24.10.2019

Size of polychaete worms. The value of polychaete worms. The class of tapeworms includes

Annelids, also called annelids or annelids, include a huge number of animal species. Their body consists of numerous repeating ones, which is why they got such a name. The general characteristics of annelids unite about 18 thousand of their different species. They live on land in the soil and on the surface in tropical rain forests, in the sea water of the oceans and fresh water rec.

Classification

Annelids are a phylum of invertebrates. Their group is called protostomes. Biologists distinguish 5 classes of annelids:

Belt, or leeches;

Small-bristle (the most famous representative of this class is the earthworm);

Polychaete (sandworm and nereid);

Mysostomides;

Dinophylides.

Considering general characteristics annelids, you understand their important biological role in the processing and aeration of soils. Earthworms loosen the soil, which is beneficial for all the surrounding vegetation of the planet. To understand how many of them there are on earth, imagine that in 1 sq. meter of soil, aeration is carried out from 50 to 500 annelids. This increases the productivity of agricultural land.

Annelids are one of the main links in the food chains of the ecosystem, both on land and in the oceans. They feed on fish, turtles, birds and other animals. Even people use them as top dressing when breeding commercial fish species in both fresh and marine waters. Fishermen put worms on their hooks as bait when fishing with a line.

Everyone knows the meaning medicinal leeches, which suck blood from sore spots, relieving a person from hematomas. Their medicinal value people have understood for a long time. Leeches are used for hypertension, increased blood clotting. Leeches have the ability to produce hirudin. This is a substance that reduces blood clotting and dilates the vessels of the human circulatory system.

Origin

Studying the general characteristics of annelids, scientists have found that they have been known since the Cambrian period. Considering their structure, biologists came to the conclusion that they originated from more ancient type lower flatworms. The similarity is evident in certain structural features of the body.

Scientists believe that the main group of polychaete worms appeared first. During evolution, when given type animals moved to life on the surface and in fresh water, there were also low-bristle, later called leeches.

Describing the general characteristics of annelids, we note that this is the most progressive type of worms. It was they who first developed the circulatory system and the ring-shaped body. Paired organs of movement appeared on each segment, which later became the prototype of the limbs.

Archaeologists have found extinct annelids that had several rows of calcareous plates on their backs. Scientists believe that there is a definite connection between them and mollusks and brachiopods.

general characteristics

In grade 7, the type of annelids is studied in more detail. All representatives have a fairly characteristic structure. Both from the front and from the back, the body looks the same and symmetrical. Conventionally, it is divided into three main sections: the head lobe, numerous segments of the central part of the body, and the posterior or anal lobe. The central segmented part, depending on the size of the worm, may include from ten to several hundred rings.

The general characteristics of annelids include information that their sizes vary from 0.25 mm to a length of 5 meters. The movement of worms is carried out in two ways, depending on its type. The first way is by contracting the muscles of the body, the second is with the help of parapodia. These are the bristles that polychaete worms have. They have lateral bilobed outgrowths on the walls of the segments. In oligochaete worms, organs such as parapodia are absent altogether or have separately growing small bundles.

The structure of the head lobe

Annelids have sensory organs located in front. These are eyes, olfactory cells, which are also found on the tentacles. The ciliary pits are organs that distinguish between the effects of various odors and chemical irritants. There are also hearing organs that have a structure resembling locators. And, of course, the main organ is the mouth.

segmented part

This part is the same general characteristic of the type of annelids. The central region of the body consists of rings, each of which is a completely independent part of the body. Such an area is called a whole. It is divided by partitions into segments. They are visible when viewed appearance. The outer rings of the worm correspond to the inner partitions. On this basis, the worms got their main name - annelids, or rings.

Such a division of the body for the life of the worm is very important. If one or more rings are damaged, the rest remain intact, and the animal regenerates in a short period of time. The internal organs are also arranged in accordance with the segmentation of the rings.

Secondary body cavity, or whole

In the structure of annelids, the following general characteristic is present: the skin-muscular sac has a coelomic fluid inside. It consists of the cuticle, skin epithelium, and circular and longitudinal muscles. In the fluid contained in the body cavity, the constancy of the internal environment is maintained. All the main functions of the body are carried out there: transport, excretory, musculoskeletal and sexual. This fluid is involved in the accumulation nutrients, brings out all the waste, harmful substances and sex products.

The type of annelids has common characteristics in the field of body cell structure. The upper (outer) layer is called the ectoderm, followed by the mesoderm with a secondary cavity lined with its cells. This is the space from the walls of the body to internal organs worm. The fluid contained in the secondary cavity of the body, due to pressure, maintains a constant shape of the worm and plays the role of a hydroskeleton. The last inner layer is called the endoderm. Since the body of annelids consists of three shells, they are also called three-layered animals.

Worm food system

General characteristics of annelids in grade 7 briefly describe the structure digestive system the bodies of these animals. In the anterior part is the mouth opening. It is located in the first segment from the side of the peritoneum. The entire digestive tract has a through system of structure. This is actually the mouth, then there is a peripharyngeal ring that separates the pharynx of the worm. The long esophagus ends in the goiter and stomach.

The intestine has a common characteristic for the class of annelids. It consists of three departments with different purposes. These are the anterior, middle and hindgut. The middle compartment is made up of endoderm, while the rest are ectodermal.

Circulatory system

The general characteristics of annelids are briefly described in the 7th grade textbook. And the structure of the circulatory system can be seen in the schematic image above. Vessels are marked in red. The figure clearly shows that the circulatory system of annelids is closed. It consists of two long longitudinal vessels. This is the dorsal and abdominal. They are connected to each other by the annular vessels present in each segment, which resemble veins and arteries. The circulatory system is closed, the blood does not leave the vessels and does not spill into the body cavity.

