In annelids, the muscles are circular and longitudinal. Type annelids general characteristics of the type

14.10.2019

general characteristics

Type Annelids are a large group (12 thousand species). It includes secondary cavity animals, whose body consists of repeating segments, or rings. The circulatory system of annelids is closed. Compared to roundworms, annelids have a more advanced nervous system and sensory 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 the 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 a constant internal environment of the body. Thanks to fluid pressure, the secondary cavity maintains a certain shape of the worm's body 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 removes 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 of 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 the coastal zone, 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 system. 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 a 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 as a whole, and then through the tubules of the 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.

Annelids are a fairly large group of invertebrate organisms. In addition, they are considered the most organized representatives of worms. They live mainly in fresh and salt water bodies, as well as in soil. Some species of tropical leeches have adapted to a terrestrial way of existence.

Type Annelids: general characteristics

The sizes of representatives of this group range from a few millimeters to six meters. A rather characteristic feature of such an organism is the presence of segmentation - their body consists of many rings, which explains the name of the type. External ringing corresponds to internal segmentation. That is why, when the body is wounded or damaged, the annelid loses only a few segments, which are soon regenerated.

From the outside, the body is covered with a cuticle that is not shed. Chitinous bristles grow from it - another characteristic feature of this species. Some representatives may have parapodia on their segments - rather primitive limbs, which in some cases are equipped with sensitive bristles or gills.

Ringed worms: structural features internal organs

For representatives of this type characterized by the presence of a secondary body cavity - the coelom. This cavity is filled with a specific fluid, thanks to which normal indicators are maintained.

There is a skin-muscular sac, which consists of epithelial balls, as well as muscles grouped into circular and longitudinal groups.

The digestive system is continuous, beginning with the mouth and ending with the anus. Annelids have three intestinal sections - anterior, middle and posterior. Some species have primitive salivary glands.

The body breathes through the skin. The only exceptions are some marine species of animals that have gills on their parapodia. As for the circulatory system, it is usually closed. It consists of the abdominal and dorsal aorta, which are connected to each other by annular vessels. These organisms do not have a heart - the movement of blood is ensured by contraction of the dorsal aorta. Blood can contain a wide variety of respiratory pigments.

Still quite simple. At the anterior end of the body there is a large nerve ganglion, which performs the functions of the brain. A nerve chain extends from it, which in each segment of the body forms a small ganglion - a collection of neurons. are represented by the eyes, organs of chemical sensitivity, as well as mechanoreceptors, which are distributed throughout the body of the worm.

Ringed worms: features of reproduction and development

Organisms of this group can be either heterosexual or hermaphrodite (they are much less common). For example, it has a hermaphroditic reproductive system, but two individuals are needed for fertilization. It can occur both in the external environment and through the introduction of sperm into the internal canals of the female.

Another interesting fact is that annelids with pronounced segmentation have a tendency to rapid and intense regeneration. Due to this, some species are characterized by direct development of organisms, without metamorphosis.

It is worth noting that the role of annelids is quite significant. For example, the well-known one is responsible for soil aeration. This group also includes leeches, which are often used in modern medicine. Hirudin, produced by the leech, is of particular value, as it thins the blood and is used in the fight against thrombosis and other dangerous diseases.

Annelids (ringworms) are a large type (about 9 thousand species) of higher free-living marine, freshwater and soil animals, having more complex organization than flatworms and roundworms. This primarily applies to marine polychaete worms, which are a key group in the evolution of higher invertebrates: mollusks and arthropods descended 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 the repetition of many internal organs, which increases the survival of the animal in case of partial damage to the body.

2. Groups of segments similar in structure and function in polychaete worms are combined into body parts - head, trunk and anal lobe. The head section was formed by the fusion of several anterior segments.

3. Secondary body cavity (whole) filled with coelomic fluid.

Figure 11.7. Head end of the Nereid: I1-eyes; 2 - tentacles; 3 - antennae; 4 - parapodia with tufts of setae.

4. Skin-muscle bag consists of a thin elastic cuticle, a single-layer epithelium located under it and two muscle layers: the outer - circular, and the inner - highly developed longitudinal.

5. First appeared specialized organs of movement - parapodia- are lateral bilobed outgrowths of the body walls of the trunk segments into which the coelom extends. Both lobes (dorsal and ventral) bear more or less number of setae (Fig. 11.7). In oligochaete worms there are no parapodia, there are only tufts with a few setae.

