The phylum annelids include protostomes, the most highly organized of all worms. The phylum includes, according to various sources, 10–18 thousand species and is divided into three classes: polychaetes (the most numerous, more than 10 thousand species), oligochaetes and leeches. These worms live in bodies of water, not excluding ocean floor And icy waters Atlantic, and in the soil. The exception is several species of leeches that have adapted to living on land in wet forests tropics. Well, the most famous and common oligochaete earthworms in our latitudes are earthworms, so named for their custom of crawling out of the soil in dozens during rain to breathe oxygen. In addition, their presence is revealed by small excavation tubercles, especially noticeable in the spring - young worms loosen the soil, simultaneously saturating it with oxygen. We will talk about earthworms in more detail later, but now we will look at important aromorphoses and structural features annelids.
Aromorphoses of annelids
1. Overall - secondary body cavity, that is, a closed, fluid-filled space that separates the intestines from the walls of the body. It is important to emphasize that, unlike the primary cavity of roundworms, the coelom of annelids has an epithelial membrane, lining. In polychaete and polychaete worms the whole occupies a fairly large volume. Cells participating in excretory, gas exchange and other processes float freely in the liquid that fills it.
2. Closed circulatory system- its occurrence is directly related to the appearance of the coelom. As the worm larva develops, the secondary cavity displaces the primary cavity, the remains of which turn into blood vessels.
3. Metamerism- segmentation, duplication of internal organs, due to which the loss of a body part is not critical for the worm. Each ring has its own sex glands, excretory organs, nerve nodes, etc.
4. Parapodia- growths on the sides of the body in polychaetes that facilitate movement.
The structure of annelids
1. Body sizes from a quarter of a millimeter to three or even more meters.
2. B cross section the body is close to a circle or oval. Three sections are distinguished: the head (head lobe), the trunk and the anal lobe. Ringworms grow by forming new segments in the area of the anal lobe.
2. The body is segmented by constrictions into many homogeneous rings(segments). It is important that the whole also has partitions, in accordance with the external division. Top layer skin-muscle bag- cuticle, the next one is single-layer epithelium. There are two types of muscles: circular on the outside, longitudinal located deeper.
3. On the head, in addition to the mouth opening, different types there may be eyes, organs of touch (various whiskers, palps, etc.).
4. They grow from the cuticle bristles, of which there can be a lot along the entire length of the body.
Organ systems
1. Digestive system not closed, it is divided into three sections, represented by the anterior, middle (here nutrients are absorbed) and hindgut. Some species of worms have acquired salivary glands.
2. Ringworms are heartless creatures, in their closed circulatory system there are only different types of vessels through which blood moves. Interestingly, red color is not necessary for the blood of worms - it all depends on the pigments.
3. Breath can be carried out in two ways - either on the surface of the body (as in oligochaete worms and leeches), or through primitive gills, which are located on parapodia outgrowths (in polychaete worms).
4. Nervous system starts in the worm's head, where two nerve ganglion, suprapharyngeal and subpharyngeal, connected by cords, form the peripharyngeal nerve ring. A pair of nerve trunks with ganglia, connected by jumpers in each segment, emerge from the ganglion under the pharynx and stretch along the body. This is the so-called ventral nerve cord.
5. Sense organs Quite well developed in active worms: tactile cells, eyes (not all species have them), chemoreceptors, balance organ.
6. Excretory system presented in all rings in pairs metanephridia: tubes located in the coelom that open outward on the surface of the body.
Origin of annelids
1. The ancestors of annelids were free-living flatworms. How can you prove this? The larvae of polychaete worms have great resemblance with planarians. What does it mean? Trochophore, a polychaete larva, has cilia, ocelli, metanephridia in the form of tubes with stellate cells and a “flickering flame” formed by the beating of the cilia. In addition, the nervous system of the trochophore is very similar to the nervous system of the planarian.
2. Oligochaetes evolved from ancient polychaetes as a result of simplification in their structure caused by life in the soil.
3. Leeches evolved from ancient oligochaete worms.
Annelids - quite large type invertebrates (about 9000 species). They are classified as free-living animals that have a very complex organization, unlike flat and roundworms. These include primary ringlets, multi- and oligochaete worms, echiurids and leeches. The most primitive look Archiannelids are considered to live in the marine environment.
Echiurids and polychaetes also live in the sea, but their organization is more complex. And leeches and oligochaete worms are inhabitants of soil and fresh water bodies.
