Polychaete worms are by far the largest group of marine organisms. Most often, representatives of the class live in a body of water and much less often lead a planktonic way of life.
Polychaete worms: body structure
The body of a representative of this class consists of a head section, a long body and a specific anal lobe. In most cases, the body of such an animal is clearly divided into several segments, to each of which a parapodium is attached.
Parapodia are nothing more than primitive limbs with small antennae and setae. Interestingly, the parapodia of some representatives of the group were transformed into gills.
Like other representatives of the annedil type (leeches, oligochaete worms), this animal’s body consists of a skin-muscular sac. From above, the body of the worm is covered with a thin protective cuticle, under which there is a single-layer epithelium. Under the skin there is muscle, which consists of longitudinal and circular muscles, which are responsible for the movement and contraction of the animal’s body.
Polychaete worms: internal structure
Representatives of this class have a fairly developed digestive system, which consists of three parts. The anterior part consists of the oral opening, which opens into the oral cavity. Then the food particle enters the muscular pharynx. By the way, it is in the pharynx that powerful jaws made of chitin are contained. Some species are even able to turn it outward.
After grinding, the food enters the esophagus, where the main glands that produce saliva open. Only some representatives have a small stomach. The animal's midgut serves to completely digest and absorb essential nutrients. The hindgut is responsible for the formation of feces and opens with the anus on the dorsal part of the anal lobe.
Polychaete worms have a closed artery that consists of a dorsal and ventral artery. By the way, the dorsal vessel is large and has contractile functions, so it works like a heart. In addition, the large arteries are connected by so-called annular vessels, which carry blood to the limbs and gills.
Representatives of this class do not have a respiratory system. The gas exchange organs are the skin and gills, which are located either on the parapodia or in the anterior, head part of the body.
The excretory system consists of small metanephridia, which remove unnecessary waste products from the coelomic fluid into external environment. Each segment has its own pair of excretory organs, which open outwards with small openings - nephropores.
As for the nervous system, it consists of a typical peripharyngeal ring, from which the ventral nerve cord extends. It is interesting that almost all representatives of this class have a highly developed sense of smell. Some species also have eyes.
Polychaete worms: and reproduction
To begin with, it is worth noting that almost all species of this group are capable of which in most cases is represented by fragmentation of the body, less often by budding.
However, animals have a well-developed reproductive system. The reproduction of worms is exclusively dioecious. The gonads form on the wall of the secondary body cavity. The release of germ cells can be carried out through tissue rupture - in this case, the adult dies. Some representatives have specific openings through which gametes are released. Fertilization occurs in an aquatic environment. From the zygote a larva develops, which in appearance bears little resemblance to an adult. Accordingly, the development of a young worm occurs with metamorphoses.
Type annelids unites about 9,000 species that have the most perfect organization among other worms. Their body consists of a large number of segments; many have bristles on the sides of each segment that play important role when moving. Internal organs are located in a body cavity called coelom. There is a circulatory system. In the anterior part there is a cluster of nerve cells that form the subpharyngeal and suprapharyngeal nerve nodes. Annelids live in fresh water bodies, seas and soil.
Most of the representatives of annelids belong to the classes: oligochaetes, polychaetes and leeches.
Class oligochaetes
Representative of the class oligochaetes - earthworm lives in burrows in moist humus soil. The worm crawls to the surface in damp weather, at dusk and at night. In an earthworm, the anterior and abdominal parts of the body can be easily distinguished. In the anterior part there is a thickening of the girdle; on the ventral and lateral sides of the body, elastic and short bristles are developed.
The body of the worm is covered with skin made of integumentary tissue, in which the cells adhere tightly to each other. The skin contains glandular cells that secrete mucus. Under the skin there are circular and deeper - longitudinal muscles, thanks to the contraction of which the body of the worm can lengthen or shorten, thereby moving through the soil.
