Annelids (annelids). Annelid worms: general characteristics of the type Nervous system of polychaete worms
The type of annelids, uniting about 12,000 species, represents, as it were, a node in the family tree of the animal world. According to existing theories, annelids trace their origins back to ancient eyelash worms(turbellar theory) or from forms close to ctenophores (trochophore theory). In turn, arthropods arose from annelids in the process of progressive evolution. Finally, in their origin, annelids are related by a common ancestor to mollusks. All this shows that great importance, which has the type in question for understanding the phylogeny of the animal world. From a medical point of view, annelids are of limited importance. Only leeches are of particular interest.
General characteristics of the type
The body of annelids consists of a head lobe, a segmented body and a posterior lobe. Segments of the body throughout almost the entire body have external appendages similar to each other and a similar internal structure. Thus, the organization of annelids is characterized by repeatability of structure, or metamerism.
On the sides of the body, each segment usually has external appendages in the form of muscular outgrowths equipped with bristles - parapodia - or in the form of bristles. These appendages are important in the movement of the worm. Parapodia in the process of phylogenesis gave rise to the limbs of arthropods. At the head end of the body there are special appendages - tentacles and sticks.
A skin-muscular sac is developed, which consists of a cuticle, an underlying layer of skin cells and several layers of muscles (see Table 1) and a secondary body cavity, or whole, in which the internal organs are located. The coelom is lined with peritoneal epithelium and divided by septa into separate chambers. Moreover, in each body segment there is a pair of coelomic sacs (only the head and posterior lobes are devoid of coelom).
The coelomic sacs in each segment are placed between the intestine and the body wall, they are filled with a watery fluid in which amoeboid cells float.
Overall it performs a supporting function. In addition, nutrients enter the coelomic fluid from the intestines, which are then distributed throughout the body. In the whole, harmful metabolic products accumulate, which are removed by the excretory organs. Male and female gonads develop in the walls of the coelom.
The central nervous system is represented by the suprapharyngeal ganglion and the ventral nerve cord. Nerves from the sensory organs pass to the suprapharyngeal node: eyes, balance organs, tentacles and palps. The abdominal nerve cord consists of nodes (one pair in each body segment) and trunks connecting the nodes to each other. Each node innervates all organs of a given segment.
Digestive system consists of the foregut, middle and hindgut. The foregut is usually divided into a number of sections: the pharynx, esophagus, crop and gizzard. The mouth is located on the ventral side of the first body segment. The hindgut opens with the anus on the posterior lobe. The intestinal wall contains muscles that move food along.
The excretory organs - metanephridia - are paired tubular organs, metamerically repeated in body segments. Unlike protonephridia, they have a through excretory canaliculus. The latter begins with a funnel that opens into the body cavity. Cavity fluid enters the nephridium through the funnel. A tubule of nephridium extends from the funnel, sometimes opening outward. Passing through the tubule, the liquid changes its composition; the final products of dissimilation are concentrated in it, which are released from the body through the external pore of nephridium.
For the first time in the phylogenesis of the animal world, annelids have a circulatory system. The main blood vessels run along the dorsal and ventral sides. In the anterior segments they are connected by transverse vessels. The dorsal and anterior annular vessels are capable of contracting rhythmically and perform the function of the heart. In most species, the circulatory system is closed: blood circulates through a system of vessels, nowhere interrupted by cavities, lacunae or sinuses. In some species the blood is colorless, in others it is red due to the presence of hemoglobin.
Most species of annelids breathe through skin rich in blood capillaries. A number of marine forms have specialized respiratory organs - gills. They usually develop on the parapodia or palps. Vessels carrying venous blood approach the gills; it is saturated with oxygen and enters the body of the worm in the form of arterial blood. Among annelids there are dioecious and hermaphroditic species. The gonads are located in the body cavity.
Annelids have the highest organization compared to other types of worms (see Table 1); For the first time, they have a secondary body cavity, a circulatory system, respiratory organs, and a more highly organized nervous system.