6. B digestive system Having three sections, the foregut is highly differentiated into a number of organs (mouth, pharynx, esophagus, crop, stomach).

7. First developed closed circulatory system. It consists of large longitudinal dorsal and abdominal vessels, connected in each segment by annular vessels (Fig. 11.8). The movement of blood is carried out due to the pumping activity of the contractile areas of the spinal cord, and less commonly of the annular vessels. Blood plasma contains respiratory pigments similar to hemoglobin, thanks to which ringworms have populated habitats with very different oxygen content.

8. Respiratory system in polychaete worms - gills; these are thin-walled leaf-shaped, feathery or bushy outer outgrowths of part of the dorsal lobes of the parapodia, penetrated by blood vessels. Oligochaete worms breathe over the entire surface of their body..

9. Excretory organs- metanephridia located in pairs in each segment, removing waste products from the cavity fluid.

10. Nervous system ganglion type. It consists of paired suprapharyngeal and subpharyngeal ganglia, connected by nerve trunks into a peripharyngeal nerve ring, and many pairs of ganglia of the ventral nerve cord, one pair in each segment (Fig. 11.8, a). Sense organs are diverse: vision (in polychaete worms), touch, chemical sense, balance.

11. Overwhelming majority kolchetsov- dioecious animals, less often hermaphrodites. In most aquatic ringlets, fertilization is external, while in soil forms it is internal. Development with metamorphosis (in polychaete worms) or direct (in polychaete worms, leeches). Some types of ringworms, in addition to sexual reproduction, also reproduce asexually (by fragmentation of the body with subsequent regeneration of the missing parts).

12. Phylum Annelids are divided into three classes - Polychaetes, Oligochaetes and Leeches.

Class Oligochaetes

Its representatives are mainly soil dwellers, but freshwater forms are also known. The structure of oligochaetes is largely determined by the soil way of life, due to which the organization of worms has been simplified. The head section has a simple structure and is devoid of sensory organs. Parapodia are absent, although a limited number of setae are preserved. All oligochaetes are hermaphrodites. The reproductive system is concentrated in a few segments of the anterior part of the body, fertilization is internal.

Earthworms live in moist, humus-rich soil. The body is elongated, the segmentation is homogeneous. On each segment, the remaining eight setae are arranged in two rows on the sides of the body. Clinging to uneven soil, the worm, with the help of the muscles of a powerful skin-muscular sac, moves forward.

The digestive system has a number of significant structural features. Its anterior section is differentiated into the muscular pharynx, esophagus, crop and muscular stomach. The ducts of the calcareous glands open into the cavity of the esophagus. Their secretions neutralize the acids that the food consumed by worms is rich in. In the midgut, food is digested and absorbed.

The movement of blood in a closed circulatory system is carried out by contraction of the five anterior Maltsev vessels (“hearts”).

Earthworms breathe through the entire surface of their wet body due to the presence of a dense subcutaneous network of blood vessels.

Earthworms are hermaphrodites. Cross fertilization. To do this, two worms are applied with their ventral sides to each other, as a result of which an exchange of seminal fluid occurs, which enters the sac-like skin invaginations - the seminal receptacles. After exchanging sperm, the earthworms disperse. After this, the girdle areas (segments 32-37) of each individual begin to form a mucous membrane into which the worms lay eggs. As the coupling moves through the segments containing the spermatheca, the eggs are fertilized by sperm belonging to another individual. The clutch with fertilized eggs is thrown off the front end of the body by the movement of the worm's muscles, becomes compacted and turns into an egg cocoon, where young worms develop.

Earthworms are characterized by a high ability to regenerate.

Soil annelids are beneficial animals. Even Charles Darwin noted their importance for soil fertility. By dragging fallen leaves into holes, they enrich the soil with humus, and by making passages in the soil, they loosen it and facilitate the penetration of air and water to the roots of plants. The amount of soil passed through the digestive tract of worms in Europe ranges from 6 to 84 t/ha, and in Cameroon it can reach 210 t/ha.

Freshwater oligochaetes play a significant role in the nutrition of bottom-dwelling fish.

Annelids apparently originate from lower segmented worms with parenchyma. The most ancient of the annelids are the marine polychaetes. From them, during the transition to a freshwater and terrestrial way of life, oligochaetes evolved, and from them leeches.

