Polychaete marine worms (polychaetes): body shape and reproduction. The value of polychaete worms The external structure of polychaete worms
Polychaete worms are by far the largest group of marine organisms. Most often, representatives of the class live on a reservoir 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 trunk and a specific anal lobe. In most cases, the body of such an animal is clearly divided into several segments, each of which is attached to a parapodia.
Parapodia are nothing more than primitive limbs with small antennae and bristles. Interestingly, the parapodia of some members of the group were transformed into gills.
Like other representatives of the annedil type (leeches, low-bristle worms), in such an animal the 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 is the musculature, 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 a mouth opening that opens into the oral cavity. Then the food particle enters the muscular pharynx. By the way, it is in the pharynx that contains powerful jaws made of chitin. Some species are even able to turn it outward.
After grinding, food enters the esophagus, where the main glands that produce saliva open. Only a few representatives have a small stomach. The midgut of the animal serves for complete digestion and absorption of essential nutrients. The posterior intestine is responsible for the formation of feces and opens with an anus on the dorsal part of the anal lobe.
Polychaete worms have a closed one which consists of the dorsal and ventral arteries. By the way, the dorsal vessel is large and has contractile functions, so it works like a heart. In addition, large arteries are connected by the so-called annular vessels, which carry blood to the limbs and gills.
The respiratory system in representatives of this class is absent. The organs of gas exchange are the skin and gills, which are located either on the parapodia or in the anterior, head section of the body.
The excretory system consists of small metanephridia, which remove waste products of metabolism from the coelomic fluid into the 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 chain departs. Interestingly, 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 body fragmentation, less often by budding.
Nevertheless, animals have a well-developed reproductive system. Reproduction of worms is exclusively dioecious. 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 takes place in the aquatic environment. A larva develops from the zygote, which outwardly bears little resemblance to an adult. Accordingly, the development of a young worm occurs with metamorphoses.
Polychaete worms are the largest group of organisms. Scientists have about 10 thousand species of the class of annelids. Common representatives: sandworm living in the Arctic, Arctic Ocean.
A distinctive feature is the numerous bristles collected in bundles 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, in polychaete worms, the body is divided into several parts:
- head
- long
- torso
- anal lobe
located at the back 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 it is possible to move forward.
The whole carcass of the worm is dressed in a muscle bag. Outside, the body is made up of a thin cuticle covering the epithelium. Under the skin of the polychaete there is a musculature consisting of longitudinal and annular muscles. Rings are from two millimeters to three meters long, and this is a fairly large value for invertebrates.
Habitat
Basically, polychaetes live in salt waters and lead a bottom lifestyle. However, there are individuals that vegetated in the zone, not located in the immediate vicinity of the bottom, these individuals include the Tomopterid family. There are also polychaetes that have adapted to fresh water, woody soil.
Nutrition
The diet of the polychaete polychaete is relatively varied. Most feed on detritus - dead organic matter, this choice is associated with an immobile lifestyle. But there are also species that eat mollusks, coelenterates, ampictinids.
Enemies
Fish, some types of crustaceans love to eat polychaete worms, because this is a tasty and healthy food. Let's talk about the use of worms for fishing by people, since this activity sharply reduces their numbers.
reproduction
Polychaete worms are heterosexual, with the exception of some hermaphrodites. The sex glands are present in both females and males. The female has eggs and the male has sperm. Due to external fertilization, a larva, a trophora, is formed from the eggs.
Trophora moves through outgrowths, sinking to the bottom, where metamorphosis proceeds into an adult. Some families of polychaete worms also reproduce asexually. There are a couple of varieties 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, the pharynx, which has chitinous teeth, the esophagus and the stomach. These unusual creatures have an intestine divided into three sections:
- front
- middle
- rear
On the last part is the anal ring.
Circulatory system
Polychaetes have a closed circulatory system, each representative of annelids, that is, blood always flows through the vessels.
There are two main vessels in the camp, connected by semicircular 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 big capillaries.
Nervous system
The freely moving polychaete worms have developed sense organs, expressed by two tentacles and antennae. A 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
The withdrawal of harmful liquid occurs with the help of 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 up the water
- Eating decaying remains
- They are food for marine life.
