Type annelids external structure. Type annelids

Let's take a closer look at a number of animals that biology studies very carefully - the type annelids. To learn a little about them, you need to consider their species components, special way of life, habitat, as well as the external and internal structure of their body.

General signs and characteristics of the type annelids

Annelids or in another way rings, annelids are one of the most numerous groups among animals, which, according to general data, contains about 18 thousand open views. Basically, these animals are presented in the form of non-skeletal vertebrates, which are capable of participating in the destruction of organic substances, and are also considered the basis of nutrition for other species of the animal world.

In what environment do ringlets primarily live? Thus, the area of ​​residence of ringlets is very wide - they include seas and land, as well as fresh water bodies. You can find a lot of annelids that live on the surface of salty seas, as well as oceans. Annelids live everywhere, they can be found at any depth of the World Ocean and can even be found at the bottom of the Mariana Trench. The population density of ocean worms is very high - up to 100,000 units of ringlets per square meter bottom surface. Marine species are considered the best food for fish and play one of the main roles in the processes of the sea ecosystem.

In the territory of fresh water bodies you can find mainly blood-sucking individuals, for example, leeches, which are very often used in medical field. In tropical latitudes, leeches can live both in the soil and on trees.

Aquatic individuals not only crawl along the bottom or burrow into the surface, but can also independently create a protective tube and live there for a long time until someone disturbs the animal.

The most popular are ringworms that live in the surface of the soil, their name is earthworms. The density of these individuals in meadow and forest soils can reach up to 600 units per square meter. Also, these worms are involved in the processes of soil and soil formation.

What classes of ringworms live on earth?

About 200 years ago, Georges Cuvier did work in the field of animal classification and brought out only 6 rows of its representatives. This number also included arthropods - creatures whose bodies were previously divided by nature into segments. This group includes: woodlice, earthworms, leeches, insects, spiders, and crayfish.

It is possible to identify a small number of features in annelids, with the help of which they were separated into a whole group. The most important thing is the presence of a coelom (secondary body cavity), metamerism (segmentation) of the body and a well-developed circulatory system. In addition to all this, annelids have unusual organs of movement - parapodia. Also, ringlets have a developed nervous system, which includes the suprapharyngeal nerve ganglion, as well as the ventral nerve cord. The structure of the excretory system in ringlets is metanephridal.

According to experts, annelids were divided into 4 main classes. Main classes of rings:

What does the appearance of an annelid worm look like?

Annelids can be characterized as the most highly organized representatives of the group of worms. Their body lengths range from a few millimeters to 2.5 meters. The body of an individual can be clearly divided into three main parts: head, trunk, anal lobe. Home distinctive feature worms is that annelids do not have a clear division into sections, as occurs in higher varieties of animals.

In the area of ​​the individual's head there are various sensory organs. Most annelids have well-developed vision. Some individuals of annelids can be proud of their special eyes, as well as very clear vision. The organ of vision in these animals can be located not only in the head, but also on the tail, body or tentacles.

Worms have particularly developed taste buds. Worms are able to sense various odors well with the help of developed olfactory cells, as well as ciliary fossae. The auditory part of the rings is created on the principle of locators. It happens that echiruids are able to hear and recognize even the quietest sound with the help of their hearing organ, which is similar in structure to the lateral line of fish.

What are the respiratory organs and the hematopoietic system of a creature?

Description of the digestive system and excretory organs of the ringlet

The digestive system of annelids can be divided into three areas. The foregut (or stomodeum) contains the mouth opening, as well as the worm's oral cavity, sharp and powerful jaws, a pharynx, salivary glands, and a very narrow esophagus.

The oral cavity, whose second name is the buccal section, can be turned inside out without any problems. Behind this section you can find powerful jaws, curved in inner side. This device is very necessary in order to quickly and deftly capture your prey.

After that comes the mesodeum - the midgut. The anatomy of this section is quite uniform throughout the entire body region. At the same time, the midgut narrows in certain places and expands again; this is where the process of digesting food takes place. The hindgut is quite short and represents the anus.

The excretory system of the worm consists of metanephridia, which are located in pairs in each segment of the ring. They help remove excess waste products from the cavity fluid.

Understanding the sense organs and nervous system of an animal

Each class of annelids has its own gangionary type system. It includes the peripharyngeal nerve ring, which is created by connecting the suprapharyngeal and subpharyngeal ganglia, as well as from pairs of the chain of abdominal ganglia that are present in each segment.

The sense organs of annelids are quite well developed. Thus, worms have acute vision, good hearing and smell, as well as touch. Some individuals of annelids may not easily capture light, but also emit it on their own.

