(Protostomia). Representatives of the class of ciliated worms live in salty and fresh waters; some species have adapted to life in moist terrestrial habitats. Representatives of other classes lead an exclusively parasitic lifestyle, parasitizing various animals, both vertebrates and invertebrates. Currently, about 25,000 species have been described, with more than 3,000 species in Russia.
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The body is bilaterally symmetrical, with clearly defined head and caudal ends, somewhat flattened in the dorsoventral direction, in large representatives it is strongly flattened. The body cavity is not developed (with the exception of some phases of the life cycle of tapeworms and flukes). Gases are exchanged across the entire surface of the body; respiratory organs and blood vessels are absent.
Coverings of the body
The outside of the body is covered with single-layer epithelium. In ciliated worms, or turbellarians, the epithelium consists of cells bearing cilia. Flukes, monogeneans, cestodes and tapeworms lack ciliated epithelium for most of their lives (although ciliated cells can be found in larval forms); their integument is represented by the so-called tegument, in a number of groups bearing microvilli or chitinous hooks. Flatworms with tegument belong to the group Neodermata. Flatworms can regenerate 6/7 of their body.
Musculature
Under the epithelium there is a muscular sac, consisting of several layers of muscle cells that are not differentiated into individual muscles (certain differentiation is observed only in the area of the pharynx and genital organs). The cells of the outer muscle layer are oriented transversely, while the cells of the inner layer are oriented along the anterior-posterior axis of the body. The outer layer is called the circular muscle layer, and the inner layer is called the longitudinal muscle layer.
Throat and gut
In all groups, except for cestodes and tapeworms, there is a pharynx leading to the gut or, as in the so-called intestinal turbellaria, to the digestive parenchyma. The intestine is blindly closed and communicates with the environment only through the mouth opening. Several large turbellarians have been noted to have anal pores (sometimes several), but this is the exception rather than the rule. In small forms the intestine is straight, in large ones (planaria, flukes) it can be highly branched. The pharynx is located on the abdominal surface, often in the middle or closer to the posterior end of the body, in some groups it is shifted forward. Cestode and tapeworms have no intestine.
Nervous system and sensory organs
The nervous system is represented by nerve ganglia located in the anterior part of the worm’s body, cerebral ganglia and nerve columns extending from them, connected by jumpers. Sense organs are usually represented by individual skin cilia - processes of sensory nerve cells. Some free-living representatives of the type, in the process of adaptation to living conditions, acquired light-sensitive pigmented eyes - primitive organs of vision and organs of balance.
Nephridia and accumulation buds
Osmoregulation is carried out with the help of protonephridia - branching channels connecting into one or two excretory channels. The release of toxic metabolic products occurs either with fluid excreted through protonephridia, or through accumulation in specialized parenchyma cells (atrocytes), which play the role of “storage buds”.
Flatworms are the first multicellular invertebrate animals to thrive in moist habitats and also live in the tissues and organs of living organisms.
Representatives of this class are studied in biology lessons in 7th grade.
General characteristics of flatworms
The type includes about 25,000 subspecies. Their origin is associated with free-living three-layer organisms close to the ribbon type.
Flatworms are divided into ciliated, flukes, and tapeworms. Their sizes are very diverse. For example, if one type of worm has a size of 1 mm, then other varieties grow up to 10 meters in length. The body of a worm has symmetry.
Different types have similar structures. But there are also some characteristic features.
Internal structure of flatworms
The body of a flatworm includes 3 layers. Each has its own functions:
- ectoderm - is the outer covering;
- endoderm - internal tissues;
- mesoderm - intercavity part.
Cross section of a planarian body
The intestine lies between the ectoderm, which serves to form the integument, and the endoderm, which makes up the intestinal system. The mesoderm is the intermediate germ layer.
The internal structure contains a skin-muscle sac, which contains epithelium and muscle tissue, presented in the form of fibers. The fibers can have a circular or longitudinal structure; they envelop the body of the worm in one continuous bag. The worms move by contracting their muscles.
The body of a flatworm has the shape of an elongated leaf, it is slightly flattened in the dorsal-abdominal region.
Parenchyma
Flatworms do not have an internal cavity, so they are called parenchymatous animals, which means cavityless.
Parenchyma is formed by mesoderm and performs the function of connective tissue. It fills all the internal space and has different meanings:
- support;
- a place to store nutrients;
- participation in metabolic processes.
Parenchyma consists of a large number of cells.