Highly organized type
Annelids can reach 2.5 meters in length. Most species are free-living forms. Any ringworm has a body consisting of 3 parts: the trunk (consists of rings), head and anal lobe. The head of the ringlets houses various sensory organs. Ringed worms for the most part have good developed eyes. Some of them have specifically acute vision, their lens is capable of accommodation. Eyes can be located in different areas: on the head, body, tentacles and even the tail. In addition, annelids have a highly developed sense of taste. Many of them have special olfactory cells on their heads or tails that perceive all kinds of odors or the effects of chemical irritants. Hearing organs are also present, and they work on the principle of locators. Not so long ago, hearing organs were discovered in many echiurids, very similar to the lateral line organ in fish. This allows them to always be ready: to hear all the rustles and splashes, because underwater audibility is much better than on land. Annelids are named for their body, which is made up of segments (rings). The number of these rings can reach a couple of hundred. However, most ringlets have only a few segments.
Basically, each ring represents independent particle the whole organism. Annelids move thanks to specific organs of movement - parapodia, which are located on the sides of each segment. They have the appearance of lobe-shaped outgrowths of the body, from which tufts of bristles protrude outward. However, not every ringlet has parapodia. They are found in polychaete worms and some primary ringworms. In oligochaete individuals, only the setae are the organs of locomotion. The body cavity of annelids is filled with a liquid similar in composition to chemical water. This liquid is constantly moving, due to which the transport process occurs nutrients throughout the body of ringlets, secretions of endocrine glands, oxygen and carbon dioxide who accept direct participation during the breathing process of any organism.
Reproduction of annelids
Annelids reproduce sexually and asexually. However, asexual reproduction dominates in aquatic ringworms. Fertilization of the egg occurs outside the female's body. The male and female synchronously release their reproductive cells into the water, where fertilization actually occurs.
FEDERAL AGENCY FOR EDUCATION
TAMBOV STATE UNIVERSITY
NAMED AFTER G.R. DERZHAVIN
Test
in biology
on the topic: General characteristics and classification of annelids
Completed by a student
1st year correspondence form training
Faculty of Geography
Petropavlovskaya Olesya Sergeevna
(For verification to V.V. Koryakin)
TYPE RINGED WORMS
( ANNELIDA )
The annelids include primary annelids, polychaete and oligochaete worms, leeches and echiurids. In the phylum of annelids there are about 8 thousand species. The most primitive marine primary ringlets are archiannelids. Polychaetes and echiurids are inhabitants of the sea. Lesser ringlets and leeches are mainly inhabitants of fresh water and soil.
Structure. Annelids are the most organized representatives of worms. The sizes of the rings range from fractions of a millimeter to 2.5 m. These are predominantly free-living forms. The body of the ringlets is divided into three parts: the head, the body, consisting of rings, and the anal lobe. Animals that are lower in their organization do not have such a clear division of the body into sections.
The ringlet's head is equipped with various sensory organs. Many ringlets have well-developed eyes. Some have particularly acute vision, and their lens is capable of accommodation. Eyes can be located not only on the head, but also on the tentacles, on the body and on the tail. Ringworms also have developed senses of taste. On the head and tentacles of many of them there are special olfactory cells and ciliary fossae that perceive various odors and the effects of many chemical irritants. The ringed birds have well-developed hearing organs, arranged like locators. Recently, hearing organs have been discovered in sea ringed echiurids, very similar to the lateral line organs of fish. With the help of these organs, the animal subtly distinguishes the slightest rustles and sounds, which are heard much better in water than in air.
The body of the ringlets consists of rings, or segments. The number of rings can reach several hundred. Other ringlets consist of only a few segments. Each segment represents to some extent independent unit the whole organism. Each segment includes parts of vital organ systems.
Special organs of movement are very characteristic of ringlets. They are located on the sides of each segment and are called parapodia. The word "parapodia" means "foot-like". Parapodia are lobe-shaped outgrowths of the body from which tufts of bristles protrude outward. In some pelagic polychaetes, the length of the parapodia is equal to the diameter of the body. Parapodia are not developed in all ringlets. They are found in primary ringworms and polychaete worms. In oligochaetes only the setae remain. The primitive leech Acanthobdella has setae. Other leeches move without parapodia and setae. Echiurids do not have parapodia and have setae only at the posterior end of the body.
Parapodia, nodes of the nervous system, excretory organs, gonads and, in some polychaetes, paired intestinal pouches are systematically repeated in each segment. This internal segmentation coincides with the external annulation. Repeated repetition of body segments is called Greek word"metamerism". Metamerism arose in the process of evolution in connection with the elongation of the body of the ancestors of ringlets. Lengthening the body necessitated repeated repetition, first of the organs of movement with their muscles and nervous system, and then of the internal organs.