The color of blood in different types of worms can be different: red, transparent and even green. It depends on the properties of the chemical structure of the respiratory pigment. It is close to hemoglobin and has a different oxygen content. Depends on the habitat of the annelids.

The movement of blood through the vessels is carried out due to the contractions of some parts of the dorsal and, less often, the annular vessels. After all, they don't. Rings contain special contractile elements in these vessels.

excretory and respiratory systems

These systems in the type of annelids (the general characteristics are briefly described in the 7th grade textbook) are associated with the skin. Respiration is carried out through the skin or gills, which in marine polychaete worms are located on the parapodia. The gills are branched thin-walled outgrowths on the dorsal lobes. They can be different shapes: foliate, pinnate or bushy. Inner part The gills are pierced by thin blood vessels. If the worms are low-bristle, then breathing occurs through the moist skin of the body.

excretory system consists of metanephridia, protonephridia and myxonefridia, arranged in pairs in each segment of the worm. Myxonephridia are the prototype of the kidneys. Metanephridia are funnel-shaped, located in the coelom, from which a thin and short canal brings excretion products out in each segment.

Nervous system

If we compare the general characteristics of round and annelids, then the latter have a more advanced nervous system and sensory organs. They have a cluster of nerve cells above the parapharyngeal ring of the anterior lobe of the body. Consists of nervous system from ganglia. These are supra-pharyngeal and sub-pharyngeal formations connected by nerve trunks into a peri-pharyngeal ring. In each segment, one can see a pair of such ganglia of the ventral chain of the nervous system.

You can see them in the picture above. They are marked in yellow. Large ganglia in the pharynx play the role of the brain, from which impulses diverge along the abdominal chain. The sense organs of the worm also belong to the nervous system. He has many of them. These are the eyes, and the organs of touch on the skin, and the chemical senses. Sensory cells are located all over the body.

reproduction

Describing the general characteristics of the type of annelids (class 7), one cannot fail to mention the reproduction of these animals. They are mostly heterosexual, but some have developed hermaphroditism. The latter include well-known leeches and earthworms. In this case, conception occurs in the body itself, without fertilization from outside.

In many polychaetes, development occurs from the larva, while in the remaining subspecies it is direct. The gonads are located under the epithelium of the coelom in each or almost in each segment. When a rupture occurs in these cells, the germ cells enter the coelom fluid and are excreted through the organs of the excretory system to the outside. In many, fertilization occurs on the outer surface, while in underground soil worms, it occurs inside.

But there is another type of reproduction. In conditions favorable for life, when there is a lot of food, individual parts of the body begin to grow in individuals. For example, multiple mouths may appear. Subsequently, the rest grows. The worm breaks up into several separate parts. This asexual species reproduction, when a certain part of the body appears, and the rest regenerate later. As an example, we can cite the ability of aulophorus to this type of reproduction.

In the article, you learned in detail all the main characteristics of annelids, which are studied in the 7th grade of the school. We hope that is detailed description these animals will help to learn knowledge more easily.

polychaete worms got its name for a large number of hard hairs - bristles. Most species belong specifically to polychaetes, which are also called polychaetes. In some species, the bristles sit on soft outgrowths - "legs". Polychaete worms live mainly in the sea and use these "legs" (parapodia), moving along the bottom or digging in sand and mud. Some polychaetes live in tubes. Compared to earthworms, most polychaete worms have pronounced head and tail ends. The head usually bears finger-like tentacles, which are arranged either in circles or collected in a corolla. Some types of polychaete worms even have eyes that can recognize the shape of objects. At the end of the tail, they may have two long thread-like processes. Nereis and sandworms live on the coast of the seas. The former usually live in burrows lined with their own mucus, in muddy sediments that accumulate at the bottom of rocky pools. At low tide, the Nereis hide. With the onset of the tide, they again leave their shelters - to look for food on the surface of the silt. They are able to move like snakes or swim by paddling with parapodia. Nereis also eat the remains of dead animals, small creatures like shrimp, and seaweed. Their strong jaws can easily bite off pieces of flesh, and they are able to suck blood even through human skin. In addition, they can shed their mucous membrane, eating along with it the microbes contained in the mucus.

Body fan worms hidden inside a protective tube, tripled on seabed from particles of silt and sand glued together with mucus. These worms got their name for the hard, feathery feeding tentacles that they release from the top of the tube, spreading them like a fan. One of the most beautiful representatives of this group is the peacock sabella worm. Its tube reaches 25 cm in length. Under favorable conditions, the ona releases tentacles fluttering in the water, covered with a thin layer of mucus, which capture food particles passing by. In addition, they perform the function of gills. If a fish or other predator swims by, the sabella retracts its tentacles and hides in the tube. Sometimes sabella and other fan worms form groups that resemble a flower carpet. At the moment of danger, they disappear inside the tubes, and the seabed becomes empty and lifeless.
At the bottom of the sea, near underwater volcanic springs emitting bubbling water rich in hydrogen sulfide Pompeian worms. They can withstand heating above 100 degrees and cooling up to +2 degrees. Their bristles are inhabited by bacteria that process hydrogen sulfide and nutrients that they share with worms.
Some polychaete worms form hard and strong tubes of limestone in sand and on stones. At high tide, the worms spread the crowns of their feeding tentacles above the tubes in search of food.
Miniature mounds of sand and mud on the coast - traces of activity sandworms. These soft-bodied worms with a thickening at the front end reach a length of 15-20 cm. They feed, like earthworms, eating sand and dirt, digesting nutrients and then throwing out the remains in the form of mounds. Sandworms hide in the sand in ∪ - shaped burrows. Moving, the worm creates a current of water through the hole, thus getting the opportunity to breathe.