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 living 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 the nereid (see Fig. 11.7) and the sand vein. They are food items for a number of commercial fish. Nereid has been successfully acclimatized in the Caspian Sea.

Related information.

Annelids belong to the subsection of coelomic animals Coelomata), a group (superphylum) of protostomes (Protostomia). For primary stomates it is characteristic:

The primary mouth (blastopore) of the embryo (gastrula) passes into the adult animal or the definitive mouth is formed in place
primary mouth.
Mesoderm is formed, as a rule, by a teloblastic method.
The covers are single-layered.
External skeleton.
Protostomes are the following types of animals: annelids (Annelida), mollusks (Mollusca), arthropods (Arthropoda), onychophorans (Onychophora).
Annelids are a large group of animals, about 12 thousand species are known. They are inhabitants of the seas, fresh water bodies, and inhabit land.

Polychaete annelids Polychaetes

Main characteristics of the type:

The body consists of a head lobe (prostomium), a segmented trunk and an anal lobe (pygidium). Characterized by metamerism of external and internal structure.
The body cavity is secondary, well developed in most animals. The blades lack a coelom.
The skin-muscular sac is developed, represented by epithelium and circular and longitudinal muscles.
The intestine consists of three sections; salivary glands are developed.
The excretory system is of the nephridial type.
The circulatory system is a closed type, absent in some groups.
The respiratory system is either absent, animals breathe with the entire surface of the body, some representatives have gills.
The nervous system consists of a paired brain and a ventral nerve cord or scala.
Annelids are dioecious or hermaphrodites.
Crushing of eggs according to a spiral type, deterministic.
Development with metamorphosis or direct.

Annelids General characteristics

Latin name Annelida

Type annelids, or rings, is a very important group for understanding the evolution of higher invertebrates. It includes about 8,700 species. Compared with the considered flat and roundworms and even with nemerteans, annelids are significantly more highly organized animals.

The main feature of the external structure of the rings is metamerism, or body segmentation. The body consists of a more or less significant number of segments, or metameres. The metamerism of the rings is expressed not only in the external, but also in the internal organization, in the repeatability of many internal organs.

They have a secondary body cavity - generally absent in lower worms. The body cavity of the ringlets is also segmented, that is, divided by partitions in greater or lesser accordance with the external segmentation.

U ringlets there is a well-developed closed circulatory system. The excretory organs - metanephridia - are located segment by segment, and therefore are called segmental organs.

Nervous system consists of a paired suprapharyngeal ganglion, called the brain, connected by peripharyngeal connectives to the ventral nerve cord. The latter consists of a pair of longitudinally contiguous trunks in each segment, forming ganglia, or nerve ganglia.

Internal structure

Musculature

Under the epithelium there is a muscular sac. It consists of external circular and internal longitudinal muscles. Longitudinal muscles in the form of a continuous layer or divided into ribbons.
Leeches have a layer of diagonal muscles, which are located between the circular and longitudinal ones. The dorso-abdominal muscles are well developed in leeches. In wandering polychaetes, flexors and extensors of parapodia are developed - derivatives of the ring muscles. The ring muscles of oligochaetes are more developed in the anterior eight segments, which is associated with the way of life.

Body cavity

Secondary or whole. The body cavity is lined with coelomic or perinoneal epithelium, which separates the cavity fluid from tissues and organs. Each body segment of polychaetes and oligochaetes has two coelomic sacs. The walls of the sacs on one side are adjacent to the muscles, forming a somatopleura, on the other side to the intestines and to each other, a splanchnopleura (intestinal leaf) is formed. The splanchnopleura of the right and left sacs forms the mesentery (mesentery) - a two-layer longitudinal septum. Either two or one septum is developed. The walls of the sacs facing adjacent segments form dissepiments. Dissepiments disappear in some polychaetes. Coelom absent from prostomium and pygidium. In almost all leeches (with the exception of bristle-bearing ones), the parenchyma between the organs is generally preserved in the form of lacunae.

The functions of the coelom are: supporting, distributive, excretory and, in polychaetes, reproductive.

Origin of the coelom. There are 4 known hypotheses: myocoel, gonocoel, enterocoel and schizocoel.

