What is the development of polychaete worms. Living earth - know your home. General characteristics of polychaete annelids
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 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 tubes 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).
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) |
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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 a subclass of 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 Type annelids unites about 9,000 species with the most perfect organization among other worms. Their body consists of a large number of segments; many have setae on the sides of each segment, which play an important role in locomotion. Internal organs are located in the body cavity, called as a whole. There is a circulatory system. In the anterior part there is an accumulation of nerve cells that form the subpharyngeal and supraesophageal ganglions. Annelids live in fresh water, seas and soil.
Most of the representatives of annelids belong to the classes: oligochaetes, polychaetes and leeches.
Low-bristle class
Representative of the low-bristle class - earthworm lives in minks in damp humus soil. The worm crawls to the surface in wet 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 girdle, on the ventral and lateral sides of the body - elastic and short setae are developed.
The body of the worm is covered with skin from the integumentary tissue, in which the cells fit tightly to each other. The skin contains glandular cells that secrete mucus. Under the skin are circular and deeper - longitudinal muscles, due to the contraction of which the body of the worm can lengthen or shorten, thereby advancing in the soil.
Skin and muscle layers form skin-muscle sac, inside which is the body cavity, where the internal organs are located. Earthworms feed on decaying plant debris. Through the mouth and pharynx, food enters the goiter 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 along with the earth are excreted through the anus.
The circulatory system of an earthworm closed and consists of dorsal and abdominal blood vessels, interconnected by annular vessels from each segment. Larger annular vessels are located around the esophagus, acting as the "hearts" of large vessels, lateral branches depart, forming a network of capillaries. Blood never mixes with body cavity fluid, so 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 sense organs. There are only various kinds of sensitive cells that perceive external stimuli (light, smell, etc.).
Earthworms are hermaphrodites. However, their insemination is cross, two individuals are involved in this process. When eggs are laid on the girdle of the worm, abundant mucus is formed, into which the eggs fall, after which the mucus darkens and hardens, forming a cocoon. Then the cocoon is dropped from 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.
Earthworms are characterized ability to regenerate. Earthworms are called soil formers, as they, making passages in the soil, loosen it, contribute to aeration, that is, the entry of air into the soil.
Polychaete class
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 setae sit in bunches. 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 the entire surface of the body. Some polychaetes on the lobes have gills- primitive respiratory organs.
belongs to the polychaete peskozhil, living in minks, in the sand, or building a plaster turtle for itself, which is attached to algae. Many marine fish feed on Nereids and other annelids.
Leech class
The most famous representative of this class is medicinal leech, which has been used to treat people since ancient times. Leeches are characterized by the presence of two suckers: the front, at the bottom of which the mouth is located, and the back.
The posterior sucker is large, its diameter exceeds half of the maximum width of the body. Leeches bite through the skin with three jaws, seated along the edges with sharp teeth (up to 100 on each jaw). Strong bloodsucker. In medicine, it is used for diseases of the blood vessels (formation of blood clots), hypertension, pre-stroke condition. Leeches are applied to a certain part of a sick person in order to suck blood, as a result, blood clots dissolve, blood pressure decreases, and the person's condition improves. In addition, the salivary glands of a medical leech produce a valuable substance - hirudin- prevents blood clotting. Therefore, after leech injections, the wound bleeds for a long time. Being in the stomach of a leech, the blood under the influence of hirudin is stored for months without being subjected to coagulation and decay.
The digestive system of the leech is built 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. Due to this feature, leeches can starve for a long time (from several months to 1 year). The leech lives up to 5 years. Leeches are hermaphrodites. I reach in nature! puberty only in the third year of life and lay cocoons once a year in the summer.
Leeches are characterized by a straight developed. Leeches include a non-bloodsucking predatory leech - big lozhnokonskaya. It eats worms (including leeches), soft-bodied, aquatic insect larvae, small vertebrates (tadpoles), which it can overcome.
Polychaete worms are of the phylum Annelids, thus being relatives of our common earthworms.
Habitat
Polychaete worms are long-bodied inhabitants of the sea. However, some species have adapted to living in fresh water, as well as on land - in deep soil layers.
Appearance and structure
The similarity with earthworms is found primarily in the structure of the body, which is divided into many segments. The length of polychaetes (the so-called polychaete worms in Greek) varies from 2 millimeters to three meters.
tubular polychaete marine worm photo
Segments in large species can be several hundred. In each segment, a set of internal organs is repeated:
- Whole bags;
- Sexual ducts;
- excretory organs.
Parapodia extend from each segment - lobe-shaped outgrowths, on which there are chitinous bristles. This feature gave the name to the whole group of worms. In some species, there is a bundle of tentacles on the head segment, which acts as gills.
Another feature of polychaete worms is their developed eyes, which have a complex structure. They also have a kind of vestibular apparatus - statocysts. These are bubbles in which there are solid spherical bodies - statoliths.
polychaete worms photo
When the body changes its position, the statoliths roll over the walls of the bubble and irritate the cilia of the epithelium, the nerve impulse from which is transmitted to the central nervous system, after which the animal restores balance.
The whole variety of polychaete worms is divided into free-swimming species and sessile - attached to the bottom of the sea.
Nutrition
Polychaete worms feed on either detritus, that is, decaying organic debris, or animal food. Sedentary species extract detritus from the water column with the help of their tentacles, which also function as gills.
polychaete annelids photo
Free-swimming worms extract detritus from the ground by eating it or digging it out with their long tentacles. Carnivorous families of polychaete worms are, for example, nereids and glycerides.
reproduction
Polychaetes in most cases are dioecious animals. However, they do not form real gonads (organs that produce germ cells). Sex cells arise from the coelomic epithelium.
Fertilization is external. The eggs hatch into larvae called trochophores. These are planktonic organisms that are microscopic in size and swim with the help of many cilia. After some time, they settle to the bottom and transform into adult animals.
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 there is a 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 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 abdominal 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.