sea mouse- this is not a rodent, but one of the polychaete worms. It grows up to 20 cm in length. Its thick, bumpy body is covered with gray-brown hairs, very similar to the fur of a real mouse. The sea mouse lives mainly on the sandy bottom in Atlantic Ocean and the Mediterranean Sea. In some worms, dorsal scales are overgrown with algae, which hide the sea mouse like a modern camouflage uniform. Settle among the algae small crustaceans and other animals. As a result, the worm becomes like a mobile sea forest, which can move closer to its prey.

polychaete worms:
- 8500 species
- Mainly marine
- There are parapodia - like legs
- Dioecious
- Representatives: sandworm, nereis, sea mouse, sabella, Pompeian worm

Polychaete worms, they are also polychaetes, belong to the class of annelids and live mainly on the bottom of the seas. Only a few species are adapted to life in fresh water. Their role in the ecosystem is significant. Polychaetes filter water, clean the soil from decaying organic residues. In turn, the worms themselves become food for many fish, crustaceans, and echinoderm marine life.

Seta worms live at the bottom of the seas, and are rarely found in freshwater bodies of water.

Description and structure

Outwardly, this representative of the ringed can be characterized as follows:

The length of polychaetes can be from 2 mm to 3 m.
The body shape of polychaete worms consists of many segments, on the sides of which there are skin-muscular outgrowths that help polychaetes move. These organs of movement are called parapodia. The worm swims near the bottom, bending its body and raking with muscular outgrowths.
In addition, the head segment (prostomium) and the caudal lobe (pygidium) are distinguished.
Tentacles, palpi and antennae may be present on the head - all of them serve as organs of touch for polychaetes.

Among the polychaete worms, there are sessile subspecies with a reduced number of parapodia, which are preserved only in the anterior part of the body. These polychaetes live inside a protective tube they have built and never leave it.
Internal organs and systems of the representative of the annelids arranged as follows:

Worm larvae lead a planktonic lifestyle in the water column and are carried by the current over long distances, this is how their settlement occurs. Initially, the trochophore consists of two hemispheres, gradually its body is elongated and takes on a worm-like shape due to the growth of larval (larval) segments. The growth zone is more often formed at the posterior end of the larva.

Reproduction of polychaetes
Most polychaete worms reproduces sexually. The females release eggs and the males release sperm. The genital organs of animals are developed in the peritoneal epithelium. Fertilization in most species occurs in the external environment.
As soon as the body segment is overflowing with mature germ cells, the epithelium breaks and they fall out. In other species, there are special funnels for this - coelomoducts. The fertilized larva is called a trochophore. Having settled to the bottom, after a while it turns into an adult.

sea worms reproduce sexually
Only a few forms have a sophisticated reproductive apparatus that allows them to copulate (for example, Saccocirrus). Many species of polychaete worms reproduce by budding. At the same time, part of the body segments separates and breaks up into separate segments.
In the future, each of them forms the head and anal parts, becoming an independent individual. This process is called archetomy. With paratomy, everything happens the other way around - a chain consisting of several individuals is separated. Later they separate, becoming individually existing worms.
Practical value
Marine polychaete worms inhabit salt water bodies in large numbers and serve as food for many commercial fish species. Polychaetes make up the main diet of stellate sturgeon and sturgeon. Only with a lack of polychaete worms does the fish switch to other types of food and begin to eat mollusks, shrimps and other crustaceans. The Caspian Sea, where sturgeon fish is fished, for a long time had only 5 species of polychaetes.
A group of Soviet scientists carried out acclimatization in the Caspian Sea of the Nereis polychaete, which was delivered from Sea of Azov. It is this worm that is distinguished by unpretentiousness and minimal requirements for the level of salinity of the water. In the 40s of the last century, 65,000 Nereis polychaetes were released into the waters of the Caspian Sea, and by the end of the first decade, the worms inhabited an area of 30,000 km2. This made it possible to significantly increase the biological value of the Caspian Sea.

Class Polychaeta (Polychaeta)

with all the colors of the rainbow bristles. Serpentine phyllodoces (Phyllodoce) swim and crawl quickly. Tomopteris (Tomopteris) hang on their long mustaches in the water column.

The class of polychaetes differs from other annulus in a well-separated head region with sensory appendages and the presence of limbs - parapodia with numerous setae. Mostly dioecious. development with metamorphosis.

General morphofunctional characteristics

External structure . The body of polychaete worms consists of a head section, a segmented trunk, and an anal lobe. The head is formed by the head lobe (prostomium) and the oral segment (peristomium), which is often complex as a result of fusion

with 2-3 trunk segments (Fig. 172). The mouth is located ventrally on the peristomium. Many polychaetes have ocelli and sensory appendages on their heads. So, in a Nereid, on the prostomium of the head there are two pairs of eyes, tentacles - tentacules and two-segmented palps, on the bottom of the peristomium there is a mouth, and on the sides there are several pairs of antennae. On the trunk segments there are paired lateral outgrowths with setae - parapodia (Fig. 173). These are primitive limbs with which polychaetes swim, crawl or burrow into the ground. Each parapodia consists of a basal part and two lobes - dorsal (notopodium) and ventral (neuropodium). At the base of the parapodia on the dorsal side there is a dorsal antennae, and on the ventral side there is a ventral antennae. These are the sensory organs of polychaetes. Often, the dorsal barbel in some species is turned into feathery gills. Parapodia are armed with tufts of setae, consisting of organic matter close to chitin. Among the setae there are several large aciculous setae, to which muscles are attached from the inside, setting the parapodia and the tuft of setae in motion. The limbs of polychaetes make synchronous movements like oars. In some species leading a burrowing or attached lifestyle, the parapodia are reduced.