Digestive system

Represented by three departments. Cavity digestion. The pharynx of predatory polychaetes is armed with chitinous jaws. The ducts of the salivary glands open into the pharynx of annelids. Leech glands contain the anticoagulant hirudin. In earthworms, ducts of calcareous (morrain) glands flow into the esophagus. The foregut of earthworms includes, in addition to the pharynx and esophagus, a crop and a muscular stomach. The absorption surface of the midgut increases due to outgrowths - diverticulum (leeches, part of the polychaetes) or typhlosol (oligochaetes).

Excretory system

Nephridial type. As a rule, each segment has two excretory canals; they begin in one segment and open with an excretory pore in the next segment of the body. The excretory organs of polychaetes are the most diverse. Polychaete worms have the following types of excretory systems: protonephridia, metanephridia, nephromyxia and myxonephridia. Protonephridia are developed in larvae; they begin with club-shaped terminal cells with a flagellum (solenocytes), then the nephridia canal. Metanephridia begins with a funnel with a nephrostomy, inside
the funnels contain the cilia, followed by the duct and nephropore. Protonephridia and metanephridia are ectodermal in origin. Nephromyxia and myxonephridia are the fusion of the ducts of the protonephridia or metanephridia with the coelomoduct - the genital funnel. Coelomoducts of mesodermal origin. The excretory organs of oligochaetes and leeches are metanephridia. In leeches, their number is significantly less than that of body segments (medicinal leeches have 17 pairs), and the funnel is separated from the canal. In the excretory canals of the nephridia, ammonia is converted into high molecular weight compounds, and water is absorbed as a whole. Annelids also have storage “buds”: chloragogenous tissue (polychaetes, oligochaetes) and botryodenic tissue (leeches). They accumulate guanine and uric acid salts, which are removed from the coelom through nephridia.

Circulatory system of annelids

Most annelids have a closed circulatory system. It is represented by two main vessels (dorsal and abdominal) and a network of capillaries. Blood movement is carried out due to contraction of the walls of the dorsal vessel; in oligochaetes, the annular hearts also contract. The direction of blood movement through the spinal vessel is from back to front, and in the abdominal vessel - in the opposite direction. The circulatory system is developed in bristle-bearing and proboscis leeches. In jaw leeches there are no vessels; the function of the circulatory system is performed by the lacunar system. The process of functional replacement of one organ with another, different in origin, is called organ substitution. The blood of annelids is often colored red due to the presence of hemoglobin. Primitive polychaetes do not have a circulatory system.

Respiratory system

Most breathe over the entire surface of the body; some polychaetes and some leeches have gills. Respiratory organs are evaginated. The gills of polychaetes are in origin a modified dorsal antennae of parapodia, while those of leeches are skin outgrowths.

Nervous system and sensory organs

The nervous system includes: the paired medullary (suprapharyngeal) ganglion, connectives, subpharyngeal ganglia and the ventral nerve cord or scalene nervous system. The abdominal trunks are connected by commissures. The evolution of the nervous system went in the direction of transforming the ladder-type nervous system into a chain, immersing the system in the body cavity. Nerves coming from central system, constitute the peripheral system. There are varying degrees of development of the suprapharyngeal ganglion; the brain is either monolithic or divided into sections. Leeches are characterized by the fusion of ganglion segments that make up the suckers. Sense organs. Polychaetes: epithelial sensory cells, antennae, nuchal organs, antennae of parapodia, statocysts, organs of vision (goblet or bubble type eyes). Sense organs of oligochaetes: light-sensitive cells, some water inhabitants have eyes, chemical sense organs, tactile cells. Leeches: goblet organs – chemical sense organs, eyes.

Classification

The type of rings is divided into several classes, of which we will consider four:

1. Polychaeta ringlets

2. Echiurida

Echiurids are a highly modified group of ringlets, internal organization which differs from those of polychaetes by an unsegmented coelom and the presence of one pair of metanephrpdia.
Highest value To establish the unity of origin of echiurids with polychaetes, the trochophore larva of echiurids has.