Nervous system
The nervous system is very primitive, it is expressed by a nerve ganglion, which is called a ganglion. It has branches: 6 nerve trunks - 2 each on the abdominal, dorsal and lateral regions.
The trunks are connected using jumpers. From the ganglion itself and the trunks, nerves extend to the internal integuments and outer skin.
Excretory system
The excretion system has a branched structure. These canaliculi are discharged into the parenchyma; connection with the external environment is ensured by excretory openings.
The excretory system is formed from the mesoderm.
Digestive system
Like the nervous system, it has a primitive structure and consists of organs such as the pharynx and intestines. It is formed from the inner layer - endoderm. The intestine is divided into anterior and middle regions. Tapeworms do not have a digestive system.
The digestive and excretory organs are closed by the oral cavity, which is located in the abdominal part. The pharynx produces suction movements during the feeding process. There are also species that feed not on oral suckers, but on the walls of the body.
Circulatory system
Absolutely all varieties of flatworms do not have a circulatory system. The function of the circulatory system is performed by the parenchyma. Due to it, nutrients are transported.
Reproductive system
The reproductive system is formed from mesoderm. One individual has both female and male gonads.
The testes are the male reproductive glands that produce seminal fluid. Female glands are represented by ovaries, which produce eggs.
Some worms can reproduce asexually - by fission. When a body is cut transversely into two parts, each half will not die, but will begin the process of regeneration of the missing parts. Thus, from one worm you will get two.
Reproduction of flatworms
Most flatworms reproduce sexually; some species are capable of asexual reproduction. In the testes, seminal fluid is produced, which penetrates through special ducts into the seminal sacs. The ovaries are located in the lateral part, from which the eggs are sent to the spermatic receptacle, where they are fertilized.
Breath
Organs of locomotion
Worms move due to the skin-muscular sac and cilia that cover their body.
Lifestyle of flatworms
Liver fluke
Flukes and tapeworms can threaten the human and animal body. The former live primarily in the liver of a person or animal; they suck blood from the walls of the liver.
Tapeworms live in the intestines, since they themselves do not have a developed digestive system; they feed on already digested food from the host.
The meaning of flatworms
Flatworms come in a wide variety of species and play a controversial role in nature.
They are of no benefit to animals or people, cause complications and unpleasant symptoms, and can lead to illness. But they participate in food chains, providing food for other living organisms.
FLAT WORMS
The organ systems of flatworms are represented by the digestive, nervous, excretory, and reproductive systems. The digestive system is closed. The nervous system consists of a paired head node and two lateral trunks extending from it, which stretch along the body with peripheral branches. Free-living forms have developed light-sensitive eyes, olfactory cells, and balance organs.
The types of flatworms include the following classes:
1. Eyelash worms
2. Flukes
3. Tapeworms
Class ciliated worms
The class ciliated worms includes free-living marine or freshwater, rarely terrestrial worms, the entire body of which is covered with ciliated epithelium. Beneath the epithelium are layers of smooth muscle running in different directions. The movement of worms is ensured by the work of cilia and muscle contraction. Many species are characterized by regeneration.
A typical representative is white planaria. Inhabits fresh standing water bodies on underwater objects and plants. Her flat body is elongated. At the anterior end two small tactile tentacle-like outgrowths and two eyes are visible.
The nervous system consists of clusters of nerve cells - the brain ganglion. Nerve trunks extend from it to the sensory organs - the eyes and organs of touch (lateral outgrowths).
Planaria is a predatory animal. Her mouth is located on the ventral side, almost in the middle of the body. With the help of a muscular pharynx protruding outward, the planaria penetrates the prey and sucks out its contents. The digestive system is closed blindly, so the mouth opening serves to throw out undigested residues. Digestion of food is carried out intracellularly (phagocytosis - capture of food particles with the help of pseudopods) and extracellularly thanks to enzymes secreted by glands located in the pharynx and intestinal walls. Phagocytosis was discovered by Ilya Ilyich Mechnikov in 1865.
Breathing is carried out over the entire surface of the body. There is no circulatory system. A selection system appears.
The excretory organs, protonephridia, are located on the sides of the body.
Reproduction:
Reproduction of planarians can occur asexually. Asexual reproduction is carried out by transverse division of the body into two parts. Division begins by transverse constriction of the body behind the pharynx. Each half regenerates missing body parts. The reproductive system of ciliated worms is hermaphroditic; cross fertilization. Small planaria develop in the cocoon.
