerves. However, differing little from ciliates in size, appearance and way of life, rotifers are very similar to the latter also in behavior and do not display higher mental abilities than ciliates. This again shows that the leading factor for the development of mental activity is not general structure, but the specific living conditions of the animal, the nature of its relationships and interactions with the environment. At the same time, this example once again demonstrates how carefully one must approach the assessment of “higher” and “lower” characters when comparing organisms occupying different phylogenetic positions, in particular when comparing protozoa and multicellular invertebrates.
The Scyphoid class includes jellyfish that inhabit the seas and oceans (they live only in salt water), which are able to move freely among the expanses of water (with the exception of the sessile jellyfish, which leads a sedentary lifestyle).
general characteristics
Scyphoid jellyfish live everywhere; they have adapted to life in cold and warm waters. There are about 200 species. They are transported over considerable distances with the current, but can also move independently. Thus, with the help of active contractions of the dome and the release of water from it, the jellyfish can develop greater speed. This method of movement was called reactive.
The jellyfish has the shape of an umbrella or a longitudinally elongated dome. There are quite large species. Some representatives of the scyphoid class reach 2 m in diameter (Cyanea arctica). Many tentacles extend from the edges of the bell, which can grow up to 15m in length. They contain stinging cells that contain toxic substances necessary for protection and hunting.
Structural features
In the middle of the inner concave part of the umbrella there is a mouth, the corners of which turn into oral lobes (necessary for capturing food). In rootostomes, they grow together and form a filtering apparatus for absorbing small plankton.
Scyphoids are endowed with a stomach with 4 pocket-like protrusions, and a system of radial tubules, with the help of which nutrients from the intestinal cavity spread throughout the body. Undigested food particles are sent back to the stomach and eliminated through the mouth.
The body of jellyfish consists of two layers of epithelial cells: ectoderm and endoderm, between them is mesoglea - jelly-like tissue. It consists of 98% water, so jellyfish quickly die under the scorching sun. Jellyfish have enormous regenerative abilities; if you cut it into 2 parts, each will grow into a full-fledged individual.
Since scyphoid jellyfish have switched to an active way of life, their nervous system has become more developed. At the edges of the umbrella there are clusters of nerve cells; nearby there are also sensory organs that perceive light stimuli and help maintain balance.
Life cycle and reproduction
Scyphoids go through two phases in their life cycle: sexual (jellyfish) and asexual (polyp).
All representatives are dioecious organisms. The germ cells originate from the endoderm and mature in the pouches of the gastric cavity.
The gametes exit through the mouth and end up in the water. During the process of fusion of germ cells and further maturation, a jellyfish larva, a planula, emerges from the egg. It sinks to depth, attaches to the bottom and enters the asexual phase.
A single polyp (scyphostoma) leads a benthic life and begins to reproduce through lateral budding. After a certain time, the scyphistoma turns into a strobila, then the tentacles begin to shorten, and transverse constrictions form on the body. This is how a division called strobilation begins. Thus, the strobila gives life to young organisms - ethers. The ethers are then converted into adults.
Lifestyle
Scyphoid jellyfish do not live in schools and do not transmit signals to each other, even when at close range. Life expectancy is about 2-3 years, sometimes it happens that a jellyfish lives only a couple of months. They are also often eaten by fish and turtles.
All jellyfish are predatory animals. They consume plankton and small fish, which are immobilized by poisonous cells. Stinging cells release poison not only during hunting, but also to all organisms passing by. Therefore, jellyfish are dangerous for people in the water. If you accidentally catch the tentacles of a jellyfish, it will burn your skin with its poison.
The most common representatives of the class of scyphoid jellyfish are Aurelia, Cyanea, which inhabits the Arctic seas, and Cornerot, which is devoid of tentacles and lives in the waters of the Black Sea.
Meaning in nature and human life
Scyphoid jellyfish are part of the food chain of the world's oceans.
In Chinese and Japanese cuisine, dishes with rhopilema or aurelia are often found. Jellyfish meat is considered a delicacy.