At the bottom of the sea, among the stones in the silt and sand, there are peculiar animals, but appearance extremely little reminiscent of annelids, primarily due to their lack of segmentation. This includes such forms as Bonellia, Echiurus and some others, about 150 species in total. The body of the female Bonellia, which lives in rock crevices, has the shape of a cucumber and carries a long, non-retractable trunk, forked at the end. The length of the trunk can be several times greater than the length of the body. A groove lined with cilia runs along the trunk, and at the base of the trunk there is a mouth. With the flow of water, small food particles are brought to the mouth along the groove. On the ventral side of the anterior part of Bonellia's body there are two large setae, and in other echiurids there is also a corolla of small setae at the posterior end. The presence of setae brings them closer to the ringlets.

3. Oligochaeta

The oligochaetes, or oligochaetes, are a large group of annelids, including about 3,100 species. They undoubtedly descend from polychaetes, but differ from them in many significant features.
Oligochaetes overwhelmingly live in the soil and at the bottom of fresh water bodies, where they often burrow into muddy soil. The Tubifex worm can be found in almost every freshwater body, sometimes in huge quantities. The worm lives in silt, and sits with its head end buried in the ground, and its back end constantly makes oscillatory movements.
Soil oligochaetes include a large group of earthworms, an example of which is the common earthworm (Lumbricus terrestris).
Oligochaetes feed mainly on plant foods, mainly on decaying parts of plants, which they find in the soil and silt.
When considering the characteristics of oligochaetes, we will mainly have in mind the common earthworm.

4. Leeches (Hirudinea) >> >>

Phylogeny

The problem of the origin of rings is very controversial; there are various hypotheses on this issue. One of the most widespread hypotheses to date was put forward by E. Meyer and A. Lang. It is called the turbellar theory, since its authors believed that polychaete ringlets originate from turbellarian-like ancestors, i.e., they associated the origin of ringlets with flatworms. At the same time, supporters of this hypothesis point to the phenomenon of so-called pseudometamerism, observed in some turbellarians and expressed in the repeatability of some organs along the length of the body (intestinal outgrowths, metameric arrangement of the gonads). They also point out the similarity of the ringlet trochophore larvae with the Müllerian turbellarian larva and the possible origin of metanephridia by changing the protonephridial system, especially since the ringlet larvae - trochophores - and the lower ringlets have typical protonephridia.

However, other zoologists believe that annelids are closer to nemerteans in a number of ways and that they descend from nemertean ancestors. This point of view is developed by N. A. Livanov.

The third hypothesis is called the trochophore theory. Its proponents produce ringlets from a hypothetical ancestor of Trochozoon, which has a trochophore-like structure and originates from ctenophores.

As for the phylogenetic relationships within the four classes of annelids considered, they currently seem quite clear.

Thus, annelids, which are highly organized protostomes, apparently originate from ancient protostomes.

Undoubtedly, not only modern polychaetes, but also other groups of annelids originated from ancient polychaetes. But it is especially important that polychaetes are a key group in the evolution of higher protostomes. Mollusks and arthropods originate from them.

The meaning of annelids

Polychaete worms.

 Food for fish and other animals. Mass species play the greatest role. Introduction of the polychaete Azov nereid into the Caspian Sea.
 Human food (palolo and other species).
 Purification of sea water, processing of organic matter.
 Settlement on the bottoms of ships (serpulids) – reduction in movement speed.

Oligochaete worms.

 Oligochaetes - inhabitants of water bodies - provide food for many animals and participate in the processing of organic matter.
 Earthworms are animal food and human food.Gallery

76. General characteristics of annelids

The type of annelids, or annelids, covers about 9 thousand species of worms, which have a much more complex organization than representatives of other types of worms.

Certain structural features of the larvae, which are very reminiscent of the larval forms of free-living flatworms (the body is not divided into segments and is covered with ciliated epithelium), suggest that ringworms, like roundworms, originated from primitive flatworms, similar in structure to modern ciliated worms . This happened more than 600 million years ago.

The body of most forms consists of separate rings - segments. Many ringlets are characterized by the presence of lateral mobile outgrowths of the body of parapodia and tufts of setae, which are the prototype of the limbs. Some annelids have skin projections called gills on the dorsal part of the parapodia.

External segmentation corresponds to the division of the internal body cavity by partitions into separate sections and the segmental arrangement of a number of internal organs. Nerve ganglia, annular blood vessels, excretory organs - metanephridia, midgut pouches and genitals are correctly repeated. The skin-muscular sac consists of the cuticle, epithelium, circular and longitudinal muscles, as well as the internal lining of the body cavity.