Lifespan
Annelida polychaete worms live for about six years.
The most famous representatives of annelids for each person are leeches (subclass Hirudinea) and earthworms (suborder Lumbricina), which are also called earthworms. But in total there are more than 20 thousand species of these animals.
Systematics
To date, experts attribute from 16 to 22 thousand modern animal species to the type of annelids. There is no single approved classification of rings. The Soviet zoologist V.N. Beklemishev proposed a classification based on the division of all representatives of annelids into two superclasses: girdleless, which includes polychaetes and echiurids, and girdle, including oligochaetes and leeches.
The following is a classification from the World Register of Marine Species website.
Table of biological taxonomy of annelids
| Class* | Subclass | Infraclass | Detachment |
| Polychaete worms, or polychaetes (lat. Polychaeta) | |||
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| Polychaeta incertae sedis (disputed species) | |||
| Sedentaria | Canalipalpata |
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| Scolecida (Scolecida) |
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| Palpata |
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| Errantia (sometimes called Aciculata) |
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| Belt class (Clitellata) | Leeches (Hirudinea) | Acanthobdellidea | |
|
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|
Small-bristle worms (Oligochaeta) |
|
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|
Echiuridae (Echiura) |
|
There is also a superclass Annelida incertae sedis, which includes controversial species. There, according to the World Register of Marine Species, such a controversial group as Myzostomidae (Myzostomida), which other classifications refer to polychaete worms or even separate into a separate class, also entered as a detachment.
- Class Polychaete(Polychaetes). Representatives of the class have connected lateral appendages (parapodia) bearing chitinous setae; the name of the group is determined by the presence of a large number of setae per segment. Head with or without appendages. In most cases - dioecious; gametes are dumped directly into the water, where fertilization and development take place; floating freely and are called trochophores. Sometimes they reproduce by budding or fragmentation. The class includes more than 6000 species, which are divided into free-living and sessile forms.
- Class Poyaskovye (Clitellata). Representatives of the class on the body have a small number or no bristles at all. Parapodia are absent. They are characterized by the presence of a unique reproductive organ - a girdle, which is formed from the remains of a cocoon and performs a protective function for fertilized eggs. The class has about 10,000 representatives.
- Subclass Small-bristle(Oligochetes). They live primarily in fresh water. They have setae that arise directly from the walls of the body, due to the small number of which (usually 4 on each segment), the subclass was called low-setae. Appendages on the body, as a rule, do not have. Hermaphrodites. Development is direct, there is no larval stage. There are about 3250 species.
- Subclass Leeches. They inhabit mainly freshwater reservoirs, but there are also terrestrial and marine forms. There is a small sucker at the anterior end of the body and a large sucker at the posterior end. The fixed number of body segments is 33. The body cavity is filled with connective tissue. Hermaphrodites. Fertilized eggs are laid in a cocoon. Development is direct, there is no larval stage. There are about 300 types of representatives.
Class Echiuridae (Echiura). This is a small group with only about 170 known species, all of which are exclusively marine life. Echiurids were recently classified as annelids after DNA examinations, but earlier it was a separate type. The reason is that their body is different - it does not have segmentation, like annelids. In some sources, the Echiurids are considered not as a separate class, but as a subclass of Polychaetes.
Spreading
Annelids, depending on the species, live on land, in fresh and salt water.
Polychaete worms, as a rule, live in sea water (with the exception of some species that can also be found in freshwater bodies). They are food for fish, crayfish, as well as birds and mammals.
Small-bristle worms, to which the earthworm belongs, live in soil fertilized with humus or fresh water.
Echiurides are distributed only in marine waters.
Morphology
The main characteristic of representatives of the Annelida type is considered to be the division of the body into a number of cylindrical segments, or metameres, the total number of which, depending on the type of worm, varies widely. Each metamere consists of a section of the body wall and a section of the body cavity with its internal organs. The number of outer rings of worms corresponds to the number of inner segments. The body of annelids consists of the region of the head (prostomium); a body consisting of metameres; and a segmented posterior lobe called the pygidium. In some primitive representatives of this type, the metameres are identical, or very similar to each other, each containing the same structures; in more advanced forms, there is a tendency to consolidate some segments and restrict certain organs to certain segments.