The skin and muscle layers form skin-muscle sac, inside which there is a body cavity, where they are located internal organs. Earthworms feed on rotting plant debris. Through the mouth and pharynx, food enters the crop and muscular stomach, where it is ground and enters the intestine and is digested there. Digested substances are absorbed into the blood, and undigested substances are excreted along with the soil through the anus.
Circulatory system of an earthworm closed and consists of dorsal and abdominal blood vessels, connected to each other by ring vessels from each segment. Around the esophagus there are larger annular vessels that act as the “hearts” of the large vessels; side branches branch off, forming a network of capillaries. The blood does not mix anywhere with the fluid of the body cavity, which is why the system is called closed.
The excretory organs are represented by convoluted tubes through which liquid and harmful substances are removed from the body.
The nervous system consists of the peripharyngeal nerve ring and the ventral nerve cord. The earthworm does not have specialized sensory organs. There are only different kinds of sensitive cells that perceive external stimuli (light, smell, etc.).
Earthworms are hermaphrodites. However, they have cross-insemination; two individuals participate in this process. When eggs are laid on the worm's belt, copious mucus is formed, into which the eggs fall, after which the mucus darkens and hardens, forming a cocoon. Then the cocoon is thrown off the worm through the head end of the body. Inside the cocoon, young worms develop from fertilized eggs.
Among the oligochaetes there are dwarfs, whose body length does not exceed a few millimeters, but there are also giants: Australian earthworm 2.5-3 m long.
Characteristic of earthworms regenerative ability. Earthworms are called soil formers, since they, by making passages in the soil, loosen it and promote aeration, that is, the entry of air into the soil.
Class Polychaetes
This includes a variety of marine worms. Among them nereid. Her body consists of a large number of segments. The anterior segments form the head section, on which the mouth and sensory organs are located: touch - tentacles, vision - eyes. On the sides of the body, each segment has lobes on which numerous bristles sit in tufts. With the help of blades and bristles, Nereids swim or move along the bottom of the sea. They feed on algae and small animals. Breathe with the entire surface of the body. Some polychaetes have gills- primitive respiratory organs.
Refers to polychaetes sandstone, living in burrows, in the sand, or building itself a gypsum turtle, which is attached to algae. Many marine fish feed on nereids and other annelids.
Leech class
The most famous representative of this class is medical leech, which was already used in ancient times to treat people. Leeches are characterized by the presence of two suckers: the front one, at the bottom of which the mouth is located, and the back one.
The posterior sucker is large, its diameter exceeding half the greatest width of the body. Leeches bite through the skin with three jaws lined with sharp teeth along the edges (up to 100 on each jaw). Strong bloodsucker. In medicine, it is used for diseases of blood vessels (formation of blood clots), hypertension, and pre-stroke conditions. Leeches are applied to a certain part of a sick person to suck out blood; as a result, blood clots dissolve, blood pressure decreases, and the person’s condition improves. In addition, the salivary glands medical leech produce a valuable substance - hirudin, - preventing blood clotting. Therefore, after leech injections, the wound bleeds for a long time. While in the leech's stomach, the blood, under the influence of hirudin, is preserved for months without being subject to coagulation and rotting.
The leech's digestive system is designed in such a way that it can accumulate large reserves of blood, preserved with the help of hirudin. The size of a leech that has sucked blood increases significantly. Thanks to this feature, leeches can starve for a long time (from several months to 1 year). A leech lives up to 5 years. Leeches are hermaphrodites. In nature I achieve! They reach sexual maturity only in the third year of life and lay cocoons once a year in the summer.
Leeches are characterized by a straight development. Leeches include the non-blood-sucking predatory leech - large pseudokonskaya. It eats worms (including leeches), soft-bodied animals, larvae of aquatic insects, small vertebrates (tadpoles), which it can overcome.
General characteristics. Polychaetes are the most ancient group of the annelids, giving rise to other classes of this type. The reason for the name of the class was the fact that these worms developed peculiar organs on the sides of the body segments - parapodia, equipped with numerous bristles.