The nervous system is represented by a peripharyngeal nerve ring with a well-developed suprapharyngeal and less pronounced subpharyngeal nerve nodes, as well as an abdominal nerve cord that forms nodes in each segment of the body. Numerous nerves arise from them. Sense organs are better developed in polychaete annelids and are represented by one or two pairs of eyes located on the dorsal side of the first segment.

The circulatory system is closed, consists of vessels, some of which have contractile walls (“hearts”), which ensures blood circulation. Some groups do not have a circulatory system. The blood of a number of forms contains hemoglobin.

Breathing is carried out in most cases over the entire surface of the body, some have special outgrowths - skin gills.

The digestive system is continuous, complex, divided into the pharynx, esophagus, stomach and intestine, sometimes having lateral outgrowths; ends with the anus.

The excretory system is represented by segmentally located metanephridia. Their funnel faces the body cavity, and the other end opens outward.

Reproduction of annelids occurs sexually and asexually by budding. Among the ringlets there are dioecious species and hermaphrodites. Some ringlets have a rather complex reproductive system, while others do not have special reproductive organs - germ cells are formed from the internal lining of the body cavity and are brought out through metanephridia.

The phylum unites several classes, of which the three main ones are Polychaetes, Oligochaetes and Leeches.

77. Characteristics and systematic position of the taxon Polychaete worms using the example of Nereids.

Class Polychaete worms

The body of polychaete ringlets has various appendages: parapodia, sensitive antennae, setae - they serve for movement and are sensory organs. The appendages on the head section are more developed. The head section is the result of the fusion of several (two or three) anterior segments. Here are located the mouth opening, a pair of palps and a pair (or more) of tactile organs - tentacles (antennas) of various sizes and shapes.

Polychaetes are characterized by the presence of paired parapodia - short muscular mobile outgrowths located on the sides of the body on each segment. The parapodium consists of a main undivided part and two branches - dorsal and ventral. From the base of the dorsal and ventral lobes, the parapodia extends along a thin tentacle-like appendage - an antennae, which performs the functions of the organs of smell and touch. Each of the branches of the parapodia contains a tuft of setae protruding from it with the ends outward, and one large supporting seta. They consist of an organic substance similar in chemical composition to chitin.

Most polychaetes are found mainly in the coastal zone of the seas. Many of them descend, however, deeper than 1000 m, and some are found even at a depth of 8 thousand m. Relatively few species lead a free-swimming lifestyle and, like other planktonic animals, have a glassy transparent body. Benthic polychaetes, for example, Nereid, Lepidonotus, Palolo, mainly crawl along the bottom among algae, but many of them lead a burrowing lifestyle, making long burrows in sand or silt. This is a large sandworm sea worm. Others lead a sedentary lifestyle: spirorbis, serpula, etc.

78. Characteristics and systematic position of the taxon Oligochaete worms using the example of an earthworm.

Class Oligochaete worms

The class of oligochaetes includes annelids, which have the basic features of the type, but with underdeveloped tentacles, parapodia and gills. This is due to adaptation to life in the sandy soils of reservoirs (tubifex) and in the soil (earthworms).

The body of oligochaete annelids is highly elongated and cylindrical. Small forms are barely 0.5 mm, most major representative– earthworm from Australia – reaches a length of 3 m. At the front end there is a small movable head lobe, devoid of eyes, antennae and tentacles. The body segments are externally identical, their number is usually large (90 - 600). Each segment, except the most anterior one, which bears the mouth opening, is equipped with small bristles protruding directly from the body wall and arranged in four tufts - a pair of lateral and a pair of ventral ones.

Earthworms are hermaphrodites, but they undergo cross-fertilization. The two worms approach each other and exchange sperm, which enter their spermatic receptacles. Then a mucous muff is formed on the body of each worm. By contracting the muscles, the worm moves it to the anterior end of the body. When the muff passes by the openings of the ovarian ducts and spermatic receptacles, eggs and sperm enter it. Then the muff slides off the worm and closes into a cocoon, where small worms develop from the fertilized eggs.

In addition to sexual reproduction, asexual reproduction is also observed in oligochaetes: the body of the worm is divided into two parts, the posterior end of the body is regenerated at the anterior one, and the anterior end at the posterior one.