| Table 1. Characteristics various types worms | ||||||
| Type | Skin-muscle bag | Digestive system | Circulatory system | Reproductive system | Nervous system | Body cavity |
| Flatworms | Includes layers of longitudinal and circular muscles, as well as bundles of dorso-abdominal and diagonal muscles | From the ectodermal foregut and endodermal midgut | Not developed | Hermaphrodite | Paired brain ganglion and several pairs of nerve trunks | Absent, filled with parenchyma |
| Roundworms | Only longitudinal muscles | From the ectodermal anterior and posterior gut and the endodermal midgut | Same | Dioecious | Peripharyngeal nerve ring and 6 longitudinal trunks | Primary |
| From the external circular and internal longitudinal muscles | From the ectodermal foregut and hindgut and the endodermal midgut | Well developed, closed | Dioecious or hermaphrodite | Paired medullary ganglion, peripharyngeal nerve ring, ventral nerve cord | Secondary | |
|
Animals belonging to the type of annelids, or ringworms, are characterized by:
Annelids live in fresh and marine waters, as well as in soil. Several species live in the air. The main classes types of annelids are:
Class polychaete ringletsFrom the point of view of phylogeny of the animal world, polychaetes are the most important group of annelids, since their progressive development is associated with the emergence of higher groups of invertebrates. The body of polychaetes is segmented. There are parapodia consisting of dorsal and ventral branches, each of which carries an antennae. The muscular wall of the parapodia contains thick supporting setae, and tufts of thin setae protrude from the apex of both branches. The function of parapodia is different. Typically these are locomotor organs involved in the movement of the worm. Sometimes the dorsal barbel grows and turns into a gill. The circulatory system of polychaetes is well developed and always closed. There are species with cutaneous and gill respiration. Polychaetes are dioecious worms. They live in the seas, mainly in the coastal zone. A typical representative of the class is the Nereid (Nereis pelagica). It is found in abundance in the seas of our country; leads a bottom lifestyle, being a predator, it captures prey with its jaws. Another representative, the sandbill (Arenicola marina), lives in the seas and digs holes. It feeds by passing sea mud through its digestive tract. Breathes through gills. Class oligochaete ringletsOligochaetes originate from polychaetes. The external appendages of the body are setae, which sit directly in the body wall; no parapodia. The circulatory system is closed; skin breathing. Oligochaete ringlets are hermaphrodites. The vast majority of species are inhabitants of fresh water and soil. A typical representative of the class is the earthworm (Lumbricus terrestris). Earthworms live in the soil; During the day they sit in holes, and in the evening they often crawl out. Rummaging in the soil, they pass it through their intestines and feed on the plant debris contained in it. Earthworms play a large role in soil-forming processes; they loosen the soil and promote its aeration; they drag leaves into holes, enriching the soil with organic matter; deep layers of soil are removed to the surface, and superficial layers are carried deeper. Structure and reproduction earthworm The earthworm has an almost round body in cross section, up to 30 cm long; have 100-180 segments or segments. In the anterior third of the earthworm's body there is a thickening - the girdle (its cells function during the period of sexual reproduction and egg laying). On the sides of each segment there are two pairs of short elastic setae, which help the animal when moving in the soil. The body is reddish-brown in color, lighter on the flat ventral side and darker on the convex dorsal side. A characteristic feature of the internal structure is that earthworms have developed real tissues. The outside of the body is covered with a layer of ectoderm, the cells of which form the integumentary tissue. The skin epithelium is rich in mucous glandular cells. Under the skin there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. When the circular muscles contract, the animal’s body elongates and becomes thinner; when the longitudinal muscles contract, it thickens and pushes the soil particles apart.
The digestive system begins at the front end of the body with the mouth opening, from which food enters sequentially into the pharynx and esophagus (in earthworms, three pairs of calcareous glands flow into it, the lime coming from them into the esophagus serves to neutralize the acids of rotting leaves on which the animals feed). Then the food passes into the enlarged crop, and a small muscular stomach (the muscles in its walls help grind the food). The midgut stretches from the stomach almost to the posterior end of the body, in which, under the action of enzymes, food is digested and absorbed. Undigested remains enter the short hindgut and are thrown out through the anus. Earthworms feed on half-rotten remains of plants, which they swallow along with the soil. As it passes through the intestines, the soil mixes well with organic matter. Earthworm excrement contains five times more nitrogen, seven times more phosphorus and eleven times more potassium than regular soil. The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it - the abdominal vessel. In each segment they are united by a ring vessel. In the anterior segments, some annular vessels are thickened, their walls contract and pulsate rhythmically, thanks to which blood is driven from the dorsal vessel to the abdominal one. The red color of blood is due to the presence of hemoglobin in the plasma. Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, therefore earthworms are very sensitive to soil moisture and are not found in dry sandy soils, where their skin quickly dries out, and after rains, when there is a lot of water in the soil, they crawl to the surface. Excretory system represented by metanephridia. Metanephridia begins in the body cavity with a funnel (nephrostom) from which a duct emerges - a thin loop-shaped curved tube that opens outward with an excretory pore in the side wall of the body. In each segment of the worm there is a pair of metanephridia - right and left. The funnel and duct are equipped with cilia, causing the movement of excretory fluid. The nervous system has a structure typical of annelids (see Table 1), two abdominal nerve trunks, their nodes are interconnected and form the abdominal nerve chain. The sense organs are very poorly developed. The earthworm does not have real organs of vision; their role is played by individual light-sensitive cells located in the skin. The receptors for touch, taste, and smell are also located there. Like hydra, earthworms are capable of regeneration. Reproduction occurs only sexually. Earthworms are hermaphrodites. At the front of their body are the testes and ovaries. Earthworms undergo cross fertilization. During copulation and oviposition, girdle cells on the 32-37th segment secrete mucus, which serves to form an egg cocoon, and protein fluid to nourish the developing embryo. The secretions of the girdle form a kind of mucous muff. The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the muff stick together and a cocoon is formed, which remains in the earthen burrow. Embryonic development of eggs occurs in a cocoon, and young worms emerge from it. Earthworm tunnels are located mainly in the surface layer of soil to a depth of 1 m; in the winter they descend to a depth of 2 m. Earthworms penetrate into the soil through burrows and tunnels atmospheric air and water necessary for plant roots and the life of soil microorganisms. During the day, the worm passes through its intestines as much soil as its body weighs (on average 4-5 g). On each hectare of land, earthworms process an average of 0.25 tons of soil every day, and over the course of a year they throw out 10 to 30 tons of soil they processed to the surface in the form of excrement. In Japan, specially bred breeds of fast-reproducing earthworms are bred and their excrement is used for biological method tillage. The sugar content of vegetables and fruits grown in such soil increases. Charles Darwin was the first to point out the important role of earthworms in soil formation processes. Annelids play a significant role in the nutrition of bottom fish, since in some places worms make up up to 50-60% of the biomass of the bottom layers of reservoirs. In 1939-1940 the Nereis worm, which now forms the basis, was resettled from the Azov Sea to the Caspian Sea diet sturgeon fish of the Caspian Sea.