Skin-muscular sac(Fig. 174). The body of polychaetes is covered with a single layer of skin epithelium, which exposes a thin cuticle to the surface. In some species, some parts of the body may have ciliated epithelium (longitudinal abdominal band or ciliary bands around segments). Glandular epithelial cells in sessile polychaetes can secrete a protective horny tube, often impregnated with lime.

Under the skin lies the annular and longitudinal muscles. The longitudinal muscles form four longitudinal bands: two on the dorsal side of the body and two on the ventral side. Longitudinal tapes may be more. On the sides there are bundles of fan-shaped muscles that set in motion the blades of the parapodia. The structure of the skin-muscular sac varies greatly depending on the lifestyle. The inhabitants of the ground surface have the most complex structure of the skin-muscular sac, close to that described above. This group of worms crawls along the surface of the substrate with the help of a serpentine bending of the body and movements of the parapodia. The inhabitants of calcareous or chitinous pipes have limited mobility, as they never leave their shelters. In these polychaetes, strong longitudinal muscle bands provide a sharp lightning-fast contraction of the body and withdrawal into the depth of the tube, which allows them to escape from the attack of predators, mainly fish. In pelagic polychaetes, the muscles are poorly developed, as they are passively carried by ocean currents.

Rice. 172. External structure of the nereid Nereis pelagica (according to Ivanov): A - anterior end of the body; B - posterior end of the body; 1 - antennae, 2 - palps 3 - peristomal antennae, 4 - eyes, 5 - prostomium, 6 - olfactory fossa, 7 - peristomium, 8 - parapodia, 9 - setae, 10 - dorsal antennae, 11 - pygidium, 12 - caudal appendages , 13 – segment

Rice. 173. Parapodia Nereis pelagica (according to Ivanov): 1 - dorsal antennae, 2 - lobes of notopodium, 3 - setae, 4 - lobes of neuropodia, 5 - ventral antennae, 6 - neuropodium, 7 - atsikula, 8 - notopodium

Rice. 174. Cross section of a polychaete worm (according to Natalie): 1 - epithelium, 2 - annular muscles, 3 - longitudinal muscles, 4 - dorsal antennae (gill), 5 - notopodium, 6 - supporting seta (acicula), 7 - neuropodium, 8 - nephridial funnel, 9 - nephridial canal, 10 - oblique muscle, 11 - abdominal vessel, 12 - ovary, 13 - abdominal antennae, 14 - setae, 15 - intestine, 16 - whole, 17 - dorsal blood vessel

Secondary body cavity- in general - polychaetes have a very diverse structure. In the most primitive case, separate groups of mesenchymal cells cover the inside of muscle ribbons and the outer surface of the intestine. Some of these cells are capable of contraction, while others are able to turn into germ cells that mature in a cavity, only conditionally called secondary B more difficult case The coelomic epithelium can completely cover the intestines and muscles. The whole is presented completely in the case of the development of paired metameric coelomic sacs (Fig. 175). When paired coelomic sacs close in each segment above the intestine and under the intestine, dorsal and abdominal mesentery, or mesentery, are formed Between the coelomic sacs of two adjacent segments, transverse partitions are formed - dissipations. , covering the intestines and forming the mesenterium, is called the visceral sheet of mesoderm. Blood vessels lie in the coelomic septa.

Rice. 175. Internal structure polychaetes: A - nervous system and nephridia, B - intestines and the whole, C - intestines, nervous and circulatory systems, side view (according to Meyer); 1 - brain, 2 - peripharyngeal connective, 3 - ganglia of the abdominal nerve chain, 4 - nerves, 5 - nephridium, 6 - mouth, 7 - whole, 8 - intestine, 9 - diosepiament, 10 - mesentery, 11 - esophagus, 12 - oral cavity, 13 - pharynx, 14 - muscles of the pharynx, 15 - muscles of the body wall, 16 - olfactory organ, 17 - eye, 18 - ovary, 19, 20 - blood vessels, 21 - network of vessels in the intestine, 22 - annular vessel , 23 - musculature of the pharynx, 24 - palp

In general, it performs several functions: musculoskeletal, transport, excretory, sexual and homeostatic. The cavity fluid supports the turgor of the body. With the contraction of the ring muscles, the pressure of the cavity fluid increases, which provides the elasticity of the worm's body, which is necessary when making passages in the ground. Some worms are characterized by a hydraulic mode of movement, in which the abdominal fluid, when the muscles contract under pressure, is distilled to the anterior end of the body, providing vigorous forward movement. In general, there is a transport of nutrients from the intestines and dissimilation products from various organs and tissues. The excretory organs of metanephridia open as a whole with funnels and ensure the removal of metabolic products, excess water. In general, there are mechanisms to maintain the constancy of the biochemical composition of the liquid and water balance. In this favorable environment, gonads are formed on the walls of the coelomic sacs, germ cells mature, and in some species, juveniles even develop. Derivatives of the coelom - coelomoducts serve to remove the reproductive products from the body cavity.