Body polychaete worms consists of a number of externally similar segments. The anterior segments, merging, form a well-defined head section, on which the mouth and various appendages, as well as some sensory organs, are located. Animals are dioecious. They develop with metamorphosis (Fig. 106). Several thousand species have been described. They are of practical importance as fish food.
Rice. 106. Forks of polychaete annelids:
/ - sand core
(Arenieola);
II yersis (Nereis); 111 -
phyllodesch1
(I"
ln/
lodoce);
IV.....-■ amphntrigo (Amphitrite);
V-
sabellarine
(Suhellariu);
VI- yerpula
(Serptilci),
VII marine mnpp>
(Aphrodite);
I- tentacles; 2 -
gills; 3
........ paranodpi
Structure and vital functions. The body length of polychaete worms ranges from a few millimeters to 1 m or more.
The organs of movement are narapodia - paired lateral outgrowths, usually consisting of an unpaired main part and two lobes: dorsal and ventral (Fig. 107). Each lobe contains a tuft of elastic bristles, as well as usually a tactile antenna. When the dorsal lobe is underdeveloped, the parapodia become single-branched. Parapodia are used by worms when crawling along the bottom of a reservoir, and when the animal swims they play the role of fins. In worms that burrow in the ground or live in tubular houses, the parapodia are completely or partially reduced.
The integument of polychaete worms leading active life at the bottom of the reservoir, they are distinguished by a well-developed cuticle. On the contrary, worms that swim in the water column, burrow into the ground or build tubular houses have a very thin cuticle. Secretions from the integument serve as a building and cementing material in the construction of tubes in which some polychaete worms live.
The sense organs of most moss-chaete worms are well developed. On the head (Fig. 108) there are usually 1-2 pairs of eyes, tactile antennae, tentacles and olfactory pits.
Respiratory organs - gills. For some, they are absent, and breathing occurs over the entire surface of the body.
Reproductive organs. Polychaete worms are usually dioecious; there are no external differences between the sexes. Parthenogenesis is observed in some species. Most lay eggs, but viviparous forms are also found. Some reproduce by budding, which can result in the formation of temporary branched colonies.
Rice. 108. Head of the polychaete annelid worm Nereis:
The development of polychaete worms occurs with or without metamorphosis. In most forms, microscopically small floating larvae emerge from the eggs - trochophores, which have a round, non-segmented body with belts of cilia. First, they have a primary body cavity, which is replaced by a secondary cavity during the development of the animal.
Polychaete worms inhabit the seas, living from shallow waters to great depths. Some representatives are found in freshwater basins, for example in lake. Baikal. Most species live on the bottom, but some live in the water column. Some are very mobile and are able to crawl along the bottom and swim, others burrow into the ground, others constantly live in a tube-shaped house built of lime or organic matter secreted by the integument of the worm.
Each of environmental groups polychaete worms have their own organizational features, feeding methods, and protective devices. Those living in houses usually have underdeveloped aranodes; their head tentacles, merging, can form a lid that covers the entrance to the house. The tentacles of some sessile species acquire a pinnately branched shape; they act as gills and are involved in obtaining food. Their surface is covered with ciliated epithelium, the cilia of which drive water with food particles suspended in it to the mouth. A free-swimming larva is used to disperse such attached worms.
Some polychaetes that burrow in the ground have a proboscis with teeth that help them burrow. Their parapodia are often atrophied, and short bristles stick out directly from the body in tufts.
Many species of polychaetes serve as the main food for commercial fish. sea fish. Therefore, their distribution and abundance are taken into account when assessing the biological productivity of water bodies and exploring reserves commercial fish. Of the annelids that are food for fish, those living in shallow water are important Nereids (Nereis). At the suggestion of Prof. L. A. Zenkevich in 1939-1941. they were resettled from the Azov Sea to the Caspian Sea, where they had not previously been found. Nereids have taken root well in the new conditions and have now become valuable food for sturgeon fish in the Caspian Sea.
A peculiar polychaete worm sandstone (Arenicolamarina) inhabits sandbanks in large numbers. It lives in silted sand, passing it through its intestines and digesting the organic matter it contains.