Reproduction process in annelids

Description by specialists ringworm reports that these individuals are capable of reproducing both sexually and asexually. Asexual reproduction occurs by dividing the body into several parts. The worm is capable of splitting into several halves, each of which subsequently becomes a full-fledged creature.

With all this, the creature’s tail is considered independent and cannot in any way grow a new head on itself. In some situations, a second head grows independently in the middle of the worm’s body even before the separation process.

Reproduction by budding is quite rare. Particularly interesting are those individuals whose budding can cover the entire body area, at which time the posterior ends bud from each segment. During reproduction, additional oral cavities may appear, which over time will become separate, full-fledged individuals.

Worms are in most cases dioecious, but some varieties (leeches and earthworms) have developed hermaphroditism - a process in which both individuals perform two functions at once, the role of a female and a male. The process of fertilization can be carried out both in the external environment and in the body of creatures.

For example, in marine worms, which reproduce only sexually, fertilization is considered external. Individuals of different sexes usually throw their reproductive cells to the surface of the water, where the process of fusion of eggs and sperm occurs. From eggs of the fertilized type, larvae emerge, which are completely distinctive in appearance from adults. Freshwater and terrestrial ringworms do not have a larval stage; they are immediately born with exactly the same structure as that of adult creatures.

Class polychaetes

Curious, sessile worms, serpulids, which live in spiral or twisted tubes of the Izvetian type. Serpulids are accustomed to only sticking out their heads with large fan-shaped gills from their house.

Leeches

All leeches are predators, which for the most part feed only on the blood of warm-blooded creatures, worms, fish and mollusks. The distribution and habitat area of ​​annelids from the class of leeches is very diverse. IN more The leech can be found in fresh water or in wet grass. But there are also marine species, and in Ceylon you can even find a terrestrial species of leeches.

This primarily applies To marine polychaete worms, which are a key group in the evolution of higher invertebrates: mollusks and arthropods evolved from their ancient ancestors.

The main progressive features of the ring structure are as follows:

1. The body consists of numerous (5-800) segments(rings). Segmentation is expressed not only in external, but also in internal organization, in repetition of many internal organs, which increases the survival rate of the animal with partial damage to the body.

2. Groups of segments similar in structure and function polychaete worms merged into body parts- head, trunk and anal lobes. The head section was formed by the fusion of several anterior segments. Body segmentation in oligochaete worms homogeneous.

3. Body cavity secondary, or in general, lined with coelomic epithelium. In each segment, the coelom is represented by two isolated sacs filled with coelomic fluid.

11.5.2. Class Polychaetes

This class is represented by marine animals. Many of them lead an active lifestyle, crawling along the bottom, burrowing into the ground or swimming in the water column. There are attached forms that live in protective tubes. The body is usually divided into the head, trunk and anal lobe. Annelids are often predators. Their pharynx is equipped with grasping appendages, sharp spines or jaws. Parapodia are present and have a variety of shapes depending on the habitat and method of movement. They breathe with gills. Polychaetes are dioecious, fertilization is external.

Typical representatives of this class are nereid(see Fig. 11.7) and sandstone They are food items for a number of commercial fish. Nereid has been successfully acclimatized in the Caspian Sea.

The most famous representatives of annelids for every 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.

Taxonomy

Today, experts classify from 16 to 22 thousand as annelids. modern species animals. There is no single approved classification of ringlets. The Soviet zoologist V.N. Beklemishev proposed a classification based on the division of all representatives of annelids into two superclasses: non-girdle worms, which includes polychaetes and echiurids, and girdle worms, which includes oligochaetes and leeches.

Below is the classification from the World Register of Marine Species website.

Table of biological taxonomy of annelids

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 Myzostomida, which other classifications classify as polychaete worms or even separate them into a separate class, was included as an order.

• Class Polychaetes(Polychaetes). Representatives of the class have connected lateral appendages (parapodia) bearing chitinous setae; the name of the group is determined by the presence large number bristles for each segment. Head with or without appendages. In most cases - dioecious; gametes are discharged directly into the water, where fertilization and development occur; free floating and are called trochophores. Sometimes they reproduce by budding or fragmentation. The class includes more than 6,000 species, which are divided into free-living and sessile forms.
• Class Girdle (Clitellata). Representatives of the class have an insignificant amount or no bristles on their body. There are no parapodia. They are characterized by the presence of a unique reproductive organ - the girdle, which is formed from the remains of the cocoon and performs a protective function for fertilized eggs. The class has about 10,000 representatives.
  • Subclass Oligochaetes(Oligochaetes). 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 is called oligochaete. As a rule, they do not have appendages on the body. Hermaphrodites. Development is direct, there is no larval stage. There are about 3250 species.
  • Leech subclass. They inhabit mainly freshwater bodies, 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 species of representatives.