Extremely characteristic of ringlets is the segmented secondary body cavity, or coelom. This cavity is located between the intestines and the body wall. The body cavity is lined with a continuous layer of epithelial cells, or coelothelium. These cells form a layer covering the intestines, muscles and all other internal organs. The body cavity is divided into segments by transverse partitions - dissepiments. By midline The body is traversed by a longitudinal septum, the mesenterium, which divides each compartment of the cavity into right and left parts.
The body cavity is filled with fluid, which chemical composition very close to sea water. The fluid filling the body cavity is in continuous motion. The body cavity and abdominal fluid perform important functions. Cavity fluid (like any fluid in general) does not compress and therefore serves as a good “hydraulic skeleton”. The movement of the cavity fluid can transport various nutritional products and gland secretions inside the body of the ringlets. internal secretion, as well as oxygen and carbon dioxide involved in the respiration process.
Internal partitions protect the body in case of severe injuries and ruptures of the body wall. For example, an earthworm cut in half does not die. The septa prevent cavity fluid from flowing out of the body. The internal partitions of the rings thus protect them from death. But not all annelids have well-developed septa in the body cavity. For example, in echiurids the body cavity does not have partitions. A puncture in the body wall of an echiurid can lead to its death. In addition to the respiratory and protective role, the secondary cavity acts as a container for reproductive products that mature there before being excreted.
Ringlets, with few exceptions, have a circulatory system. However, they have no heart. The walls of large vessels themselves contract and push blood through the thinnest capillaries. In leeches, the functions of the circulatory system and the secondary cavity are so identical that these two systems are combined into a single network of lacunae through which blood flows. In some rings the blood is colorless, in others it is colored green a pigment called chlorocruorin. Often ringlets have red blood, similar in composition to the blood of vertebrates. Red blood contains iron, which is part of the hemoglobin pigment. Some ringworms, burrowing into the ground, experience an acute deficiency of oxygen. Therefore, their blood is adapted to bind oxygen especially intensively. For example, the polychaete Magelonapapillicornis has developed the pigment hemerythrin, which contains five times more iron than hemoglobin.
In ringlets, compared to lower invertebrates, metabolism and respiration are much more intense. Some polychaete ringlets develop special respiratory organs - gills. A network of blood vessels branches out in the gills, and through their wall oxygen penetrates into the blood and is then distributed throughout the body. Gills can be located on the head, on the paropodia and on the tail. The through intestine of ringlets consists of several sections. Each section of the intestine performs its own special function. The mouth leads into the throat. Some ringlets have strong horny jaws and teeth in their throats, which help them grasp live prey more firmly. In many predatory ringlets, the pharynx serves as a powerful weapon of attack and defense. The pharynx is followed by the esophagus. This section is often supplied with a muscular wall. Peristaltic movements of the muscles slowly push food into the next sections. In the wall of the esophagus there are glands whose enzyme serves for the primary processing of food. Following the esophagus is the midgut. IN in some cases goiter and stomach are developed. The wall of the midgut is formed by epithelium, very rich in glandular cells that produce digestive enzymes. Other cells of the midgut absorb digested food. Some ringlets have a midgut in the form of a straight tube, in others it is curved in loops, and still others have metameric outgrowths on the sides of the intestine. The hindgut ends at the anus.
Special organs - metaniphridia - serve to release germ cells - sperm and eggs. Metanephridia begins as a funnel in the body cavity; from the funnel there is a convoluted channel, which opens outward in the next segment. Each segment contains two metanephridia.
Reproduction. Rings reproduce asexually and sexually. Asexual reproduction is common in aquatic ringworms. At the same time, their long body breaks up into several parts. After some time, each part restores its head and tail. Sometimes a head with eyes, tentacles and a brain forms in the middle of the worm's body before it splits into parts. In this case, the separated parts already have a head with all the necessary sensory organs. Polychaetes and oligochaetes are relatively good at restoring lost body parts. Leeches and echiurids do not have this ability. These ringlets have lost their segmented body cavity. This is partly why, apparently, the ability to asexual reproduction and they lack the ability to restore lost parts.
Fertilization of eggs in ringed fish most often occurs outside the body of the mother's body. In this case, males and females simultaneously release reproductive cells into the water, where fertilization occurs.
In marine polychaetes and echiurids, the crushing of fertilized eggs leads to the development of a larva, which is not at all similar to adult animals and is called a trochophore. Trochophore short time lives in the surface layers of water, and then settles to the bottom and gradually turns into an adult organism. Freshwater and terrestrial ringworms are most often hermaphrodites and have direct development. Dense shells here protect the eggs from mechanical damage and from drying out under the scorching rays of the sun.