Digestive system consists of three departments (Fig. 175). The entire anterior section consists of derivatives of the ectoderm. The anterior begins mouth opening located on the peristomium from the ventral side. The oral cavity passes into a muscular pharynx, which serves to capture food objects. In many species of polychaetes, the pharynx can turn outward, like a finger of a glove. In predators, the pharynx consists of several layers of annular and longitudinal muscles, armed with strong chitinous jaws and rows of small chitinous plates or spikes that can firmly hold, injure and crush captured prey. In herbivorous and detritivorous forms, as well as in seston-eating polychaetes, the pharynx is soft, mobile, adapted to swallowing liquid food. The pharynx is followed by the esophagus, into which the ducts open. salivary glands also of ectodermal origin. Some species have a small stomach

The middle section of the intestine is a derivative of the endoderm and serves for the final digestion and absorption of nutrients. In predators, the middle section of the intestine is relatively shorter, sometimes equipped with paired blind side pockets, while in herbivores, the middle section of the intestine is long, tortuous, and usually filled with undigested food debris.

The posterior intestine is of ectodermal origin and can perform the function of regulating the water balance in the body, since there water is partially absorbed back into the coelom cavity. Fecal masses are formed in the hindgut. The anal opening usually opens on the dorsal side of the anal lobe.

Respiratory system. Polychaetes mainly have cutaneous respiration. But a number of species have dorsal skin gills, which are formed from the parapodial antennae or appendages of the head. They breathe oxygen dissolved in water. Gas exchange occurs in a dense network of capillaries in the skin or gill appendages.

Circulatory system closed and consists of the dorsal and abdominal trunks connected by annular vessels, as well as peripheral vessels (Fig. 175). The movement of blood is carried out as follows. Through the dorsal, the largest and pulsating vessel, blood flows to the head end of the body, and through the abdominal - in the opposite direction. Through the annular vessels in the anterior part of the body, blood is distilled from the dorsal vessel to the abdominal one, and vice versa in the posterior part of the body. Arteries depart from the annular vessels to parapodia, gills and other organs, where a capillary network is formed, from which blood is collected into venous vessels that flow into the abdominal bloodstream. In polychaetes, the blood is often red in color due to the presence of the respiratory pigment hemoglobin dissolved in the blood. Longitudinal vessels are suspended on the mesentery (mesenterium), annular vessels pass inside the dissipations. Some primitive polychaetes (Phyllodoce) have no circulatory system, and hemoglobin is dissolved in nerve cells.

excretory system polychaetes are most often represented by metanephridia. This type of nephridia appears for the first time in the type of annelids. Each segment has a pair of metanephridia (Fig. 176). Each metanephridium consists of a funnel lined with cilia and open as a whole. The movement of cilia into the nephridium drives solid and liquid metabolic products. A channel departs from the funnel of nephridium, which penetrates the septum between the segments and in another segment opens outwards with an excretory opening. In convoluted channels, ammonia is converted into macromolecular compounds, and water is absorbed as a whole. In different types of polychaetes, the excretory organs can be different origin. So, some polychaetes have protonephridia of ectodermal origin, similar in

Rice. 176. Excretory system of polychaetes and its relationship with coelomoducts (according to Briand): A - protonephridium and genital funnel (in a hypothetical ancestor), B - nephromixium with protonephridium, C - metanephridium and genital funnel, D - nephromixium; 1 - whole, 2 - genital funnel (coeloduct), 3 - protonephridium, 4 - metanephridium

structure with those of flat and roundworms. Most species are characterized by metanephridia of ectodermal origin. Individual representatives form complex organs - nephromixia - the result of the fusion of protonephridia or metanephridia with genital funnels - coelomoducts of mesodermal origin. Additionally, the excretory function can be performed by chloragogenic cells of the coelomic epithelium. These are peculiar accumulation kidneys in which excreta grains are deposited: guanine, salts of uric acid. Subsequently, chloragogenic cells die and are removed from the coelom through nephridia, and new ones are formed to replace them.

Nervous system. Paired supraglottic ganglia form the brain, in which three sections are distinguished: proto-, meso- and deutocerebrum (Fig. 177). The brain innervates the sense organs on the head. Near-pharyngeal nerve cords depart from the brain - connectives to the abdominal nerve chain, which consists of paired ganglia, repeating segment by segment. Each segment has one pair of ganglia. Longitudinal nerve cords connecting the paired ganglia of two adjacent segments are called connectives. The transverse cords connecting the ganglia of one segment are called commissures. When paired ganglia merge, a nerve chain is formed (Fig. 177). In some species, the nervous system is complicated by the fusion of the ganglia of several segments.

sense organs most developed in mobile polychaetes. On the head they have eyes (2-4) of a non-inverted type, goblet-shaped or in the form of a complex eye bubble with a lens. Many sessile tube-dwelling polychaetes have numerous eyes on the feathery gills of the head region. In addition, they have developed organs of smell, touch in the form of special sensory cells located on the appendages of the head and parapodia. Some species have balance organs - statocysts.

reproductive system. Most polychaete worms have separate sexes. Their gonads develop in all segments of the body or only in some of them. Sex glands of mesodermal origin and are formed on the wall of the coelom. Sex cells from the gonads fall into the whole, where their final maturation takes place. Some polychaetes do not have reproductive ducts, and the germ cells enter the water through ruptures in the body wall, where fertilization occurs. In this case, the parental generation dies. A number of species have genital funnels with short channels - coelomoducts (of mesodermal origin), through which the reproductive products are brought out into the water. In some cases, germ cells are removed from the coelom through nephromixia, which simultaneously perform the function of the genital and excretory ducts (Fig. 176).