In the Pacific worm palolo (Euniceviridis) During the breeding season, segments of the posterior part of the body, filled with reproductive products, break off and float to the surface of the sea. By breaking the walls of the segments, the eggs contained in them or
sperm go into the water, where they unite. The zygotes develop into floating larvae, and from them into adult worms that sink to the bottom. Such reproduction promotes the spread of sedentary worms. During the period of mass ascent, the local population obtains them in large quantities and is used as a food product.
Nereids. Syllides. Palolo
Polychaetes- marine free-living annelids. They lead a benthic, interstitial, and rarely planktonic lifestyle.
rice. 1.
A - top view, B - side view (pharynx extended
state), B - side view (pharynx retracted
condition): 1 - prostomium, 2 - ocelli, 3 - peristomium,
4 - palps, 5 - jaws, 6 - pharynx, 7 - parapodium,
8 - tentacles, 9 - setae, 10 - mouth.
Like all annelids, 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). On the head of many polychaetes there are eyes and sensory appendages (tentacles, palps, antennae), and a mouth is located on the peristomium below (Fig. 1).
In most species, each of the body segments bears a pair of primitive limbs - parapodia (Fig. 2). Each parapodia consists of a basal part, a dorsal lobe and a ventral lobe. The dorsal lobe has a dorsal "barbel", the ventral lobe has a ventral "barbel". The dorsal "antennae" of some species is transformed into feathery gills. The parapodia are permeated with setae; muscles that cause the parapodia to move are attached to the large aciculum setae. The movement of parapodia is synchronous.
rice. 2.
1 - dorsal lobe, 2 - abdominal
lobe, 3 - dorsal antenna, 4 - ventral
antenna, 5 - setae, 6 - acicula.
The skin-muscle sac has a structure typical of annelids, which includes a cuticle, a single-layer epithelium and two layers of muscles (Fig. 3). The circular muscles are located under the epithelium, the longitudinal muscles are located under the circular muscles. The longitudinal muscles are located in four “ribbons”, two of these ribbons are on the dorsal side of the body, two on the abdominal side. On the sides of the body there are bundles of muscles that drive the parapodia.
The inner side of the longitudinal muscles is lined with epithelium of mesodermal origin. The body cavity is not limited by muscles, as in roundworms, but has its own epithelial lining - coelomic epithelium. Due to this coelomic epithelium, two-layer transverse partitions between segments (dissepiments) are formed. The secondary cavity is divided into chambers by dissepiments; each segment contains a pair of coelomic sacs filled with fluid. Coelomic fluid performs transport, excretory, homeostatic and musculoskeletal functions.
Fig.3.
1 - epithelium, 2 - circular muscles, 3 - longitudinal
muscles, 4 - gills, 5 - dorsal lobe of parapodia,
6 - supporting seta (acycula), 7 - funnel
metanephridia, 8 - parapodia muscles, 9 - canal
metanephridia, 10 - oblique muscle, 11 - abdominal
blood vessel, 12 - ovary, 13 - abdominal antenna
parapodia, 14 - ventral lobe of parapodia, 15 -
intestine, 16-coelom, 17-dorsal blood vessel.
The digestive system consists of three sections. The anterior section is of ectodermal origin. It begins with the oral opening located on the peristomium on the ventral side. The oral cavity continues into the muscular pharynx. U predatory species the pharynx consists of several layers of circular and longitudinal muscles, is armed with strong chitinous jaws and can turn outward (Fig. 1B). The pharynx is followed by the esophagus, into which the ducts of the salivary glands come off. Some species have a small stomach. The middle section is of endodermal origin. Serves for final digestion of food and absorption of nutrients. In the hindgut, which is of ectodermal origin, fecal masses are formed. The anal opening is usually located on the dorsal side of the anal blade.