Class Echiura. This is a small group, numbering only about 170 known species, all of which are exclusively marine inhabitants. Echiurids were recently classified as annelids after DNA examinations, but previously it was a separate type. The reason is that their body is different - it does not have segmentation, like those of ringed animals. In some sources, the Echiurides are considered not as a separate class, but as a subclass of the Polychaetes.

Spreading

Annelids, depending on the species, live on land, in fresh and salt water.

Polychaete worms usually live in sea ​​water(with the exception of some species that may also be found in freshwater bodies). They are food for fish, crayfish, as well as birds and mammals.

Oligochaete worms, to which the earthworm belongs, live in soil fertilized with humus or fresh water bodies.

Echiurids are distributed only in marine waters.

Morphology

The main characteristic of representatives of the Annelida phylum is the division of the body into a number of cylindrical segments, or metameres, total quantity which varies widely depending on the type of worm. Each metamer consists of a section of the body wall and a compartment of the body cavity with its internal organs. The number of outer rings of worms corresponds to the number of internal segments. The annelid body consists of a head region (prostomium); a body consisting of metameres; and a segmented posterior lobe called the pygidium. In some primitive representatives of this type the metamers are identical, or very similar to each other, each containing the same structures; in more advanced forms there is a tendency to consolidate certain segments and restrict certain organs to certain segments.

The outer shell of the annelid body (muscular sac) includes the epidermis surrounded by a cuticle, as well as well-developed, segmentally located muscles - circular and longitudinal. Most annelids have short external setae composed of chitin. In addition, on each metamere, some representatives of this type of animal may have primitive limbs called parapodia, on the surface of which bristles and sometimes gills are located. Spatial movement worms are carried out either through muscle contraction or parapodia movements.

The body length of annelids varies from 0.2 mm to 5 m.

Digestive system annelids consists of an unsegmented gut that passes through the middle of the body from the mouth, 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 coelom. The segmented compartments of the coelom are usually separated from each other by thin sheets of tissue called septa, which perforate the intestine and blood vessels. With the exception of leeches, the whole of annelids is filled with fluid and functions as a skeleton, providing muscle movement, as well as transport, sexual, and excretory functions of the body. If the integrity of the worm's body is damaged, it loses the ability to move properly, since the functioning of the body's muscles 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 channels for the release of germ cells and paired excretory organs (nephridia). In more complex species and excretory and reproductive functions are sometimes served by one type of canals (and canals may be absent in certain segments).

Circulatory system. Annelids developed a circulatory system for the first time in the process of evolution. Blood typically contains hemoglobin, a red respiratory pigment; however, some annelids contain chlorocruorin, a green respiratory pigment that gives the 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 cavity fluid mix directly in the sinuses of the body cavity). The main vessels - abdominal and dorsal - are connected to each other 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 respiration 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 ganglion.

The sense organs of ringed fish usually include eyes, taste buds, tactile tentacles and statocysts - organs responsible for balance.

Reproduction Annelids occur sexually or asexually. Asexual reproduction is possible through fragmentation, budding or fission. Among the worms that reproduce sexually, there are hermaphrodites, but most species are dioecious. Fertilized ringed eggs usually develop into free-swimming larvae. The eggs of terrestrial forms are enclosed in cocoons and larvae, like miniature versions of the adults.

The ability to restore lost body parts is highly developed in many multi- and oligochaete representatives of annelids.

Ecological significance

Charles Darwin book The Formation of Vegetable Mold through the Action of Worms (1881) presented the first scientific analysis of the effect of earthworms on soil fertility. Some of the worms dig burrows in the soil, while others live exclusively on the surface, usually in damp 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 They 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 glacial areas of North America, for example, scientists believe that almost all native earthworms were killed off by glaciers and the worms currently found in these areas (such as Amynthas agrestis) were introduced from other areas, primarily Europe , and in lately, from Asia. Northern deciduous forests have been particularly affected negative impact invasive worms through loss of leaf litter, decreased soil fertility, changes in chemical composition soils and loss of ecological diversity.

Marine annelids may account for more than one-third of benthic animal species around coral reefs and in intertidal zones. Burrowing annelid species increase the penetration of water and oxygen into seafloor sediment, which promotes the growth of populations of aerobic bacteria and small animals.

Human interaction

Anglers find 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 jar filled with damp moss.

Scientists study aquatic annelids to monitor oxygen levels, salinity and pollution environment in fresh and sea water.