Fluke class
2. Various organs of attachment to the host’s body (suction cups, hooks, etc.)
3. Regressive development of the nervous system and sensory organs.
4. Simply arranged digestive system or lack thereof.
5. Extremely high fertility.
A typical representative of the class isliver fluke. As an adult, it lives in the bile ducts of the liver, in the gall bladder of herbivores and in humans. Feeds on bile and blood, causes diseasefascioliasis.
The body shape is leaf-shaped. With the help of suction cups it is held in the host's body. The skin-muscle sac consists of epithelium, devoid of cilia, and three layers of muscle. The digestive system is represented by a mouth located at the front end of the body, a muscular pharynx, an esophagus and a branched intestine. Excretory system of protonephridial type. The nervous system consists of the peripharyngeal nerve ring and three pairs of nerve trunks extending from it, interconnected by jumpers. Sense organs are poorly developed.
Developmental cycle of the liver fluke
Class tapeworms
Tapeworms are hermaphrodites. Immature segments are located in the front part of the body, in the middle - hermaphroditic, in the end - mature. A mature segment is a sac filled with eggs (up to 175,000 eggs in each), it is rejected and, together with feces, is excreted into the external environment.
Bovine tapeworm development cycle
Meat containing viable Finns that has not undergone veterinary inspection serves as a source of human infection: when eating undercooked or undercooked beef, as well as raw beef. In the human intestine, the finna shell dissolves, the head of the young worm turns out and attaches to the mucous membrane of the small intestine with the help of suction cups. A sexually mature individual produces mature segments with eggs.
Classification of flatworms
Eyelash worms(Turbellaria) - the most primitive group of lower worms; represented mainly by free-living forms. Body length varies from 5 mm to 50 cm. Turbellaria have the shape of a spindle, ribbon or drop and are covered with ciliated epithelium; glandular cells on the surface of the body secrete mucus. In small forms, cilia are used for movement; large worms move using muscles.
Having caught the prey, the turbellaria presses against it and with sucking movements tears the prey into pieces, after which it swallows them. If the prey is too large, digestive enzymes may be released. In primitive turbellarians there is no intestine, and digestion occurs in parenchyma cells that fill the space between the internal organs. The rest have pouch-shaped or branched intestines. Excretory organs are protonephridia, the structural unit of which is the so-called “flickering flame cells.” Primitive forms do not have them. The nervous system in the most primitive forms lies deep in the skin epithelium and is a network of nerve cords. In more highly organized ciliated worms, it consists of head nodes with longitudinal trunks extending from them.
Eyelash worms have both female and male reproductive organs. After copulation in each partner, sperm fertilize the eggs. After a few weeks, young turbellaria are born. Most ciliated worms develop directly; some have a larval stage. Some species are capable of asexual reproduction by transverse division; the resulting halves regenerate the missing parts. It has been experimentally proven that even 1/279th of a worm’s body can restore the entire organism. Planarians are capable of autotomy; in moments of danger they can fall apart, and when the danger has passed, each “piece” grows into a new worm. During prolonged fasting, turbellaria feed on their own body (up to 6/7 of their mass); when favorable conditions occur, they are restored again.
Male and female reproductive organs develop in the joints. Many cestodes live for years and even decades; During this time they manage to produce billions of eggs. The egg is released with the host's excrement and ingested by an intermediate host - an annelid, arthropod, mollusk or mammal. Subsequently, the Finn larva emerges from the egg. In the Finn stage, some cestodes are the size of a match head, others grow as big as a child's head, weighing up to 50 kg. Once they reach the final host along with the intermediate host, the larvae attach to the intestinal wall and grow into an adult worm.
Theory for preparation for block No. 4 of the Unified State Exam in biology: with system and diversity of the organic world.
Type Flatworms
Flatworms- a type of the most primitive three-layered animals. Unlike coelenterates, they develop a third (middle) germ layer - mesoderm.
The body shape of flatworms, as the name of the type suggests, is flattened. They are bilaterally symmetrical, that is, only one plane of symmetry can be drawn through the body. This type of symmetry first appears during evolution in flatworms.
The body is not segmented; at the anterior end there is a mouth opening that leads into the intestinal cavity. In this, flatworms are similar to coelenterates. However, unlike them, in the body of flatworms one can distinguish not just diffusely scattered cells of different types, but already clearly formed tissues. Tissues form organs, organs form systems: digestive, excretory, nervous And sexual.
Respiratory organs and circulatory system are absent. The exchange of gases occurs directly through the integument of the body, so the flat shape of the body advantageously increases the surface area for gas exchange.