Rice. 177. Nervous system of polychaetes: 1 - nerves of antennae, 2 - neonatal palps, 3 - mushroom body, 4 - eyes with lens, 5 - nerves of peristomal antennae, 6 - mouth, 7 - peripharyngeal ring, 8 - abdominal ganglion of peristomium, 9 - 11 - nerves of the parapodia, 12 - ganglia of the abdominal nerve chain, 13 - nerve endings of the nuchal organs

reproduction polychaetes can be sexual and asexual. In some cases, there is an alternation of these two types of reproduction (metagenesis). asexual reproduction usually occurs by transverse division of the body of the worm into parts (strobilation) or by budding (Fig. 178). This process is accompanied by the regeneration of the missing parts of the body. Sexual reproduction is often associated with the phenomenon of epitokia. Epitoky is a sharp morphophysiological restructuring of the worm's body with a change in the shape of the body during the maturation of the reproductive products: the segments become wide, brightly colored, with swimming parapodia (Fig. 179). In worms that develop without epitokia, males and females do not change their shape and reproduce in bottom conditions. Species with epitokia may have several variants life cycle. One of them is observed in Nereids, the other in Palolo. So, in Nereis virens, males and females become epitonic and float to the surface of the sea for reproduction, after which they die or become prey to birds and fish. From eggs fertilized in water, larvae develop, settling to the bottom, from which adults are formed. In the second case, as in the palolo worm (Eunice viridis) from the Pacific Ocean, sexual reproduction is preceded by asexual reproduction, in which the anterior end of the body remains at the bottom, forming an atopic individual, and the posterior end of the body is transformed into an epitonic tail filled with sexual products. The backs of the worms break off and float to the surface of the ocean. Here the reproductive products are released into the water and fertilization takes place. Epitoke individuals of the entire population emerge for reproduction at the same time, as if on a signal. This is the result of the synchronous biorhythm of puberty and biochemical communication of sexually mature individuals of the population. The mass appearance of breeding polychaetes in the surface layers of water is usually associated with the phases of the moon. So, the Pacific palolo rises to the surface in October or November on the day of the new moon. The local population of the Pacific Islands knows these palolo breeding dates, and fishermen en masse catch palolos stuffed with "caviar" and use them for food. At the same time, fish, gulls, and sea ducks feast on worms.

Development. A fertilized egg undergoes uneven, spiral fragmentation (Fig. 180). This means that as a result of crushing, quartets of large and small blastomeres are formed: micromeres and macromeres. In this case, the axes of the spindles of cell fragmentation are arranged in a spiral. The inclination of the spindles is reversed with each division. Due to this, the crushing figure has a strictly symmetrical shape. Cleavage of the egg in polychaetes is deterministic. Already at the stage of four blastomeres, determination is expressed. Quartets of micromeres give derivatives of the ectoderm, and quartets of macromeres give derivatives

Rice. 178. Development of polychaetes (family Sylhdae) with metagenesis (according to Barnes): A - budding, B - multiple budding, C - alternation of sexual reproduction with asexual

Rice. 179. Reproduction of polychaetes: A - budding of the polychaete Autolytus (no Grasse), B, C - epitonic individuals - female and male Autolytus (according to Sveshnikov)

endoderm and mesoderm. The first mobile stage - blastula - a single-layered larva with cilia. The macromeres of the blastula at the vegetative pole sink into the embryo and a gastrula is formed. At the vegetative pole, the primary mouth of the animal, the blastopore, is formed, and at the animal pole, an accumulation of nerve cells and a ciliary crest are formed - the parietal sultan of cilia. Further, a larva develops - a trochophore with an equatorial ciliary belt - a troch. Trochophore has spherical shape, a radially symmetrical nervous system, protonephridia and the primary body cavity (Fig. 180). The blastopore at the trochophore is displaced from the vegetative pole closer to the animal along the ventral side, which leads to the formation of bilateral symmetry. The anus erupts later at the vegetative pole, and the intestine becomes through.

Previously, there was a point of view that in all polychaetes, the mouth and anus are formed from the blastopore. But, as was shown by the research of V. A. Sveshnikov, a specialist in polychaetes, such a situation is only special case development of polychaetes, and in most cases only the mouth is formed from the blastopore, and the anus forms independently in the later phases of development. In the region of the posterior end of the larva, in the immediate vicinity of the anus, on the right and left sides of the intestine, a pair of cells appears - teloblasts, located in the growth zone. This is the beginning of the mesoderm. The trochophore consists of three sections: the head lobe, the anal lobe, and the growth zone. - In this area, a zone of future growth of the larvae is formed. The structural plan of the trochophore at this stage resembles the organization of lower worms. The trochophore successively transforms into a metatrochophore and a nektochaete. In the metatrochophore, larval segments are formed in the growth zone. Larval, or larval, segmentation captures only ectodermal derivatives: ciliary rings, protonephridia, rudiments of bristle sacs of future parapodia. Nektochaet differs in that it forms the brain, the abdominal nerve chain. The setae from the bristle sacs are exposed, and a parapodial complex is formed. However, the number of segments remains the same as in the metatrochophore. There may be different numbers of them in different types of polychaetes: 3, 7, 13. After a certain temporary pause, post-larval segments begin to form and the juvenile stage of the worm is formed. In contrast to larval segmentation, postlarval segments in juvenile forms include derivatives not only of the ectoderm, but also of the mesoderm. At the same time, in the growth zone, teloblasts sequentially separate the rudiments of paired coelomic sacs, in each of which a metanephridial funnel is formed. The secondary body cavity gradually replaces the primary one. Dissepiments and mesentery are formed at the boundaries of contact of the coelomic sacs.