Leech classThe body is segmented. In addition to true metamerism, there is false ringing - several rings in one segment. There are no parapodia or setae. The secondary body cavity was reduced; instead there are sinuses and gaps between organs. The circulatory system is not closed; the blood passes only part of its path through the vessels and pours out of them into the sinuses and lacunae. There are no respiratory organs. The reproductive system is hermaphroditic. Medical leeches are specially bred and then sent to hospitals. They are used, for example, in the treatment of eye diseases associated with increased intraocular pressure (glaucoma), cerebral hemorrhage and hypertension. For thrombosis and thrombophlebitis, hirudin reduces blood clotting and promotes the dissolution of blood clots. |
TYPE RINGED WORMS
Annelids are animals that have a long, segmented body. The body segments look like rings. They live in seas, fresh waters, and soil.
Annelids have 3 aromorphoses:
· Metamerism (organ systems are repeated in different segments of the body).
· Overall(secondary body cavity, own epithelial lining).
· Lateral outgrowths of the body ( parapodia) - organs of movement, primitive limbs.
The sizes of ringed fish range from fractions of a millimeter to 3 m. The body is divided into three sections: the head. Trunk and anal lobe. The head was formed by the fusion of several body segments. The head contains the mouth opening, eyes, organs of touch (antennae, palps, etc.). The body consists of homogeneous segments, covered with a skin-muscular sac consisting of a thin cuticle, single-layer epithelium and two layers of muscles - external circular and internal longitudinal. In the anterior and middle sections of the intestine there are differentiated areas (crop, stomach). The circulatory system is closed. Blood moves only through blood vessels. Respiration is carried out either over the entire surface of the body (oligochaetes and leeches), or with the help of gills located on the parapodia (polychaetes). The excretory system is presented metanephridia. The nervous system is represented by a peripharyngeal nerve ring, consisting of the suprapharyngeal and subpharyngeal nodes connected by nerve cords, and two nerve trunks with ganglia, connected to each other by jumpers. Sense organs are more developed in movable rings.
The type of annelids is divided into classes:
1. Polychaetes
2. Oligochaetes
3. Leeches
Class polychaetes (polychaetes)
Representatives lead a free-swimming and attached lifestyle. Movement is carried out by parapodia equipped with tufts of bristles. Parapodia are prototypes of arthropod limbs. In some polychaetes, parapodia have gill apparatus that ensures gas exchange in aquatic environment. Representatives of the class have a well-separated head section, where the sensory organs are located: tentacles, light-sensitive eyes, olfactory fossa. In the structure of the nervous, circulatory, excretory and digestive systems, polychaetes repeat the characteristics of their type. Dioecious, development proceeds with metamorphosis (there is a larval stage).
Polychaetes are a progressive branch of animals from which arthropods descend. Serve as food for marine animals. Nereids are specially acclimatized in the Caspian Sea as food for sturgeon. Palolo, found in tropical waters Pacific Ocean, is of commercial importance.
Class oligochaetes
Representatives live in the soil or fresh water. The head end is not expressed. Sense organs are poorly developed. There are no parapodia and few setae. Hermaphrodites, direct development.
Class representative - earthworm. Earthworms live deep in burrows, which they dig by swallowing soil. As a result, the head section is very weakly expressed, there are no parapodia, tentacles and ocelli. The skin is permeated with blood capillaries and moistened with mucus, which makes gas exchange easier, movement in the soil easier, and mucus also has small bactericidal properties. Earthworms have a girdle that is lighter than the rest of the body.
The digestive system consists of the mouth opening, oral cavity, pharynx, long esophagus, which has an extension - goiter, muscular stomach, and intestines. It all ends with the anus. Earthworms feed on rotting plant debris, passing a lot of earth through the digestive tract.
There is no respiratory system as such; gas exchange occurs over the entire surface of the body.