The closed circulatory system includes dorsal, abdominal, annular and peripheral blood vessels. Through a large and pulsating dorsal blood vessel, blood flows to the head end of the body, through the abdominal end - to reverse direction. In the anterior part of the body, blood is distilled through ring vessels from the dorsal vessel to the abdominal, in the posterior part of the body - from the abdominal to the dorsal. Arteries extend from the annular vessels to the parapodia and gills (Fig. 4B).
rice. 4. Internal structure diagram
polychaete worms:
A - nervous and excretory systems (top view),
B - digestive system and whole (top view),
B - circulatory, digestive and nervous systems
(side view): 1 - suprapharyngeal cephalic ganglion, 2 -
peripharyngeal connective, 3 - ventral nervous ganglia
chains, 4 - nerves, 5 - metanephridia, 6 - mouth, 7 - oral
cavity, 8 - pharynx, 9 - esophagus, 10 - intestine, 11 - muscles
pharynx, 12 - whole, 13 - dissepiment, 14 - ovary, 15 -
dorsal blood vessel, 16 - abdominal
blood vessel, 17 - ring blood vessels.
Gas exchange occurs in the blood capillaries of the integument or gills. In some species, gills are formed from parapodial “antennae”, in others - from head appendages.
The excretory organs are metanephridia, each segment has a pair of metanephridia. The metanephridium consists of a funnel (nephrostomy) and a canal. The funnel is lined with cilia and is located in the coelomic chamber. The canal extending from the funnel penetrates the septum between the segments and in the adjacent segment opens outwards with an excretory opening (nephropore). The job of metanephridia is to remove unnecessary waste products from the coelomic fluid. An additional excretory function is performed by chloragogenic cells of the coelomic epithelium, in which grains of guanine and uric acid salts are deposited.
The nervous system consists of the peripharyngeal nerve ring with ganglia and the ventral nerve cord (Fig. 4A). The suprapharyngeal paired ganglion is more developed than the subpharyngeal ganglion, which is why it is sometimes called the “brain”. The nerve chain originates from the subpharyngeal node and consists of segmentally located pairs of nerve nodes connected to each other by transverse and longitudinal commissures. Nerves extend from the ganglia to various organs. The sense organs are developed to varying degrees. Many species have eyes, and all have olfactory and tactile receptors.
Polychaetes are dioecious animals. Gonads are formed on the wall of the coelom and are of mesodermal origin. In some species, gonads develop in all segments of the body, in others - in some segments. The germ cells from the gonads first enter the secondary cavity. From the coelom, gametes enter the water either through breaks in the body (the parent generation dies) or through special ducts (coelomoducts or nephromyxia). Fertilization is external. Development - with transformation. The polychaete larva is called a trochophore. The trochophore has a rounded shape, a parietal plume of cilia, an equatorial ciliary belt, a radially symmetrical nervous system, protonephridia and a primary body cavity (Fig. 5). At the posterior end of the larval body, two cells, teloblasts, appear on the right and left sides of the intestine. The teloblasts will form the mesoderm and then the mesodermal organs. The trochophore sequentially turns into a metatrochophore, then into a nectochaete. The metatrochophore produces larval segments. In Nektochaete, the cephalic ganglia and ventral nerve cord are formed. Nektochaete transforms into a young worm. The larvae lead a planktonic lifestyle, performing the function of dispersal.
rice. 5.
A - appearance trochophores, B - diagram of the structure of a trochophore,
C - diagram of the structure of the metatrochophore, D - diagram of the structure
nektochaetes: 1 - parietal plume of cilia, 2 - equatorial
ciliary girdle, 3 - mouth, 4 - protonephridia, 5 - intestines,
6 - teloblasts, 7 - anus, 8 - setae, 9 - ocelli.
rice. 6.
Sexual reproduction can be accompanied by the phenomenon of epitoky. Epitoky is a sharp morphophysiological restructuring of the body of a polychaete worm with a change in body shape (expansion of segments, appearance of swimming parapodia and bright coloring) during the period of maturation of reproductive products.
Polychaetes can reproduce not only sexually, but also asexually by budding (Fig. 6) or fragmentation.