The jaws of polychaetes are very strong. These advantages attracted the attention of engineers. Research has shown that the jaws of this genus of worms are composed 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 local residents. In Korea and Japan, the worms Urechis unicinctus from the class Echiuridae are eaten.

Cases of using leeches for medical purposes were known in China around 30 AD, India around 200 AD, ancient Rome around 50 AD and then throughout Europe. In 19th-century medical practice, the use of leeches was so widespread that supplies in some areas of the world were depleted, and some regions imposed restrictions or bans on their export (with medicinal leeches themselves considered an endangered species). More recently, leeches have been used in microsurgery for transplantation of organs and their parts, and skin areas. In addition, scientists claim that the saliva of medicinal 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 to humans.

Leeches can attach to a person’s skin from the outside, or penetrate internal organs (for example, the respiratory system or gastrointestinal tract). In this regard, two types of this disease are distinguished - internal and external hirudinosis. With external hirudinosis, leeches most often attach to human skin in the armpits, neck, shoulders, and calves.

Annelids have been known since the Middle Cambrian period. It is believed that they descended from lower flatworms, since certain features of their structure indicate the similarity of these groups of animals. Polychaete worms are distinguished as the main class of the annelid type. Later in the course of evolution, in connection with the transition to a terrestrial and freshwater lifestyle, oligochaetes evolved from them, which gave rise to leeches.

All annelids have a characteristic structure. Main characteristic: their bilaterally symmetrical body can be divided into a head lobe, a segmented body and a posterior (anal) lobe. The number of body segments can range from tens to several hundred. Dimensions vary from 0.25 mm to 5 m. At the head end of the rings there are sensory organs: eyes, olfactory cells and ciliary fossae, which react to the action of various chemical stimuli and perceive odors, as well as hearing organs, which have a structure similar to locators. Sensory organs can also be located on the tentacles. The body of annelids is divided into segments in the form of rings. Each segment, in a certain sense, represents an independent part of the whole organism, since the coelom (secondary body cavity) is divided by partitions into segments in accordance with the outer rings. Therefore, this type is given the name “ringed worms.” The significance of this division of the body is enormous. When damaged, the worm loses the contents of several segments, the rest remain intact, and the animal quickly regenerates. Metamerism (segmentation) of internal organs, and, accordingly, organ systems of annelids is due to the segmentation of their bodies. The internal environment of the annular organism is coelomic fluid, which fills the coelom in the skin-muscular sac, consisting of the cuticle, skin epithelium and two groups of muscles - circular and longitudinal. In the body cavity, the biochemical constancy of the internal environment is maintained, and the transport, sexual, excretory, and musculoskeletal functions of the body can be realized. More ancient polychaete worms have parapodia (paired primitive limbs with bristles) on each body segment. Some types of worms move by contracting muscles, while others use parapodia.

The oral opening is located on the ventral side of the first segment. Digestive system of annelids end-to-end The intestine is divided into the foregut, midgut and hindgut. The circulatory system of annelids is closed, consisting of two main vessels - dorsal and abdominal, which are connected to each other by ring vessels like arteries and veins. The blood of this type of worms can be different colors at various types: red, green or transparent. It depends chemical structure respiratory pigment in the blood. The respiration process is carried out over the entire surface of the worm's body, but some types of worms already have gills. The excretory system is represented by paired protonephridia, metanephridia or myxonephridia (prototypes of the kidneys) present in each segment. The nervous system of annelids includes a large nerve ganglion (the prototype of the brain) and a ventral nerve cord of smaller ganglia in each segment. Most annelids are dioecious, but some have secondarily developed hermaphroditism (as in earthworm and leeches). Fertilization occurs inside the body or in the external environment.

The importance of annelids is very great. It is worth noting their important role in food chains in their natural habitat. On the farm, people began to use marine species of ringed fish as a food source for growing valuable commercial fish species, for example sturgeon. The earthworm has long been used as bait for fishing as bird food. The benefits of earthworms are enormous, as they aerate and loosen the soil, which increases crop yields. In medicine, leeches are widely used for hypertension and increased blood clotting, as they secrete a special substance (hirudin) that has the property of reducing blood clotting and dilating blood vessels.

Annelids have the highest organization compared to other types of worms; for the first time they have a secondary body cavity, a circulatory system that is more highly organized nervous system. In annelids, inside the primary cavity, another, secondary cavity has formed with its own elastic walls made of mesoderm cells. It can be compared to airbags, one pair in each segment of the body. They “swell”, fill the space between the organs and support them. Now each segment received its own support from the bags of the secondary cavity filled with liquid, and the primary cavity lost this function.

They live in soil, fresh and sea water.