Cornerot – largest jellyfish Black Sea with a dome diameter of about 40 cm. Thus, it serves as a shelter for fish fry and protects against predators and unfavorable conditions environment. Sometimes, when the fry grow up, they begin to bite off small pieces of the jellyfish, or may eat it altogether.
Scyphoid jellyfish filter water, clearing it of contaminants.
For humans, the dangerous poison of jellyfish, which causes skin burns, sometimes provokes a painful shock and a person, being at depth, can no longer surface on his own. It is not safe to touch a jellyfish even when it is dead. Develops when touched allergic reaction, disruption of the nervous and cardiovascular systems, convulsive attacks occur.
Jellyfish, with the exception of some deviations in the organization of the digestive system, are built according to the same pattern as polyps, but are often strongly flattened in a plane perpendicular to the main axis of the body (Fig. 96).
The jellyfish has the appearance of a bell or an umbrella; the outer convex side is called exumbrella, the inner concave side is called subumbrella (Fig. 97). In the middle of the latter, a more or less long oral stalk protrudes with a mouth at the free end. The mouth leads into the digestive, or gastric, cavity, consisting of a central stomach and radial canals diverging from it to the edges of the umbrella, a number equal to or a multiple of four, and connected in the thickness of the mesoglea by a continuous endodermal plate. At the edge of the umbrella, all radial channels communicate with each other through an annular channel. The stomach and canals together form the gastrovascular (i.e., intestinal) system.
A thin ring-shaped muscular membrane is attached along the free edge of the umbrella, narrowing the entrance to the bell cavity. It is called a sail and is characteristic feature hydroid jellyfish, distinguishing them from jellyfish belonging to Scyphozoa. The sail plays an important role in the movement of jellyfish. There are tentacles on the edge of the umbrella. They, like radial channels, are available in a certain number, most often a multiple of four. Due to the correct arrangement of radial canals and tentacles, the radiant symmetry of jellyfish is pronounced.
The body of jellyfish is characterized by the strong development of mesoglea, which becomes very thick and contains a large amount of water, acquiring a gelatinous jelly-like appearance. Thanks to this, the entire body of jellyfish is almost glassy and transparent. Transparency, characteristic of many planktonic animals, is considered as a special kind of protective coloration that hides the animal from enemies.
The nervous system of jellyfish is much more complex than that of polyps. In jellyfish, in addition to the common subcutaneous nerve plexus, along the edge of the umbrella, clusters of ganglion cells are observed, which, together with the processes, form a continuous nerve ring. It innervates the muscle fibers of the sail, as well as special sensory organs located along the edge of the umbrella. In some hydroid jellyfish these organs look like ocelli, in others they are so-called statocysts, or balance organs (Fig. 97, Fig. 98).
The eyes of jellyfish in their most primitive form are arranged like simple eyespots. At the base of some tentacles there is a small area of ectodermal epithelium, consisting of cells of two genera. Some of them are high - sensitive, or retinal, cells; others contain numerous brown or black grains of pigment and alternate with sensitive cells, the totality of which corresponds to the retina of higher animals. The presence of pigment is generally characteristic of the organs of vision throughout the animal kingdom.
The structure of the eye pits is more complex, where the pigmented area of the epithelium lies at the bottom of a small invagination of the integument. Such withdrawal of the eye from the surface of the body into the depths protects it from various purely mechanical irritations, for example, friction with water, touch of foreign objects, etc. In addition, invagination of the eye leads to an increase in the surface of the photosensitive layer and the number of retinal cells. Finally, in some jellyfish, the cavity of the eye fossa is filled with a transparent secretion of the ectoderm, which takes the form of a refractive lens. In this way, a lens appears, concentrating light rays on the retina of the eye.