The outer shell of the body of annelids (skin-muscular sac) includes the epidermis surrounded by the cuticle, as well as well-developed, segmentally located muscles - annular and longitudinal. Most annelids have external short setae composed of chitin. In addition, on each metamere, some representatives of this type of animals may have primitive limbs called parapodia, on the surface of which setae and sometimes gills are located. The spatial movement of the worms is carried out either through muscle contraction or movements of the parapodia.
The body length of annelids ranges from 0.2 mm to 5 m.
The main general anatomical features of annelids in cross section Digestive system Annelids consists of an unsegmented intestine that runs through the middle of the body from the oral cavity, located on the underside of the head, to the anus, located on the anal lobe. The intestine is separated from the body wall by a cavity called the whole. The segmented compartments of the coelom are usually separated from each other by thin sheets of tissue called septa that perforate the gut and blood vessels. With the exception of leeches, in general, representatives of annelids are filled with liquid and function as a skeleton, providing muscle movement, as well as transport, sexual, and excretory functions of the body. When the integrity of the body of the worm is damaged, it loses the ability to move properly, since the functioning of the muscles of the body depends on maintaining the volume of coelomic fluid in the body cavity. In primitive annelids, each compartment of the coelom is connected to the outside by means of channels for the release of germ cells and paired excretory organs (nephridia). In more complex species, both excretory and reproductive functions are sometimes served by the same type of canals (the canals may be absent in certain segments).
Circulatory system. In annelids, for the first time in the process of evolution, a circulatory system appeared. Blood usually contains hemoglobin, a red respiratory pigment; however, some annelids contain chlorocruorin, a green respiratory pigment that gives blood its color.
The circulatory system is usually closed, i.e. enclosed in well-developed blood vessels; in some species of polychaetes and leeches, an open-type circulatory system appears (blood and abdominal fluid mix directly in the sinuses of the body cavity). The main vessels - the abdominal and dorsal - are interconnected by a network of annular vessels. Blood is distributed in each segment of the body along the lateral vessels. Some of them contain contractile elements and serve as a heart, i.e. play the role of pumping organs that move the blood.
Respiratory system. Some aquatic annelids have thin-walled, feathery gills through which gases are exchanged between the blood and the environment. However, most representatives of this type of invertebrates do not have any special organs for gas exchange, and breathing occurs directly through the surface of the body.
Nervous system, as a rule, consists of a primitive brain, or ganglion, located in the head region, connected by a ring of nerves to the ventral nerve cord. In all metameres of the body there is a separate nerve node.
The sense organs of annelids typically include eyes, taste buds, tactile tentacles, and statocysts, organs responsible for balance.
reproduction annelides occur either sexually or asexually. Asexual reproduction is possible through fragmentation, budding, or division. Among worms that reproduce sexually, there are hermaphrodites, but most species are dioecious. The fertilized eggs of marine annelids usually develop into free-swimming larvae. The eggs of terrestrial forms are encased in cocoons and larvae, like miniature versions of the adults.
The ability to restore lost body parts is highly developed in many annelids with many and few bristles.
Ecological significance
The earthworm is very important for maintaining the condition of the soil Charles Darwin, in The Formation of Vegetable Mold through the Action of Worms (1881), presented the first scientific analysis of the influence of earthworms on soil fertility. Some of the worms burrow in the soil, while others live exclusively on the surface, usually in wet leaf litter. In the first case, the animal is able to loosen the soil so that oxygen and water can penetrate into it. Both surface and burrowing worms help improve soil in several ways:
- by mixing organic and mineral substances;
- by accelerating the decomposition of organic substances, which in turn makes them more accessible to other organisms;
- by concentrating minerals and converting them into forms that are more easily absorbed by plants.
Earthworms are also important prey for birds ranging in size from robins to storks, and for mammals ranging from shrews to badgers, in some cases.