Practical significance. In Russia, for the first time in the history of world science, the acclimatization of some invertebrates was carried out to strengthen the food supply of the sea. For example, the polychaete Nereis, acclimatized in the Caspian Sea, became the most important food item for sturgeon and other fish.
Earthworms not only serve as bait for fishing and food for birds. They bring great benefits to humans by loosening the soil, making it more porous. This facilitates the free penetration of air and water to the roots of plants and increases crop yields. Rummaging in the ground, worms swallow pieces of soil, crush them and throw them to the surface well mixed with organic matter. The amount of soil brought to the surface by worms is amazingly large. If we were to distribute the soil plowed by earthworms every 10 years over the entire surface of the land, we would get a layer fertile land 5 cm thick.
Annelids (Annelida)- a type of invertebrates that numbers about 12,000 known to science species of polychaete and polychaete worms, leeches and mysostomids. Annelids live in marine environments, typically in the intertidal zone and near hydrothermal vents, freshwater bodies, and also on land.
Description
Annelids have bilateral symmetry. Their body consists of a head region, a tail region and middle region numerous repeated segments.
The segments are separated from each other by partitions. Each segment contains full set organs and has a pair of chitinous bristles, and marine species parapodia (muscular appendages used for locomotion). The mouth is located on the first segment in the head region, the intestines pass through the entire body to the anus, located in the tail segment. In many species, blood circulates through blood vessels. The body of annelids is filled with fluid, which creates hydrostatic pressure and gives animals shape. Most annelids live in soils or muddy sediments at the bottom of freshwater or marine bodies of water.
The outer layer of the body of annelids consists of two layers of muscles, one layer has fibers that work in the longitudinal direction, and the second layer has muscle fibers that work in a circular pattern. Annelids move by coordinating their muscles along the entire length of their body.
Two layers of muscles (longitudinal and circular) are able to work in such a way that parts of the body of annelids can be alternately long and thin or short and thick. This allows the annelids to create a wave of motion along their entire body, which allows them to move through loose soil (in the case of earthworm). They stretch out to penetrate the soil and build new ones. underground passages and ways.
Reproduction
Many species of annelids use asexual form reproduction, but there are species that reproduce sexually. Most species develop from larvae.
Nutrition
Classification
Annelids are divided into the following taxonomic groups.
General characteristics
Type Annelids are a large group (12 thousand species). It includes secondary cavity animals, whose body consists of repeating segments, or rings. Circulatory system in annelids it is closed. Compared to roundworms ringed ones have more perfect nervous system and sense organs. The main features of this group need to be described in more detail.
The secondary body cavity, or coelom (from the Greek koiloma - “recess”, “cavity”), develops in the embryo from the mesoderm layer. This is the space between the body wall and internal organs. Unlike the primary body cavity, the secondary cavity is lined from the inside with its own internal epithelium. The whole is filled with liquid, creating permanence internal environment body. Due to fluid pressure, the secondary cavity maintains a certain form the body of the worm and serves as a support when moving. In other words, the whole serves as a hydroskeleton. Coelomic fluid is involved in metabolism: it transports nutrients, accumulates and excretes harmful substances, and also removes reproductive products.
Annelids have a segmented body: it is divided into successive sections - segments, or rings (hence the name - annelids). There may be several or hundreds of such segments in different species. The body cavity is internally divided into segments by transverse partitions. Each segment is an independent compartment: it has its own external outgrowths, nodes of the nervous system, excretory organs and gonads.
The phylum Annelids include Polychaete worms and Oligochaete worms.
Habitats, structure and life activity of polychaete worms
About 7,000 species of polychaete worms are known. Most of them live in the seas, a few live in fresh waters, in the litter tropical forests. In the seas, polychaete worms live on the bottom, where they crawl among stones, corals, thickets of marine vegetation, and burrow into silt. Among them there are sessile forms that build a protective tube and never leave it (Fig. 62). There are planktonic species. Polychaete worms are found mainly in coastal strip, but sometimes at a depth of up to 8000 m. In some places, up to 90 thousand polychaete worms live per 1 m2 of the seabed. They are eaten by crustaceans, fish, echinoderms, intestinal cavities, and birds. Therefore, some polychaete worms were specially bred in the Caspian Sea as food for fish.