The organs of balance can be arranged differently: in the form of sensitive tentacles, but most often in the form of deep epithelial pits, which can detach from the surface of the body and turn into closed vesicles, or statocysts (Fig. 98). The vesicle is lined with sensitive ectodermal epithelium and filled with fluid. One of the cells of the vesicle protrudes inside it in the form of a club swollen at the end, inside of which one or several concretions of lime carbonate are secreted. These are statoliths, or auditory pebbles, and they are as characteristic of the organs of balance as pigment is for the organs of vision. The sensitive cells of the vesicle are each equipped with a long sensitive hair directed towards the club located in the center. The structure of the hair is similar to the structure of the cnidocil of stinging cells. The function of jellyfish statocysts more or less corresponds to the functions of the semicircular canals of the human ear. The hairs of sensitive cells in the statocysts of jellyfish are built according to the same type as the sensitive hairs of the receptor organs of more highly organized animals, including vertebrates.
Statocysts of jellyfish are considered not only organs of balance, but also devices that stimulate contractile movements of the edges of the umbrella: if you cut out all the statocysts from a jellyfish, it will stop moving.
Jellyfish swim in the water column, partly carried by sea currents, partly moving actively through the action of muscle fibers located along the edge of the umbrella and in the sail. By simultaneous contraction of the umbrella and sail and their subsequent relaxation, the water that is in the concavity of the umbrella is either pushed out of it or passively fills it again. When the water is pushed out, the animal receives a reverse push and moves forward with the convex side of the umbrella. Due to the alternating contractions and relaxations of the umbrella and sail, the movement of jellyfish consists of a series of intermittent shocks.
Jellyfish are predators. With their tentacles they capture and kill various small animals, swallow them and digest them in the gastric cavity.
Jellyfish have muscles. True, they are very different from human muscles. How are they structured and how does a jellyfish use them for movement?
Jellyfish are fairly simple creatures compared to humans. Their body lacks blood vessels, heart, lungs and most other organs. Jellyfish have a mouth, often located on a stalk and surrounded by tentacles (visible below in the picture). The mouth leads into a branched intestine. A b O Most of the jellyfish's body is made up of an umbrella. Tentacles also often grow on its edges.
The umbrella may shrink. When the jellyfish contracts the umbrella, water is released from under it. A recoil occurs, pushing the jellyfish in the opposite direction. Often such movement is called reactive (although this is not entirely accurate, but the principle of movement is similar).
The umbrella of a jellyfish consists of a gelatinous elastic substance. It contains a lot of water, but also strong fibers made from special proteins. The upper and lower surfaces of the umbrella are covered with cells. They form the integument of the jellyfish - its “skin”. But they are different from our skin cells. Firstly, they are located in only one layer (we have several dozen layers of cells in the outer layer of skin). Secondly, they are all alive (we have dead cells on the surface of our skin). Third, the integumentary cells of jellyfish usually have muscular processes; That's why they are called dermal-muscular. These processes are especially well developed in cells on the lower surface of the umbrella. Muscle processes stretch along the edges of the umbrella and form the circular muscles of the jellyfish (some jellyfish also have radial muscles, located like spokes in an umbrella). When the circular muscles contract, the umbrella contracts and water is thrown out from under it.
It is often written that jellyfish do not have real muscles. But it turned out that this was not the case. In many jellyfish, under the layer of skin-muscle cells on the lower side of the umbrella, there is a second layer - real muscle cells (see figure).
Humans have two main types of muscles - smooth and striated. Smooth muscles consist of ordinary cells with a single nucleus. They ensure contraction of the walls of the intestines and stomach, bladder, blood vessels and other organs. Striated (skeletal) muscles in humans consist of huge multinucleated cells. They are responsible for the movement of our arms and legs (as well as our tongue and vocal cords when we speak). Striated muscles have a characteristic striation and contract faster than smooth muscles. It turned out that in most jellyfish, movement is also ensured by striated muscles. Only their cells are small and mononuclear.
In humans, striated muscles are attached to the bones of the skeleton and transmit forces to them during contraction. And in jellyfish, the muscles are attached to the gelatinous substance of the umbrella. If a person bends his arm, then when the biceps relaxes, it extends due to the action of gravity or due to the contraction of another muscle - the extensor. Jellyfish do not have “umbrella extensor muscles.” After the muscles relax, the umbrella returns to its original position due to its elasticity.