Due to the remaining primary body cavity, longitudinal vessels of the circulatory system are formed in the lumen of the mesenterium, and annular ones are formed in the lumen of the septa. Due to the mesoderm, the muscles of the skin-muscular sac and intestines, the lining of the coelom, gonads and coelomoducts are formed. From the ectoderm, the nervous system, metanephridial canals, the anterior and hindgut are formed. Due to the endoderm, the middle intestine develops. After completion of metamorphosis, an adult animal develops with a certain number of segments for each species. The body of an adult worm consists of a head lobe, or prostomium, developed from the head lobe of a trochophore, several larval segments with a primary cavity, and many postlarval segments with a coelom, and from an anal lobe without a coelom.

Thus, the most important features of the development of polychaetes are spiral, determined fragmentation, teloblastic anlage of the mesoderm, metamorphosis with the formation of trochophore larvae, metatrochophores, nektochaetes, and juvenile forms. The phenomenon of the dual origin of metamerism in annelids with the formation of larval and postlarval segments was discovered by the great Soviet embryologist P. P. Ivanov. This discovery shed light on the origin of annelids from oligomeric ancestral forms.

The successive change in the phases of the individual development of polychaetes from oligomeric to polymeric reflects a phylogenetic pattern. Comparative morphological data indicate that the ancestors of polychaetes had a small number of segments, i.e., were oligomeric. Among modern polychaetes, the closest to ancestral forms are some primary annuluses of the class Archiannelida, in which the number of segments usually does not exceed seven. Manifestations of primitive features of organization at the stages of trochophore and metatrochophore (primary cavity, protonephridia, orthogon) indicate the relationship of coelomic animals with a group of lower worms.

The biological significance of the development of polychaete worms with metamorphosis lies in the fact that swimming larvae (trochophores, metatrochophores) ensure the resettlement of species that, in adulthood, lead a predominantly bottom lifestyle. In some polychaete worms, care for offspring is observed and their larvae are inactive and lose the function of settling. In some cases, live birth is observed.

The value of polychaete worms. The biological and practical importance of polychaete worms in the ocean is very great. The biological significance of polychaetes lies in the fact that they represent an important link in the trophic chains, and are also important as organisms involved in cleaning sea water and processing of organic

substances. Polychaetes are of food value. To strengthen the food base of fish in our country, for the first time in the world, the acclimatization of nereids (Nereis diversicolor) in the Caspian Sea, which were brought from the Sea of Azov, was carried out. This successful experiment was carried out under the guidance of Academician L. A. Zenkevich in 1939-1940. Some polychaetes are used as food by humans, such as the Pacific palolo worm (Eunice viridis).

Class Polychaeta (Polychaeta)

with all the colors of the rainbow bristles. Serpentine phyllodoces (Phyllodoce) swim and crawl quickly. Tomopteris (Tomopteris) hang on their long mustaches in the water column.

General morphofunctional characteristics

External structure. The body of polychaete worms consists of a head section, a segmented trunk, and an anal lobe. The head is formed by the head lobe (prostomium) and the oral segment (peristomium), which is often complex as a result of fusion

Secondary body cavity- in general - polychaetes have a very diverse structure. In the most primitive case, separate groups of mesenchymal cells cover the inside of muscle ribbons and the outer surface of the intestine. Some of these cells are capable of contraction, while others are able to turn into germ cells that mature in a cavity, only conditionally called secondary B in a more complex case, the coelomic epithelium can completely cover the intestines and muscles. The whole is presented completely in the case of the development of paired metameric coelomic sacs (Fig. 175). When paired coelomic sacs close in each segment above the intestine and under the intestine, dorsal and abdominal mesentery, or mesentery, are formed Between the coelomic sacs of two adjacent segments, transverse partitions are formed - dissipations. , covering the intestines and forming the mesenterium, is called the visceral sheet of mesoderm. Blood vessels lie in the coelomic septa.

Rice. 175. Internal structure of polychaetes: A - nervous system and nephridia, B - intestines and whole, C - intestines, nervous and circulatory systems, side view (according to Meyer); 1 - brain, 2 - peripharyngeal connective, 3 - ganglia of the abdominal nerve chain, 4 - nerves, 5 - nephridium, 6 - mouth, 7 - whole, 8 - intestine, 9 - diosepiament, 10 - mesentery, 11 - esophagus, 12 - oral cavity, 13 - pharynx, 14 - muscles of the pharynx, 15 - muscles of the body wall, 16 - olfactory organ, 17 - eye, 18 - ovary, 19, 20 - blood vessels, 21 - network of vessels in the intestine, 22 - annular vessel , 23 - musculature of the pharynx, 24 - palp