Rice. 62. Various polychaete annelids: 1 - sessile form of the sea worm: 2 - nersis; 3 - sea mouse; 4 - sand core
The body of polychaete worms is elongated, slightly flattened in the dorsal-abdominal direction, or cylindrical, from 2 mm to 3 m. Like all annelids, the body of polychaetes consists of segments, the number of which in different species ranges from 5 to 800. In addition to many body segments there is a head section and an anal lobe.
On the head of these worms there are a pair of palps, a pair of tentacles and antennae. These are the organs of touch and chemical sense (Fig. 63, A).
Rice. 63. Nersis: A - head section; B - parapodya (cross section); B - larva; 1 - tentacle; 2 - palp; 3 - antennae; 4 - eyes: 5 - bristles
On the sides of each body segment, dermal-muscular outgrowths are noticeable - organs of movement, which are called parapodia (from the Greek para - “near” and podion - “leg”) (Fig. 63, B). Parapodia have a kind of reinforcement within them - bundles of bristles that contribute to the rigidity of the organs of movement. The worm rakes its parapodia from front to back, clinging to uneven surfaces of the substrate, and thus crawls forward.
In sessile forms of worms, a partial reduction (shortening) of the parapodia occurs: they are often preserved only in the anterior part of the body.
The body of polychaete worms is covered with a single-layer epithelium. In sessile forms of worms, epithelial secretions can harden, forming a dense protective cover around the body. The skin-muscle sac consists of a thin cuticle, skin epithelium and muscles (Fig. 64, A). Under the skin epithelium there are two layers of muscles: transverse, or circular, and longitudinal. Under the muscle layer there is a single-layer internal epithelium, which lines the secondary body cavity from the inside and forms partitions between the segments.
Rice. 64. Transverse (A) and longitudinal (B) sections through the body of Nereis (arrows show the movement of blood through the vessels): 1 - parapodim; 2 - longitudinal muscles; 3 - circular muscles: 4 - intestine; 5 - abdominal nerve chain; 6 - dorsal blood vessel; 7 - abdominal blood vessel; 8 - mouth opening; 9 - pharynx; 10 - brain
Digestive system begins with the mouth, which is located on the ventral side of the head lobe. In the section next to the mouth, the muscular pharynx, many predatory worms have chitinous teeth that serve to grasp prey. The pharynx is followed by the esophagus and stomach. The intestine consists of three sections: the foregut, middle and hind intestine (Fig. 64, B). The midgut looks like a straight tube. Digestion and absorption of nutrients occurs in it. Fecal matter forms in the hindgut. The anal opening is located on the anal blade. Vagrant polychaete worms are mainly predators, while sessile ones feed on small organic particles and plankton suspended in water.
Respiratory organs. In polychaete worms, gas exchange (oxygen absorption and carbon dioxide release) is carried out either over the entire surface of the body or through areas of parapodia into which blood vessels extend. In some sessile forms, the respiratory function is performed by the corolla of the tentacles on the head lobe.
The circulatory system of annelids is closed: in any part of the worm’s body, blood flows only through the vessels. There are two main vessels - dorsal and abdominal. One vessel passes above the intestine, the other - under it (see Fig. 64). They are connected to each other by numerous semi-circular vessels. There is no heart, and the movement of blood is ensured by contractions of the walls of the spinal vessel, in which blood flows from back to front, in the abdominal - from front to back.
Excretory system represented by paired tubes located in each body segment. Each tube begins with a wide funnel facing the body cavity. The edges of the funnel are lined with flickering cilia. The opposite end of the tube opens outward on the side of the body. With the help of the system of excretory tubules, waste products that accumulate in the coelomic fluid are excreted outside.
Nervous system consists of paired suprapharyngeal, or cerebral, nodes (ganglia), connected by cords into a peripharyngeal ring, a paired abdominal nerve cord and nerves extending from them.
Sense organs most developed in wandering polychaete worms. Many of them have eyes. The organs of touch and chemical sense are located on the antennae, antennae and parapodia. There are organs of balance. Touch and other irritants act on sensitive skin cells. The excitation that arises in them is transmitted along the nerves to the nerve nodes, from them through other nerves to the muscles, causing them to contract.
Reproduction. Most polychaete worms are dioecious. Gonads are present in almost every segment. Mature germ cells (in females - eggs, in males - sperm) enter first in general, and then through the tubules excretory system- into the water. Fertilization is external. A larva develops from the egg (see Fig. 63, B), which swims with the help of cilia. Then it settles to the bottom and turns into an adult worm. Some species also reproduce asexually. In some species, the worm is divided crosswise, and each half restores the missing part. In others, the daughter individuals do not disperse, and as a result, a chain is formed, including up to 30 individuals, but then it breaks up.