External structure

The earthworm has an almost round body in cross section, up to 30 cm long; have 100-180 segments, or segments. In the anterior third of the body there is a thickening - the girdle (its cells function during the period of sexual reproduction and egg laying). On the sides of each segment there are two pairs of short elastic setae, which help the animal when moving in the soil. The body is reddish-brown in color, lighter on the flat ventral side and darker on the convex dorsal side.

Internal structure

Characteristic feature internal structure is that earthworms have developed real tissues. The outside of the body is covered with a layer of ectoderm, the cells of which form the integumentary tissue. The skin epithelium is rich in mucous glandular cells.

Muscles

Under the cells of the skin epithelium there is a well-developed muscle, consisting of a layer of circular muscles and a more powerful layer of longitudinal muscles located under it. Powerful longitudinal and circular muscles change the shape of each segment separately.

The earthworm alternately compresses and lengthens them, then expands and shortens them. Wave-like contractions of the body allow not only crawling through the burrow, but also pushing the soil apart, expanding the movement.

Digestive system

The digestive system begins at the front end of the body with the mouth opening, from which food enters sequentially into the pharynx and esophagus (in earthworms, three pairs of calcareous glands flow into it, the lime coming from them into the esophagus serves to neutralize the acids of rotting leaves on which the animals feed). Then the food passes into the enlarged crop and a small muscular stomach (the muscles in its walls help grind the food).

The midgut stretches from the stomach almost to the posterior end of the body, in which, under the action of enzymes, food is digested and absorbed. Undigested remains enter the short hindgut and are thrown out through the anus. Earthworms feed on half-rotted plant remains, which they swallow along with the soil. When passing through the intestines, the soil mixes well with organic substances. Earthworm excrement contains five times more nitrogen, seven times more phosphorus and eleven times more potassium than regular soil.

Circulatory system

The circulatory system is closed and consists of blood vessels. The dorsal vessel stretches along the entire body above the intestines, and below it is the abdominal vessel.

In each segment they are united by a ring vessel. In the anterior segments, some annular vessels are thickened, their walls contract and pulsate rhythmically, thanks to which blood is driven from the dorsal vessel to the abdominal one.

The red color of blood is due to the presence of hemoglobin in the plasma. It plays the same role as in humans - nutrients dissolved in the blood are distributed throughout the body.

Breath

Most annelids, including earthworms, are characterized by cutaneous respiration; almost all gas exchange is provided by the surface of the body, therefore the worms are very sensitive to moist soil and are not found in dry sandy soils, where their skin quickly dries out, and after rains, when in the soil a lot of water, crawling to the surface.

Nervous system

In the anterior segment of the worm there is a peripharyngeal ring - the largest cluster nerve cells. The abdominal nerve cord with nodes of nerve cells in each segment begins with it.

This nodular type nervous system was formed by the fusion of nerve cords on the right and left sides of the body. It ensures the independence of the joints and the coordinated functioning of all organs.

Excretory organs

The excretory organs look like thin, loop-shaped, curved tubes, which open at one end into the body cavity and at the other outside. New, simpler funnel-shaped excretory organs - metanephridia - remove harmful substances into the external environment as they accumulate.

Reproduction and development

Reproduction occurs only sexually. Earthworms are hermaphrodites. Their reproductive system is located in several segments of the anterior part. The testes lie in front of the ovaries. During mating, the sperm of each of the two worms is transferred to the seminal receptacles (special cavities) of the other. Cross fertilization of worms.

During copulation (mating) and oviposition, girdle cells on the 32-37 segment secrete mucus, which serves to form an egg cocoon, and protein fluid to nourish the developing embryo. The secretions of the girdle form a kind of mucous coupling (1).

The worm crawls out of it with its back end first, laying eggs in the mucus. The edges of the coupling stick together and a cocoon is formed, which remains in the earthen hole (2). Embryonic development of eggs occurs in a cocoon, from which young worms emerge (3).

Sense organs

The sense organs are very poorly developed. The earthworm does not have real organs of vision; their role is played by individual light-sensitive cells located in the skin. The receptors for touch, taste, and smell are also located there. Earthworms are capable of regeneration (easily restore the back part).

Germ layers

The germ layers are the basis of all organs. In annelids, the ectoderm ( outer layer cells), endoderm (inner layer of cells) and mesoderm (intermediate layer of cells) appear early in development as three germ layers. They give rise to all major organ systems, including the secondary cavity and the circulatory system.

These same organ systems are subsequently preserved in all higher animals, and they are formed from the same three germ layers. Thus, higher animals in their development repeat the evolutionary development of their ancestors.