But in order to swim, it is not enough to have muscles. We also need nerve cells that give the muscles the order to contract. It is often believed that the nervous system of jellyfish is a simple nervous network of individual cells. But this is also wrong. Jellyfish have complex sensory organs (eyes and balance organs) and clusters of nerve cells - nerve ganglia. You could even say that they have a brain. Only it is not like the brain of most animals, which is located in the head. Jellyfish have no head, and their brain is a nerve ring with nerve ganglia on the edge of an umbrella. Nerve cell processes extend from this ring, giving commands to the muscles. Among the cells of the nerve ring there are amazing cells - pacemakers. An electrical signal (nerve impulse) appears in them at certain intervals without any external influence. Then this signal spreads around the ring, is transmitted to the muscles, and the jellyfish contracts the umbrella. If these cells are removed or destroyed, the umbrella will stop contracting. Humans have similar cells in their heart.
In some respects, the nervous system of jellyfish is unique. The well-studied jellyfish has aglanta ( Aglantha digitale) there are two types of swimming - normal and “flight reaction”. When swimming slowly, the muscles of the umbrella contract weakly, and with each contraction the jellyfish moves one body length (about 1 cm). During the “flight reaction” (for example, if you pinch a jellyfish’s tentacle), the muscles contract strongly and frequently, and for each contraction of the umbrella, the jellyfish moves forward 4–5 body lengths, and can cover almost half a meter in a second. It turned out that the signal to the muscles is transmitted in both cases along the same large nerve processes (giant axons), but with at different speeds! The ability of the same axons to transmit signals at different speeds has not yet been discovered in any other animal.
summary of other presentations“Characteristics of coelenterates” - general characteristics type. Class Coral polyps. Class Scyphoid. Layers of the body. Hydroid class. Crossword. Type of lower multicellular animals. Trematodes. Knowledge and skills of students. Marine Coelenterates. The wind blows across the sea. Type Coelenterates. Meaning of Coelenterates. Types of Hydra cells. Terms. Multicellular animals. Oral burn. Fish. The only book.
"Coral Polyps" - The name Anthozoa means "animal-flowers." Antipataria squad. Tree-like and whip-like colonies. Order Madreporia corals (Madreporaria or Scleractinia). The same number of radial partitions divides the chambers and the intestinal cavity. Subclass Eight-rayed corals (Octocorallia). Subclass Six-rayed corals (Hexacorallia). Order Horn corals (Gorgonacea). The surface of the colony is covered with small spines.
“Structure of Hydra” - Classification. The vessel in which the hydra lives. The structure and vital functions of coelenterates. Why is hydra called polyp? Reproduction methods. Regeneration. Why is hydra a two-layered animal? Why is hydra a multicellular animal? Hydra. Attached lifestyle. Leads an attached lifestyle. Cellular structure. Body symmetry. Nervous system. Composition of the ectoderm. Habitat and external structure.
“Coelenterate organisms” - Type coelenterates. Common features coelenterates. Subkingdom multicellular animals. Coelenterates are multicellular animals with radial symmetry.
"Coral reefs" - Coral reefs. Coral polyps. The barrier reef is usually divided into three parts. A comprehensive view of the coral islands. Atolls. Six-rayed corals. Many kilometers of beauty. Asexual reproduction. Multibeam Starfish. coral reef. Biologically active substances. The meaning of corals. Relief-forming role. Ocean Pollution industrial waste. Sexual products. Shape and color of coral.
"Hydra" - Freshwater hydra. In the spring, a new generation develops from overwintered eggs. Topic: Diversity of coelenterates. The similarity in the structure and life processes of hydra with unicellular animals indicates the relationship between coelenterates and protozoa. Late autumn Hydras die. Hydras reproduce both asexually and sexually. Cross fertilization). Most representatives reproduce sexually and have planktonic or crawling larvae.