Terrestrial annelids in some cases can be invasive (brought into a certain area by people). In the glacial regions of North America, for example, scientists believe that almost all native earthworms were killed by glaciers and the worms currently found in these regions (such as Amynthas Agrestis) were introduced from other areas, primarily from Europe. , and more recently, from Asia. Northern hardwood forests have been particularly affected by invasive worms through loss of leaf litter, reduced soil fertility, changes in soil chemistry, and loss of ecological diversity.
Marine annelids can make up over one-third of benthic animal species around coral reefs and in intertidal areas. Burrowing annelids increase the infiltration of water and oxygen into the seabed sediment, which promotes the growth of populations of aerobic bacteria and small animals.
Human interaction
Anglers believe that worms are more effective baits for fish than artificial fly baits. In this case, the worms can be stored for several days in a tin can filled with wet moss.
Scientists study aquatic annelids to monitor oxygen levels, salinity and environmental pollution in fresh and sea water.
The jaws of polychaetes are very strong. These advantages have attracted the attention of engineers. Research has shown that the jaws of this genus of worms are made up of unusual proteins that bind strongly to zinc.
On the island of Samoa, catching and eating one of the representatives of annelids - the Palolo worm - is a national holiday, and the worm itself is considered a delicacy by the locals. In Korea and Japan, Urechis unicinctus worms from the Echiuridae class are eaten.
Representatives of annelids, which are eaten Cases of using leeches for medicinal purposes were known as early as China around 30 AD, India around 200 AD, ancient Rome around 50 AD, and then throughout Europe. In the medical practice of the 19th century, the use of leeches was so widespread that their stocks in some areas of the world were depleted, and some regions imposed restrictions or bans on their export (while the medicinal leeches themselves were considered an endangered species). More recently, leeches have been used in microsurgery for transplantation of organs and their parts, skin areas. In addition, scientists argue that the saliva of medical leeches has an anti-inflammatory effect, and some anticoagulants contained in it prevent the growth of malignant tumors.
About 17 species of leeches are dangerous for humans.
Medical leeches are used for hirudotherapy, and a valuable remedy is extracted from pharmacies - hirudin Leeches can attach to the skin of a person from the outside, or penetrate into internal organs (for example, the respiratory or gastrointestinal tract). In this regard, there are two types of this disease - internal and external hirudinosis. With external hirudinosis, leeches are most often attached to human skin in the armpits, neck, shoulders, and calves.
Misostomida on sea lily Polychaete worms (polychaetes)- This is a class belonging to the type of annelids and including, according to various sources, from 8 to 10 thousand species.
Representatives of polychaetes: nereid, sandworm.
The length of polychaete worms varies from 2 mm to 3 m. The body consists of a head lobe (prostomium), trunk segments, and a caudal lobe (pygidium). The number of segments is from 5 to hundreds. On the head are palps (palps), tentacles (antennae) and antennae. These formations play the role of organs of touch and chemical sense.
Almost every segment of the trunk of a polychaete worm has skin-muscular outgrowths (on the sides). These are parapodia - organs of locomotion. Their rigidity is provided by a bundle of bristles, among which there are support ones. In sessile forms, the parapodia are mostly reduced. Each parapodia consists of upper and lower branches, on which, in addition to setae, there is a tendril that performs tactile and olfactory functions.
With the help of muscles attached to the walls of the secondary cavity, parapodia perform rowing movements.
Polychaete worms swim by moving the parapodia and bending the body.
The body is covered with a single-layered epithelium, the secretions of which form cuticles. In sessile species, the epithelium secretes substances that harden to form a protective sheath.
The skin-muscular sac consists of the skin epithelium, cuticle and muscles. There are transverse (ring) and longitudinal muscles. Under the muscles there is another layer of a single-layer epithelium, which is the lining of the coelom. Also, the inner epithelium forms partitions between the segments.
The mouth is located at the head of the worm. There is a muscular pharynx that can protrude from the mouth in many carnivorous species with chitinous teeth. In the digestive system, the esophagus and stomach are separated. The intestine consists of the anterior, middle and hindgut.
The midgut looks like a straight tube. It digests and absorbs nutrients into the blood. Fecal masses are formed in the hindgut. The anal opening is located on the caudal lobe.