Closed circulatory system. Blood moves through blood vessels, of which two are especially developed - the dorsal and abdominal. They communicate with each other through annular vessels located in each segment. There is an extensive network of capillaries. The movement of blood is determined by the rhythmic contractions of blood vessels from the 7th to the 11th segment. Blood plasma contains respiratory pigments similar to hemoglobin.
The excretory system consists of paired convoluted tubes (metanephridia), which begin as a funnel with ciliated cells on the walls in the body cavity, and end with an excretory pore that opens outward.
The nervous system is represented by a peripharyngeal nerve ring, consisting of 2 suprapharyngeal and 2 subpharyngeal nodes, connected by nerve cords. Two nerve trunks depart from the subpharyngeal node, having thickenings in each segment - ganglia, which are connected to each other by jumpers.
The earthworm is a hermaphrodite. Fertilization is cross, development is direct. One worm has both male and female reproductive organs: female oviducts, male testes, vas deferens and spermatheca. The girdle forms a special mucus from which the muff is formed. The coupling begins to move towards the head ring, passing by the ducts of the oviducts, where the eggs enter. The muff then passes through the seminal receptacles, where the sperm is released.
Ringed animals are capable of regeneration. Earthworms influence the properties of the soil; By digging numerous burrows, they improve its structure, loosen it, and enrich it with organic matter.
Polychaete polychaete worms are the largest group of organisms. Scientists count about 10 thousand species of the annelid class. Common representatives: sandworm, living in the Arctic and Arctic Ocean.
A distinctive feature is the numerous bristles collected in tufts located on the sides of each segment.
Appearance
The body of a polychaete worm is divided into a large number of divisions, ranging from five to eight hundred pieces, but sometimes there are exceptions.
Description
Like similar worms, the body of polychaete worms is divided into several parts:
- head
- long
- torso
- anal blade
located at the rear of the mill.
They are inhabitants of the water depths; they are covered with skin-muscular processes - organs of movement, which are called parapodia, it is with the help of them that movement forward is possible.
The entire carcass of the worm is dressed in a muscle sac. The outside of the body consists of a thin cuticle covering the epithelium. Under the skin of the polychaete there is a musculature consisting of longitudinal and circular muscles. The ringlets are from two millimeters to three meters long, which is quite large for invertebrates.
Habitat
Polychaetes mainly live in salty waters and lead a bottom-dwelling lifestyle. However, there are individuals that vegetated in a zone not located in close proximity to the bottom; these individuals include the Tomopterid family. There are also polychaetes that have adapted to fresh water and woody soil.
Nutrition
The diet of the polychaete polychaete worm is relatively varied. Most feed on detritus - dead organic matter; this choice is associated with a sedentary lifestyle. But there are also species that eat mollusks, coelenterates, and ampictinids.
Enemies
Fish and some types of crustaceans love to eat polychaete worms, because they are tasty and healthy food. Let's talk about people's use of worms for fishing, since this activity sharply reduces their numbers.
Reproduction
Polychaete worms are heterosexual, with the exception of some hermaphrodites. Both females and males have gonads. The female has eggs, and the male has sperm. Due to external fertilization, a larva is formed from the eggs - trophora.
The trophora moves through outgrowths, sinking to the bottom, where metamorphosis into an adult takes place. Some families of Polychaete worms also reproduce asexually. There are a couple of types of asexual reproduction: archetomy and paratomy .
In the first case, the body is divided into dozens of segments, which later grow to a normal state, and in the second variation everything happens exactly the opposite.
Digestive system
Worms and their system are very curious; the system responsible for receiving energy is represented by the mouth, pharynx, which has chitinous teeth, esophagus and stomach. These unusual creatures have an intestine divided into three sections:
- front
- average
- rear
The last part contains the anal ring.
Circulatory system
Polychaetes have a closed circulatory system, each representative of annelids has a closed circulatory system, that is, blood always flows through the vessels.
There are two main vessels in the camp, connected by semi-circular formations: dorsal and abdominal. There is no heart, but its duties are performed by the folding of the walls of the spinal vessel and other rather large capillaries.
Nervous system
Freely moving polychaete worms have developed sensory organs, expressed by two tentacles and antennae. The smaller part for polychaetes has vision and balance organs. And all this is achievable thanks to the nerve nodes and nerves that permeate the entire body.
Excretory system
Harmful liquid is removed using paired tubes located in each segment of the carcass.
Meaning, interesting facts
Despite their small size, they perform many important functions for nature:
- They clean the pond
- Eats decaying remains
- They are food for marine life.
Lifespan
Polychaete annelid worms live about six years.
Type Annelids
The most important aromorphoses of the type:
1) organs of movement appear - parapodia,
2) the first respiratory organs appear,
3) secondary body cavity – in general,
4) the circulatory system appears.
The circulatory system in animals can be of 2 types: closed And open. In a closed circulatory system, blood flows only through the vessels and does not flow out of them. In an open circulatory system there are only large vessels; they open into the body cavity. Therefore, the blood pours out of the vessels, washes the internal organs and then collects again in the vessels.
In annelids closed circulatory system.