Nereids (Nereis sp.)(Fig. 7) lead a benthic lifestyle, can bury themselves in silt, and can swim above the surface of the bottom. Predators. Due to their active lifestyle, they have well-developed muscles and sensory organs. Some Nereids sexual reproduction is accompanied by epitocy: Nereis virens float to the surface of the water during the breeding season, sweep out reproductive cells, after which they die or are eaten by birds and fish. From fertilized eggs, larvae develop, which, after swimming, settle to the bottom and develop into adults.
rice. 7. Nereid
(left) and
sandstone
(on right)
Nereids have food value. To strengthen the food supply of sturgeon, Nereis diversicolor was brought from the Azov Sea to the Caspian Sea, which took root and successfully reproduced in the new place.
Sand veins (Arenicola sp.)(Fig. 7) settle on flat sandbanks and burrow deep into the sand. The body shape and feeding method of sandworms are similar to those of an earthworm. Parapodia are reduced due to the burrowing lifestyle. Digging uses strong body muscles and a hydraulic method of movement by pushing cavity fluid from one end of the body to the other. Just like Nereids, sandworms are a favorite food of fish.
Fig.8. sedentary
polychaetes:
A - sertularia,
B - Spirobranchus.
Sessile polychaetes (Fig. 8) are a collective group of polychaete worms leading an attached lifestyle. They have glandular cells in their epithelium that secrete a secretion from which a protective convoluted or spirally twisted horny tube is built. As construction progresses, the tube is soaked in lime. Polychaetes of this group never leave their shelters. Only the head ends with fan-shaped gills protrude from the tubes. Gills - often brightly colored, are modified appendages of the head. Many species of sessile polychaetes have numerous eyes on their gills. When a predator approaches, these polychaetes contract their bodies with lightning speed and hide deep into the tube.
Parapodia in most species of sessile polychaetes are reduced due to their attached lifestyle.
rice. 9. Lateral
budding polychaete
Syllis ramosa
In Trypanosyllis, the budding zone is located at the caudal end of the parent organism. Here a “bundle” of sexual individuals is formed of different ages. As they mature, older individuals bud off and swim away.
In Autolytes (Fig. 10) there is an alternation of asexual and sexual generations. The asexual generation leads a benthic lifestyle, reproduces by budding, and in some species - longitudinal multiple budding. The sexual generation is epitocous, with pronounced sexual dimorphism. Females and males perform a “mating dance” at the surface of the water; after the release of sperm, the males die. Females carry the eggs on themselves, and after the larvae hatch, they also die.
rice. 10. Reproduction
Polychaetes autolites:
A - multiple budding, B -
"mating dance": 1 - parent
individual, 2 - daughter individuals (“buds”),
3 - female, 4 - male.
Palolo (Eunice viridis) live in Pacific Ocean. Sexual reproduction of these worms is preceded by asexual reproduction. In this case, the front part of the body remains at the bottom, and the rear budded part of the body is transformed into epitocous individuals filled with reproductive products and floats to the surface of the ocean. Here, germ cells are released into the water and fertilization occurs. In the entire population, the emergence of epitocine individuals occurs simultaneously, as if on a signal. The mass appearance of breeding polychaetes occurs in October or November on the day of the new moon. Knowing the timing of the reproduction of palolo, fishermen en masse catch polychaetes stuffed with “caviar”, which are used as food.
Class Polychaete worms (Polychaeta)
Audio fragment "Class Polychaete worms" (00:57)
About 7 thousand species of polychaete worms are known. Most of them live in the seas. Few live in fresh waters, in the litter tropical forests. In the seas, polychaete worms live on the bottom, where they crawl among stones, corals, thickets of marine vegetation, and burrow into silt. Among these worms there are sessile forms that build a protective tube and never leave it. There are also planktonic species among them. Polychaete worms are found mainly in the coastal zone, but sometimes at a depth of up to 8 thousand m. In some places, 1 m2 seabed up to 90 thousand polychaete worms live. They are eaten by crustaceans, fish, echinoderms, coelenterates, and birds. Therefore, some polychaete worms were specially bred in the Caspian Sea as food for fish.