Breathing is carried out through the entire surface of the body or by folded protrusions of parapodia, in which there are many blood vessels (peculiar gills). In addition, outgrowths that perform a respiratory function can form on the head lobe.
The circulatory system is closed. This means that the blood moves only through the vessels. Two large vessels - dorsal (above the intestine, blood moves towards the head part) and abdominal (under the intestine, blood moves towards the tail part). The dorsal and abdominal vessels are interconnected in each segment by smaller annular vessels.
There is no heart, the movement of blood is provided by contractions of the walls of the spinal vessel.
The excretory system of polychaete worms is represented in each segment of the body by paired tubules (metanefridia), which open outward in the adjacent (behind) segment. In the body cavity, the tubule expands into a funnel. Along the edge of the funnel there are ciliated cilia, which ensure that waste products from the coelom fluid enter it.
Paired supraesophageal ganglia are connected to form a peripharyngeal ring. There are a pair of ventral nerve trunks. In each segment, nerve knots are developed on them, thus abdominal nerve chains are formed. Nerves depart from the ganglia and abdominal nodules. The distance between the abdominal chains is different in different species of polyshedines. The more evolutionarily progressive the species, the closer the chains are, one might say, merge into one.
Many mobile polychaete worms have eyes (several pairs, including eyes, are on the caudal lobe). In addition to antennae and antennae, there are organs of touch and chemical sense on the parapodia. There are organs of balance.
Most are segregated. Usually the sex glands are present in each segment. The eggs and spermatozoa first appear in the whole, from where they enter the environment through the tubules of the excretory system or gaps in the body wall. Thus, fertilization in polychaete worms is external.
A trochophore larva develops from a fertilized egg, swimming with the help of cilia, having a primary body cavity and protonephridia as excretory organs (in this way it resembles the structure of ciliary worms). Settling on the bottom of the trochophore turns into an adult worm.
There are polychaete species that can reproduce asexually (by dividing across).
Annelida Polychaeta. Photo: Paul.Paquette
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. 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 (perestomium), which is often complex as a result of fusion with 2–3 trunk segments. The mouth is located ventrally on the perestomium. 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 perestomium 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. 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-muscle bag. 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). The glandular cells of the epithelium of 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.
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 a cavity, only conditionally called secondary. In a more complex coelomic epithelium may completely cover the intestines and muscles. The coelom is fully represented in case of development of paired metameric coelomic sacs. When paired coelomic sacs close in each segment above and below the intestine, the dorsal and abdominal mesentery, or mesentery, are formed. Between the coelomic sacs of two adjacent segments, transverse partitions are formed - dissipations. The wall of the coelomic sac, lining the inside of the muscles of the body wall, is called the parietal mesoderm, and the coelomic epithelium that covers the intestines and forms the mesenterium is called the visceral mesoderm. The coelomic septa contain blood vessels.
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 body of the worm, 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 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. 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.
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. 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 in 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 contains a pair of metanephridia. 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. Thus, some polychaetes have protonephridia of ectodermal origin, similar in structure to those of flatworms 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. An additional 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 supraesophageal ganglia form the brain, in which three sections are distinguished: proto-, meso- and deutocerebrum. The brain innervates the sense organs on the head. Near-pharyngeal nerve cords depart from the brain - connectives to the ventral 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 neural chain is formed. 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 organs of balance - 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 reproductive and excretory ducts.
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. This combing is accompanied by the regeneration of the missing parts of the body. Sexual reproduction is often associated with the phenomenon of epitokia. Epitokia is a sharp morphophysiological restructuring of the worm's body with a change in the shape of the body during the maturation of reproductive products: the segments become wide, brightly colored, with swimming parapodia. 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 palolo stuffed with "caviar" and use them for food. At the same time, fish, gulls, sea ducks feast on worms.
Development. The fertilized egg undergoes uneven, spiral crushing. 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 of the endoderm and mesoderm. The first mobile stage is the 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 tuft, the parietal sultan of cilia, are formed. Then 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. The blastopore at the trochophore shifts 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.