Animals of this type are characterized by segmentation - their body is divided into repeating sections - segments that look like rings. Hence the name of the type. Moreover, the segments have exactly the same external and internal structure. And the body cavity is also divided into compartments by partitions.
The body of a worm can contain from 5 to 800 segments. Among them, only the first segment stands out, bearing the mouth and, in some, sensory organs, as well as the anal lobe.
The phylum Annelids includes several classes, the most important of which are Polychaetes, Oligochaetes and Leeches.
Class Polychaetes (Polychaetes)
Most polychaetes live in the seas. They live on the bottom, where they crawl between vegetation and rocks. Among them there are also sessile forms - they are attached to the bottom and form a protective tube around themselves.
Let's consider polychaete worms using the Nereid as an example. Its body is reddish or green in color. Nereid is a predator; it feeds on organic debris and plankton.
On the head lobe of the Nereid, antennae (organs of touch), tentacles, 2 pairs of eyes and olfactory pits are visible. The segments of the body have muscular outgrowths - parapodia. Parapodia have bristles that allow the worms to cling to the bottom like claws. They move either with the help of parapodia along the bottom, relying on them as levers, or they swim, bending their whole body in waves.
The body wall of the Nereid, like other worms, is formed by a skin-muscle sac. It consists of a single-layer epithelium covering the outside of the worm, 2 layers of muscles (circular and longitudinal) and epithelium lining the body cavity.
Also, in each segment of the nereid, special muscle groups are formed that control the parapodia.
Body cavity Nereids secondary (overall)– has an epithelial lining and is filled with fluid. The coelom is located between the organs and is an epithelial sac filled with fluid. The secondary cavity serves as a hydroskeleton (creates support during movement), transports nutrients, metabolic products, and also serves as a place for the formation of germ cells.
Cross section of a Nereid's body
Digestive system. The nereid develops tentacles on the head lobe, which serve to transfer prey to the mouth. The digestive system begins with the mouth, then the pharynx, equipped with chitinous outgrowths that act as teeth → esophagus → crop → stomach → tubular midgut, hindgut → anus. The esophagus and midgut contain glands that secrete digestive juices.
Respiratory system first appears in annelids. Most often, the respiratory organs are represented by outgrowths of the dorsal branch of the parapodia and have a branched structure. But not everyone has gills. The Nereid breathes throughout the entire surface of its body.
Internal structure ringlets using the example of an earthworm
Circulatory system also occurs for the first time in annelids. She is a closed type. There are 2 main vessels in the circulatory system: dorsal and abdominal. Along the entire length of the body, they are connected by transverse bridges and branch into capillaries - the smallest vessels that carry blood to all cells. Thanks to the reduction spinal vessel(no heart) blood moves through the worm's body.
Excretory system Nereids are represented by metanephridia. They form paired excretory tubes in each body segment. Metanephridia consist of a funnel that bears cilia and opens into a whole. The beating of the cilia forces body cavity fluid into the funnel and then into the convoluted tubule. The tubule is densely entwined with blood capillaries, which take everything back into the blood useful material (the right water, vitamins and nutrients), and metabolic products and excess water are thrown out through the excretory pores.
It is characteristic that the funnel opens as a whole in one segment, and the excretory canaliculus
Metanephridia
sometimes opens outward in another segment.
Nervous system – ventral nerve cord. Consists of the peripharyngeal nerve ring and the ventral nerve cord, which forms a ganglion in each segment (therefore resembling beads or a chain).
Sense organs are developed quite well in the Nereid. There are organs of touch and chemical sense (“taste”) - these are various outgrowths of the head lobe (antennae, tentacles, antennae). 4 eyes are well developed, and there are also balance organs - statocysts.
Reproduction. Nereids are dioecious, but their sexual dimorphism is not pronounced. The reproductive cells of worms are formed directly in the coelom - eggs in females, sperm in males. They are excreted through the channels of the excretory system. Fertilization is external—male and female gametes fuse in water.
Development proceeds with metamorphosis - the trochophore larva is completely different from the adult. It swims with the help of cilia, and after a while it settles to the bottom and turns into an adult worm.
In polychaete worms, asexual reproduction occurs - by budding and fragmentation. Fragmentation is the division of a worm in half, after which each half restores the missing part. Sometimes this creates a whole temporary chain of 30 worms.
Class Polychaeta
with all the colors of the rainbow bristles. Serpentine phyllodoces (Phyllodoce) swim and crawl quickly. Tomopteris (Tomopteris) hang in the water column on their long whiskers.
The class of polychaetes differs from other ringlets by a well-separated head section 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 body 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 body segments (Fig. 172). The mouth is located ventrally on the peristomium. Many polychaetes have eyes and sensory appendages on their heads. Thus, in a Nereid, on the prostomium of the head there are two pairs of ocelli, tentacles - tentacles and two-segmented palps, on the peristomium below there is a mouth, and on the sides there are several pairs of antennae. The trunk segments have paired lateral projections 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, there is a dorsal barbel on the dorsal side, and a ventral barbel on the ventral side. These are the sensory organs of polychaetes. Often the dorsal barbel in some species is transformed into feathery gills. Parapodia are armed with tufts of bristles consisting of an organic substance close to chitin. Among the setae there are several large setae-acicules, to which muscles are attached from the inside, driving the parapodia and tuft of setae. The limbs of polychaetes make synchronous movements like oars. In some species leading a burrowing or attached lifestyle, parapodia are reduced.