Digestive system consists of three departments (Fig. 175). The entire anterior section consists of derivatives of the ectoderm. The anterior section begins with a mouth opening located on the peristomium from the ventral side. The oral cavity passes into a muscular pharynx, which serves to capture food objects. In many species of polychaetes, the pharynx can turn outward, like a finger of a glove. In predators, the pharynx consists of several layers of annular and longitudinal muscles, armed with strong chitinous jaws and rows of small chitinous plates or spikes that can firmly hold, injure and crush captured prey. In herbivorous and detritivorous forms, as well as in seston-eating polychaetes, the pharynx is soft, mobile, adapted to swallowing liquid food. The pharynx is followed by the esophagus, into which the ducts of the salivary glands, also of ectodermal origin, open. Some species have a small stomach

excretory system polychaetes are most often represented by metanephridia. This type of nephridia appears for the first time in the type of annelids. Each segment has a pair of metanephridia (Fig. 176). Each metanephridium consists of a funnel lined with cilia and open as a whole. The movement of cilia into the nephridium drives solid and liquid metabolic products. A channel departs from the funnel of nephridium, which penetrates the septum between the segments and in another segment opens outwards with an excretory opening. In convoluted channels, ammonia is converted into macromolecular compounds, and water is absorbed as a whole. In different types of polychaetes, the excretory organs can be of different origin. So, some polychaetes have protonephridia of ectodermal origin, similar in

reproduction polychaetes can be sexual and asexual. In some cases, there is an alternation of these two types of reproduction (metagenesis). Asexual reproduction usually occurs by transverse division of the body of the worm into parts (strobilation) or by budding (Fig. 178). This process is accompanied by the regeneration of the missing parts of the body. Sexual reproduction is often associated with the phenomenon of epitokia. Epitoky is a sharp morphophysiological restructuring of the worm's body with a change in the shape of the body during the maturation of the reproductive products: the segments become wide, brightly colored, with swimming parapodia (Fig. 179). In worms that develop without epitokia, males and females do not change their shape and reproduce in bottom conditions. Species with epitokia may have several life cycle variants. One of them is observed in Nereids, the other in Palolo. So, in Nereis virens, males and females become epitonic and float to the surface of the sea for reproduction, after which they die or become prey to birds and fish. From eggs fertilized in water, larvae develop, settling to the bottom, from which adults are formed. In the second case, as in the palolo worm (Eunice viridis) from the Pacific Ocean, sexual reproduction is preceded by asexual reproduction, in which the anterior end of the body remains at the bottom, forming an atopic individual, and the posterior end of the body is transformed into an epitonic tail filled with sexual products. The backs of the worms break off and float to the surface of the ocean. Here the reproductive products are released into the water and fertilization takes place. Epitoke individuals of the entire population emerge for reproduction at the same time, as if on a signal. This is the result of the synchronous biorhythm of puberty and biochemical communication of sexually mature individuals of the population. The mass appearance of breeding polychaetes in the surface layers of water is usually associated with the phases of the moon. So, the Pacific palolo rises to the surface in October or November on the day of the new moon. The local population of the Pacific Islands knows these palolo breeding dates, and fishermen en masse catch palolos stuffed with "caviar" and use them for food. At the same time, fish, gulls, and sea ducks feast on worms.

Rice. 178. Development of polychaetes (family Sylhdae) with metagenesis (according to Barnes): A - budding, B - multiple budding, C - alternation of sexual reproduction with asexual

Rice. 179. Reproduction of polychaetes: A - budding of the polychaete Autolytus (no Grasse), B, C - epitonic individuals - female and male Autolytus (according to Sveshnikov)

endoderm and mesoderm. The first mobile stage - blastula - a single-layered larva with cilia. The macromeres of the blastula at the vegetative pole sink into the embryo and a gastrula is formed. At the vegetative pole, the primary mouth of the animal, the blastopore, is formed, and at the animal pole, an accumulation of nerve cells and a ciliary crest are formed - the parietal sultan of cilia. Further, a larva develops - a trochophore with an equatorial ciliary belt - a troch. The trochophore has a spherical shape, a radially symmetrical nervous system, protonephridia, and a primary body cavity (Fig. 180). The blastopore at the trochophore is displaced from the vegetative pole closer to the animal along the ventral side, which leads to the formation of bilateral symmetry. The anus erupts later at the vegetative pole, and the intestine becomes through.

Previously, there was a point of view that in all polychaetes, the mouth and anus are formed from the blastopore. But, as was shown by the studies of V. A. Sveshnikov, a specialist in polychaetes, this situation is only a particular case of the development of polychaetes, and in most cases only the mouth is formed from the blastopore, and the anus is formed independently in the later phases of development. In the region of the posterior end of the larva, in the immediate vicinity of the anus, on the right and left sides of the intestine, a pair of cells appears - teloblasts, located in the growth zone. This is the beginning of the mesoderm. The trochophore consists of three sections: the head lobe, the anal lobe, and the growth zone. - In this area, a zone of future growth of the larvae is formed. The structural plan of the trochophore at this stage resembles the organization of lower worms. The trochophore successively transforms into a metatrochophore and a nektochaete. In the metatrochophore, larval segments are formed in the growth zone. Larval, or larval, segmentation captures only ectodermal derivatives: ciliary rings, protonephridia, rudiments of bristle sacs of future parapodia. Nektochaet differs in that it forms the brain, the abdominal nerve chain. The setae from the bristle sacs are exposed, and a parapodial complex is formed. However, the number of segments remains the same as in the metatrochophore. There may be different numbers of them in different types of polychaetes: 3, 7, 13. After a certain temporary pause, post-larval segments begin to form and the juvenile stage of the worm is formed. In contrast to larval segmentation, postlarval segments in juvenile forms include derivatives not only of the ectoderm, but also of the mesoderm. At the same time, in the growth zone, teloblasts sequentially separate the rudiments of paired coelomic sacs, in each of which a metanephridial funnel is formed. The secondary body cavity gradually replaces the primary one. Dissepiments and mesentery are formed at the boundaries of contact of the coelomic sacs.

The value of polychaete worms. The biological and practical importance of polychaete worms in the ocean is very great. The biological significance of polychaetes lies in the fact that they represent an important link in the trophic chains, and are also important as organisms that take part in the purification of sea water and the processing of organic matter.

Life

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