The space between the internal organs and the body wall is filled parenchyma – unspecialized tissue from the middle germ layer, mesoderm. Parenchyma serves to store and transport substances, maintains the shape of the worm's body and serves as a support for internal organs.
Integument and muscles
Epithelial and muscle tissues are separate, separated by a layer of connective tissue. Together these three tissues form the body wall of the worms, called skin-muscle bag. Typically, the outer layers of muscle cells are ring-shaped, that is, when they contract, the body of the worm narrows and elongates. The inner layers of muscles have a longitudinal arrangement, with their help the worm can shorten and bend in different directions. In addition, there are dorsoventral (dorsoventral) bunches muscles - they connect the abdominal and dorsal parts of the animal. When they contract, the body flattens.
Digestive system
The digestive system consists of the foregut ( throats), formed by the ectoderm, and the middle endodermal intestine, in which digestion actually occurs. There is no hindgut or anus, so the remains of undigested food are returned to the environment through the mouth.
The nervous system of flatworms is much more complex than that of coelenterates. Here are its characteristic differences:
- nerve cells are collected in ganglia, which in turn are connected into nerve trunks;
- nerve cells are located deeper in the body, which allows them to be protected;
- is happening cephalization, that is, ganglia located closer to the head play a more important role in controlling the body;
- oligomerization nerve centers, that is, their number decreases as the body becomes more complex.
In the anterior part of the body there is a large cerebral ganglion, from which two nerve trunks extend posteriorly. The trunks are connected by transverse bridges, which is why this system received the name orthogon(meaning orthogonal, that is, perpendicular arrangement of nerve trunks).
Excretory system
Waste products, often toxic to cells, accumulate in tissue fluid. Unlike coelenterates, flatworms do not have the ability to secrete metabolic products directly into the external environment; this requires a separate system.
The excretory system consists of branching tubules of ectodermal origin - protonephridia. Each tubule ends with a star-shaped cell - cyrtocyte. There are bundles of cilia on the cyrtocytes. When the cilia beat, reminiscent flickering flame, there is a movement of tissue fluid into the protonephridia tubules. All tubules empty into larger ducts that open on the surface of the body excretory openings. Thus, the liquid with metabolic products gets out.
In some species, the excretory ducts expand at the back of the body to form the bladder. Metabolic products accumulate and concentrate in it. With the help of the excretory system, excess fluid can also be removed from the worm’s body, which is especially important for freshwater forms. Without this mechanism, freshwater worms simply would not be able to maintain water-salt balance.
Reproductive system
Most flatworms are hermaphrodites. Their gonads are located deep in the body, and the germ cells are excreted through ducts. The organization of the reproductive system can vary significantly among representatives of different species.
Male gonads - testes. From them to the copulatory organ ( cirrus) there are vas deferens. The female reproductive system contains ovaries, zheltochniks, oviducts and vagina, opening into the genital cloaca. The yolk sacs are similar in structure to the ovaries, but contain yolk cells– sterile eggs with a large supply of nutrients for the future egg.
Classification
Flatworms include five classes, of which only three are considered in the school course.
Class Ciliated worms (Turbellaria)
There are more than 3,500 species in the class. Unlike other flatworms, most turbellarians are free-living. Characteristic representatives of the class are planaria (milk, brown, mourning, black, etc.). They live in fresh water, are found in large numbers in stagnant and slow-flowing bodies of water, and hide under stones or plant leaves. The sizes of eyelash worms range from 2-3 mm to 30 cm.
The body is flat, thickened in the middle. There may be outgrowths at the anterior end. With the help of cilia and a skin-muscle sac, worms can crawl on various surfaces or swim. The mouth opening is usually located in the middle part of the body.
The turbellarian epithelium contains scattered unicellular glands that secrete a mucous or protein secretion. The mucus probably helps in movement and attachment to the substrate and serves for protection. The protein secretion can be toxic, which repels other predatory animals.
Most eyelash worms are predators. They have a retractable throat, with which they can swallow prey or tear off pieces from it. If the victim's body is covered with a chitinous shell, the worm throws digestive enzymes out and softens the hard covers. Interestingly, planarians can use the “weapons” of coelenterates: when a worm eats a hydra, its stinging cells do not split, but migrate through the body wall, ending up in the epithelium of the worm, protecting it from enemies.
Since turbellaria lead an active lifestyle, their sense organs are quite well developed. The entire body is covered with special long sensitive cilia, sensilla. They perceive mechanical or chemical irritations. Also, almost all ciliated animals have balance organs and two or more photosensitive eyes, which are located in the head area or evenly along the edge of the body.