Skin-muscle bag(Fig. 174). The body of polychaetes is covered with a single-layer dermal epithelium, which secretes a thin cuticle onto the surface. In some species, certain parts of the body may have ciliated epithelium (a longitudinal ventral stripe or ciliated bands around the segments). Glandular epithelial cells in sessile polychaetes can secrete a protective horny tube, often impregnated with lime.
Under the skin lies circular and longitudinal muscles. The longitudinal muscles form four longitudinal ribbons: two on the dorsal side of the body and two on the abdominal side. There may be more longitudinal strips. On the sides there are bundles of fan-shaped muscles that drive the parapodium blades. The structure of the skin-muscle sac varies greatly depending on lifestyle. The inhabitants of the ground surface have the most complex structure of the skin-muscle sac, close to that described above. This group of worms crawls along the surface of the substrate using serpentine body bending and parapodia movements. The inhabitants of calcareous or chitinous tubes 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 retreat into the depths of the tube, which allows them to escape from attacks by predators, mainly fish. In pelagic polychaetes, the muscles are poorly developed, since they are passively transported 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 of Nereis pelagica (according to Ivanov): 1 - dorsal antenna, 2 - notopodium lobes, 3 - setae, 4 - neuropodium lobes, 5 - ventral antenna, 6 - neuropodium, 7 - acicula, 8 - notopodium
Rice. 174. Cross section of a polychaete worm (according to Natalie): 1 - epithelium, 2 - circular muscles, 3 - longitudinal muscles, 4 - dorsal antennae (gill), 5 - notopodium, 6 - supporting seta (acicula), 7 - neuropodium, 8 - funnel of nephridium, 9 - canal of nephridium, 10 - oblique muscle, 11 - abdominal vessel, 12 - ovary, 13 - abdominal antenna, 14 - setae, 15 - intestine, 16 - coelom, 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 the muscle bands 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 the cavity, only conventionally called secondary B more difficult case The coelomic epithelium can completely cover the intestines and muscles. The coelom is fully represented in the case of the development of paired metameric coelomic sacs (Fig. 175). When paired coelomic sacs close in each segment above and below the intestine, the dorsal and abdominal mesentery, or mesenteries, are formed. Between the coelomic sacs of two adjacent segments, transverse partitions are formed - dissepiments. The wall of the coelomic sac, lining the inside muscles of the body wall, is called the parietal layer of mesoderm, and the coelomic epithelium , covering the intestine and forming the mesentery, is called the visceral layer of mesoderm. Blood vessels lie in the coelomic septa.
Rice. 175. Internal structure of polychaetes: A - nervous system and nephridia, B - intestine and whole, C - intestine, 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 - coelom, 8 - intestine, 9 - diosepiment, 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 - muscles of the pharynx, 24 - palps
The whole performs several functions: musculoskeletal, transport, excretory, sexual and homeostatic. Cavity fluid maintains body turgor. When the circular muscles contract, 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 method of movement, in which the cavity fluid, when muscles contract under pressure, is driven to the front end of the body, providing energetic forward movement. Overall there is transport nutrients from the intestines and dissimilation products from various organs and tissues. The organs for excreting metanephridia by funnels open as a whole and ensure the removal of metabolic products and excess water. In the whole, there are mechanisms to maintain the constancy of the biochemical composition of the fluid and water balance. In this favorable environment Gonads form on the walls of coelomic sacs, germ cells mature, and in some species even juveniles develop. Derivatives of the coelom - coelomoducts - serve to remove sexual products from the body cavity.
Digestive system consists of three sections (Fig. 175). The entire anterior section consists of derivatives of the ectoderm. The anterior section begins with the oral opening located on the peristomium on the ventral side. Oral cavity passes into the muscular pharynx, which serves to capture food objects. In many species of polychaetes, the pharynx can turn outward, like the finger of a glove. In predators, the pharynx consists of several layers of circular and longitudinal muscles, is armed with strong chitinous jaws and rows of small chitinous plates or spines, capable of firmly holding, wounding and crushing captured prey. In herbivorous and detritivorous forms, as well as in sestivorous polychaetes, the pharynx is soft, mobile, adapted for swallowing liquid food. Following the pharynx is the esophagus, into which 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 final digestion and absorption of nutrients. In carnivores, the midgut is relatively shorter, sometimes equipped with paired blind side pouches, while in herbivores, the midgut is long, convoluted, and usually filled with undigested food debris.
The hind intestine is of ectodermal origin and can perform the function of regulating water balance in the body, since there water is partially absorbed back into the coelom cavity. Fecal matter forms in the hindgut. The anal opening usually opens on the dorsal side of the anal blade.