The length of polychaete worms is from 2 mm to 3 m. The body is elongated, slightly flattened in the dorsoventral direction or cylindrical. Like all annelids, the body of polychaetes consists of segments, the number of which is different types ranges from 5 to 800. In addition to many body segments, there are head department
And anal paddle
.
These worms have a pair of palps (palps) , pair tentacles (antennas) And mustache . These are the organs of touch and chemical sense.
On the sides of each body segment there are noticeable muscular outgrowths - organs of movement, which are called parapodia
(from Greek pair- "near" and podion- "leg"). Parapodia contain a kind of reinforcement - bunches of bristles that contribute to the rigidity of the organs of movement. The worm rakes its parapodia from front to back, clinging to uneven surfaces of the substrate, and thus crawls forward.
In sessile forms of worms, the parapodia are partially reduced: often they remain only in the anterior part of the body.
The body of oligochaete worms is covered with single-layer epithelium. In sessile forms of worms, epithelial secretions can harden, forming a dense protective cover around the body. The skin-muscle sac consists of a thin cuticle, skin epithelium, circular and longitudinal muscles.
Under the skin epithelium there are two layers of muscles: transverse, or circular, and longitudinal. Under the muscle layer there is a single-layer epithelium, which from the inside lines the secondary body cavity, or coelom, and also forms partitions between the segments.
Digestive system begins with the mouth, which is located on the ventral side of the head lobe. The intestine consists of three sections: foregut, midgut and hindgut.
In the muscular pharynx, many predatory worms have chitinous teeth , serving for grasping prey. The midgut is in the form of a straight tube. The anal opening is located on the anal blade. Vagrant polychaete worms are mainly predators, while sessile ones feed on small organic particles and plankton suspended in water.
Respiratory system. In polychaete worms, gas exchange occurs either over the entire surface of the body or through areas of parapodia into which blood vessels extend. In some sessile forms, the respiratory function is performed by the corolla of the tentacles on the head lobe.
Circulatory system in annelids it is closed. This means that in any part of the worm’s body, blood flows only through the vessels. There are two main vessels - dorsal And abdominal .
One vessel passes above the intestine, the other below it. They are connected to each other by numerous semi-circular vessels. There is no heart, and the movement of blood is ensured by contractions of the walls of the spinal vessel, in which blood flows from back to front, in the abdominal - from front to back.
Excretory system presented paired tubes located in each body segment. Each tube begins with a wide funnel, the edges of which are lined with flickering cilia. The funnel faces the body cavity, and the opposite end of the tube opens outward on the side of the body. With the help of a system of tubes, decay products that accumulate in the coelomic fluid are removed to the outside.
Nervous system consists of paired suprapharyngeal, or cerebral, nodes, a paired abdominal nerve trunk and nerves extending from them.
Sense organs most developed in wandering polychaete worms. Many of them have eyes (in some species even capable of accommodation). The organs of touch and chemical sense are located on the antennae, palps, antennae and parapodia. Polychaete worms have balance organs (statocysts). Some species are capable of luminescence.
Reproduction. Most polychaete worms dioecious . Gonads are formed in almost every segment. Mature germ cells (in females - eggs, in males - sperm) enter first as a whole, and then through the tubules excretory system are brought out into the water. Fertilization in polychaete worms external ; the parents then die. After crushing, the eggs develop into a planktonic larva, which swims using cilia. After some time, it settles to the bottom and then turns into an adult worm. In some species, mating games and competition for territory are observed.
Some polychaete worms also have asexual reproduction . The worm is divided crosswise, and then each half restores the missing part of the body. In this case, sometimes a temporary chain is formed, including up to 30 worms.
Polychaete worms (eg. Nereis, 2 ) serve as food for many fish. Some worms ( palolo) are consumed by humans.
Bristles located along the sides kam of each segment are organ us movements. Against the backdrop of many trunk segments stand out fishing department It contains sense organs (palps, antennae, legs) yes eyes). Body ends in anal noah blade.