Ciliated worms are hermaphrodites, fertilization is internal, most often cross-fertilization, that is, partners take turns fertilizing each other. Sperm is usually injected into the genital cloaca, but sometimes directly into the body of the worm (in this case, the copulatory organ pierces the partner’s integument). After this, the sperm move towards the eggs and fertilize them.
Development can be direct (an individual similar to an adult emerges from the egg) or with metamorphosis (a larva with cilia emerges from the egg).
Turbellaria regenerate well: from a small piece of the body a full-fledged adult organism can develop. When unfavorable conditions occur, planaria tend to disintegrate into parts and wait in this form for a long time. After conditions improve, new organisms regenerate from the pieces. This is an example of asexual reproduction in ciliated worms.
Class Flukes (Trematoda)
The nervous system is formed by a pair of cephalic ganglia. Two bridges connecting the ganglia form the peripharyngeal nerve ring. Nerve trunks extend forward and backward from the ring.
Trematodes are hermaphrodites. In all flukes, the female reproductive system is represented by one branching ovary, vitelline and shell glands. Their ducts empty into a sac-like cavity, which continues into the uterus. The uterus opens into the genital cloaca. Nearby is the copulatory organ, which receives sperm from two testes (rarely from one).
During fertilization, the seed enters the genital cloaca, from where sperm move towards the eggs. Fertilized eggs are surrounded by yolk cells, covered with a shell and begin to move out of the uterus.
The life cycle of flukes is complex: the worm goes through several stages of development with a change of hosts. Adult animal ( Marita), capable of sexual reproduction, lives in the main host - a vertebrate. After fertilization, the eggs are released into the external environment and enter the water (most often with the host’s feces). Comes out of an egg in water miracidium, a larva with cilia.
Miracidium actively swims and searches for an intermediate host, a mollusk of a certain species. For example, for the liver fluke the intermediate host is small pond snail. Having penetrated the mollusk with the help of a special proboscis, the larva loses its cilia and becomes immobile. sporocyst. The sporocyst divides asexually, resulting in the formation of many larvae of a new generation. They feed on the tissues of the mollusk and continue to reproduce. As a result, they emerge from the mollusk cercariae- larvae with tails, similar to adult maritas. Cercariae attach to the leaves of coastal plants and become encysted. Cyst can wait for a long time until the host animal eats it. A person can become infected if they drink raw water with broken cysts.
The body resembles a thin ribbon and consists of a head, neck and many segments. Due to their segmented structure, tapeworms are also called tapeworms. The length of worms can reach 20-30 m. Such large individuals are called tapeworms, because they usually occur only singly.
On the head there are suction cups and hooks, with the help of which the worm clings tightly to the intestinal wall. The neck is followed by many segments, each of which lives and develops independently.
The digestive system of tapeworms is completely reduced: animals live in the intestines and absorb food processed by the host’s enzymes through the body surface.
Respiration is anaerobic, so when nutrients are oxidized, glucose is not completely broken down. Products of incomplete breakdown are excreted and poison the host's body.
Each segment of the worm contains organs of the excretory and reproductive systems. The nervous system is extremely poorly developed: two nerve trunks run along the sides, and tactile cells are scattered in the epithelium.
Tapeworms are hermaphrodites. The genital organs develop gradually: the youngest segments located next to the head may not have them at all. A large number of testes with ducts are formed in the parenchyma, which merge into a common vas deferens. The ovary is one, large, consisting of several lobules.
Both cross-fertilization and self-fertilization are possible, in which sperm are introduced into the vagina of an adjacent or even one’s own member. As the eggs mature, the segment matures and may eventually become detached from the worm's body. The eggs are shed in the host's feces and can settle on plant leaves.
When an egg is ingested by an intermediate host, it produces oncosphere, a larva with six hooks. For bovine tapeworm (Taeniarhynchus saginatus) intermediate hosts are artiodactyls, for pork tapeworm (Taenia solium)– pigs, dogs, hares and rabbits. Once in the animal's intestine, the oncosphere drills through its wall and enters the bloodstream, settling in some organ. There the larva transforms into Finn and waits for it to enter the body of the next owner. Infection usually occurs when the primary host eats the intermediate host. A person can become infected by eating undercooked meat.
In the intestine, the head of the worm turns out of the finna and attaches to the intestinal wall. Young segments separate from the neck, the body of the tapeworm grows.