Respiratory system. Polychaetes mainly have cutaneous respiration. But a number of species have dorsal cutaneous gills formed from parapodial antennae or head appendages. 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 ventral trunks, connected by annular vessels, as well as peripheral vessels (Fig. 175). Blood movement is carried out as follows. Through the dorsal, largest and most pulsating vessel, blood flows to the head end of the body, and through the abdominal - in the opposite direction. Through the ring vessels in the front part of the body, blood is distilled from the dorsal vessel to the abdominal one, and in the back part of the body - vice versa. Arteries extend from the annular vessels to parapodia, gills and other organs, where a capillary network is formed, from which blood collects into venous vessels that flow into the abdominal bloodstream. In polychaetes, the blood is often red due to the presence of the respiratory pigment hemoglobin dissolved in the blood. Longitudinal vessels are suspended on the mesentery (mesentery), annular vessels pass inside the dissepiments. Some primitive polychaetes (Phyllodoce) lack a 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 phylum annelids. Each segment contains a pair of metanephridia (Fig. 176). Each metanephridia consists of a funnel, lined inside with cilia and open as a whole. The movement of the cilia drives solid and liquid metabolic products into the nephridium. A canal extends from the funnel of the nephridium, which penetrates the septum between the segments and in another segment opens outwards with an excretory opening. In the convoluted channels, ammonia is converted into high-molecular compounds, and water is absorbed as a whole. U different types Polychaetes' excretory organs can be of different origins. Thus, some polychaetes have protonephridia of ectodermal origin, similar in
Rice. 176. The excretory system of polychaetes and its relationship with coelomoducts (according to Briand): A - protonephridia and genital funnel (in a hypothetical ancestor), B - nephromyxium with protonephridium, C - metanephridia and genital funnel, D - nephromyxium; 1 - coelom, 2 - genital funnel (coelomoduct), 3 - protonephridia, 4 - metanephridia
structure with those of flatworms and roundworms. Most species are characterized by metanephridia of ectodermal origin. In some representatives, complex organs are formed - nephromyxia - the result of the fusion of protonephridia or metanephridia with the genital funnels - coelomoducts of mesodermal origin. Additionally, the excretory function can be performed by chloragogenic cells of the coelomic epithelium. These are peculiar storage buds in which grains of excreta are deposited: guanine, uric acid salts. Subsequently, chloragogenic cells die and are removed from the coelom through nephridia, and new ones are formed to replace them.
Nervous system. Paired suprapharyngeal 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. Periopharyngeal nerve cords extend from the brain - connectives to the abdominal nerve cord, which consists of paired ganglia, repeating in segments. Each segment has one pair of ganglia. The 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 the paired ganglia merge, a nerve chain is formed (Fig. 177). In some species, the nervous system becomes more complex due to the fusion of ganglia from several segments.
Sense organs most developed in motile 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 polychaetes living in tubes have numerous eyes on the feathery gills of the head. In addition, they have developed organs of smell and 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 are dioecious. Their gonads develop in all segments of the body or only in some of them. The gonads are of mesodermal origin and form on the wall of the coelom. The germ cells from the gonads enter the whole, where their final maturation occurs. Some polychaetes do not have reproductive ducts and the germ cells enter the water through breaks in the body wall, where fertilization occurs. In this case, the parent generation dies. A number of species have genital funnels with short channels - coelomoducts (of mesodermal origin), through which the reproductive products are excreted out into the water. In some cases, germ cells are removed from the coelom through nephromyxia, which simultaneously perform the function of the reproductive and excretory ducts (Fig. 176).
Rice. 177. Nervous system of polychaetes: 1 - nerves of the antennae, 2 - neopalps, 3 - mushroom body, 4 - eyes with a lens, 5 - nerves of the peristomal antennae, 6 - mouth, 7 - peripharyngeal ring, 8 - abdominal ganglion of the peristomium, 9- 11 - parapodia nerves, 12 - ganglia of the ventral nerve chain, 13 - nerve endings of the nuchal organs
Reproduction Polychaetes can be sexual or asexual. In some cases, alternation of these two types of reproduction (metagenesis) is observed. Asexual reproduction usually occurs by transverse division of the worm's body into parts (strobilation) or budding (Fig. 178). This process is accompanied by the regeneration of missing body parts. Sexual reproduction often associated with the phenomenon of epitoky. Epitoky is a sharp morphophysiological restructuring of the worm's body with a change in body shape during the period of maturation of reproductive products: segments become wide, brightly colored, with swimming parapodia (Fig. 179). In worms that develop without epitocy, males and females do not change their shape and reproduce in benthic conditions. Species with epitocy may have several variants life cycle. One of them is observed in Nereids, the other in Palolos. Thus, in Nereis virens, males and females become epitocous and float to the surface of the sea to reproduce, 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 atokny individual, and the posterior end of the body is transformed into an epitokny tail part filled with sexual products. The back parts of the worms break off and float to the surface of the ocean. Here the reproductive products are released into the water and fertilization occurs. Epitocene individuals of the entire population emerge to reproduce simultaneously, 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 massive appearance of reproducing polychaetes in the surface layers of water is usually associated with the phases of the Moon. Thus, 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 periods of reproduction of palolos, and fishermen en masse catch palolos stuffed with “caviar” and use them for food. At the same time, fish, seagulls, and sea ducks feast on worms.
Development. The fertilized egg undergoes uneven, spiral crushing (Fig. 180). This means that as a result of fragmentation, quartets of large and small blastomeres are formed: micromeres and macromeres. In this case, the axes of the cell cleavage spindles are arranged in a spiral. The inclination of the spindles changes to the opposite with each division. Thanks to this, the crushing figure has a strictly symmetrical shape. Egg crushing in polychaetes is determinate. Already at the stage of four blastomeres, determination is expressed. Quartets of micromeres give derivatives of 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 - epitocous individuals - female and male Autolytus (according to Sveshnikov)
endoderm and mesoderm. The first mobile stage is the blastula - a single-layer larva with cilia. The blastula macromeres at the vegetative pole plunge into the embryo and the gastrula is formed. At the vegetative pole, the primary mouth of the animal is formed - the blastopore, and at the animal pole, a cluster of nerve cells and a ciliated crest - the parietal plume of cilia - is formed. Next, the 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 of the trochophore shifts from the vegetative pole closer to the animal along the ventral side, which leads to the formation of bilateral symmetry. The anal opening breaks through later at the vegetative pole, and the intestines become 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 polychaete specialist V.A. Sveshnikov, this situation represents only a special case of the development of polychaetes, and in most cases only a mouth is formed from the blastopore, and the anus forms independently at later phases of development. In the area 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 rudiment of mesoderm. The trochophore consists of three sections: the head lobe, the anal lobe and the growth zone. -In this area, the zone of future growth of the larva is formed. The structural plan of the trochophore at this stage resembles the organization of lower worms. The trochophore successively turns into a metatrochophore and a nectochaete. In the metatrochophore, larval segments are formed in the growth zone. Larval, or larval, segmentation involves only ectodermal derivatives: ciliary rings, protonephridia, rudiments of the setal sacs of future parapodia. Nektochaete is distinguished by the fact that it develops a brain and an abdominal nerve cord. The setae from the setal sacs are exposed, and the parapodial complex is formed. However, the number of segments remains the same as in the metatrochophore. Different types of polychaetes may have different numbers of them: 3, 7, 13. After a certain time pause, postlarval segments begin to form and the juvenile stage of the worm is formed. In contrast to larval segmentation, postlarval segments in juvenile forms capture derivatives of not only ectoderm, but also mesoderm. At the same time, in the growth zone, teloblasts sequentially separate the rudiments of paired coelomic sacs, in each of which a metanephridia funnel is formed. The secondary body cavity gradually replaces the primary one. At the borders of contact of the coelomic sacs, dissepiments and mesenterium are formed.
Due to the remaining primary body cavity, longitudinal vessels are formed in the lumen of the mesentery circulatory system, and in the gaps of the septa they are ring-shaped. Due to the mesoderm, the muscles of the skin-muscular sac and intestines, the lining of the coelom, gonads and coelomoducts are formed. The nervous system, metanephridia channels, foregut and hindgut are formed from the ectoderm. The midgut develops from the endoderm. After completion of metamorphosis, an adult animal develops with a certain number segment for each type. The body of an adult worm consists of a head lobe, or prostomium, developed from the head lobe of the trochophore, several larval segments with a primary cavity, and many postlarval segments with a coelom and an anal lobe without a coelom.
Thus, the most important features The development of polychaetes is spiral, determinate fragmentation, teloblastic anlage of 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 prominent Soviet embryologist P. P. Ivanov. This discovery shed light on the origin of annelids from oligomeric ancestral forms.
The consistent change in the phases of 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. they were oligomeric. Among modern polychaetes, the closest to ancestral forms are some primary ringlets of the class Archiannelida, in which the number of segments usually does not exceed seven. Manifestations of primitive organizational features at the trochophore and metatrochophore stages (primary cavity, protonephridia, orthogon) indicate the relationship of coelomic animals with the group of lower worms.
The biological significance of the development of polychaete worms with metamorphosis lies in the fact that the floating larvae (trochophores, metatrochophores) ensure the dispersal of species that, as adults, lead a predominantly bottom lifestyle. Some polychaete worms show care for their offspring and their larvae are inactive and lose their distribution function. In some cases, live births are observed.
The meaning of polychaete worms. Biological and practical significance Polychaete worms are very numerous in the ocean. The biological significance of polychaetes lies in the fact that they represent an important link in trophic chains, and are also important as organisms that take part in the purification of sea water and the processing of organic matter.
substances. Polychaetes have food value. To strengthen the food supply 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 Azov Sea, was carried out. This successful experiment was carried out under the leadership of Academician L.A. Zenkevich in 1939-1940. Some polychaetes are used as food by humans, such as the Pacific palolo worm (Eunice viridis).