Oligochaetes live in soil and fresh water bodies, only extremely rarely found in the seas. Freshwater forms either crawl along the bottom or, like tubeweeds, sit in burrows dug in the silt, protruding only the back half of the body from them into the water. Terrestrial forms, as a rule, lead a burrowing lifestyle. For example, an earthworm lives in various soils, loosening and cultivating them (this activity is especially favorable for the soils of vegetable gardens and orchards). These animals, passing the soil through their intestines, constantly improve it, saturating it with organic residues and mixing, loosening it, providing air access to deeper layers, and increasing fertility. In some cases, the transfer of earthworms to soils where they were not previously present increases the yield of garden crops. In countries with a humid climate, there are more earthworms. However, the earthworm does not live in waterlogged soils, as well as in swamps, especially peat ones. Annelids that live in the soil serve as food for many animals. They are eaten by moles, frogs and some reptiles.

79. Morphophysiological characteristics, systematic position, significance for humans of the leech.

In total, about 250 species of leeches are known, the vast majority of which live in fresh water bodies.

Veils. The body is covered on the outside with a cuticle. The underlying epithelium is rich in glandular mucous cells, and numerous pigment cells are scattered near the axis of the epithelial cells, causing the color of leeches.

Rice. 116. Anatomy of a medicinal leech:

/ – suprapharyngeal nerve ganglion; 2 pharynx; 3 ■-esophagus; 4 – stomach;
5 – the posterior will dig out the stomach; V- midgut; 7 – hindgut; N- anus; U posterior sucker; 10- ganglion of the ventral nerve cord; // – meta-pefrndpi; 12 - soaked bladder; 13 – seed sacs; 14 – vas deferens; 15 - meta-nephridnev funnels; 16
- vagina; 17 ovaries; IS epididymis; 19 - conulular organ; 20 - prostate; 21 bokonalakuna

The muscles are very developed. The skin-muscle sac contains three layers of muscle fibers that stretch in transverse, diagonal and longitudinal directions to the axis of the body.

The body cavity is greatly reduced and has the appearance of a system of lacunae.

Nervous system. There is a ventral nerve cord.

Sense organs. Eyes, if present, are distinguished by their primitive structure.

The integument contains sensory cells and nerve endings.

The mouth leads into the oral cavity, in which in some species (for example, the medicinal leech) there are

three jaws equipped with many teeth (jaw leeches), others have a proboscis with which they penetrate the integument of the victim (proboscis leeches).

The oral cavity leads to the pharynx, which plays the role of a sucking apparatus. Single-celled salivary glands open into the pharynx.

In medicinal leeches, the salivary glands secrete a special substance - hirudin, which has the property of preventing blood clotting.

The anterior section of the intestinal tract has several pairs of pocket-like lateral projections that increase their volume, which allows for a large supply of blood, which is enough for a medicinal leech for 2-3 months. Thanks to the admixture of hirudin, the blood of leeches does not clot and remains fresh for a long time. Digestion occurs in the endodermal part of the intestine.

Respiration in most species occurs through the body, but some species have gills.

The excretory organs are metanephridia.

Reproductive system. Leeches are hermaphrodites. Mating of medicinal leeches occurs in the spring near a reservoir in damp soil above the water level. Their large cocoons resemble acorns. They are formed by the end of June. The development of leeches in the cocoon lasts about 5 weeks. Leeches reach sexual maturity by 5 years. They live up to 20 years.

Of practical interest is the medicinal leech used to treat sick people. Hirudotherapy - treatment with leeches. An attached leech causes local capillary bleeding, which can eliminate venous congestion, increase blood supply to the body area, in addition, substances that have an analgesic and anti-inflammatory effect enter the blood. As a result, blood microcirculation improves, the likelihood of thrombosis decreases, and swelling subsides. A reflexogenic effect is expected.

In medical practice, a leech is removed after use by applying an alcohol swab to its head end. Getting rid of an unwanted leech is quite simple - you just need to pour a little salt on the suction cup.

It should also be noted that leeches, when attacking a person, cause hirudinosis.

80. General characteristics and taxonomy of Mollusks.

SHELLFISH, a type of invertebrate animal. Distributed throughout the globe. They live in the seas (especially numerous in the coastal zone of tropical seas), fresh waters and on land. Diverse in appearance and size. As a rule, they have a bilaterally symmetrical, non-segmented body, consisting of three sections: head, torso and legs.
In most mollusks, the body is covered with a calcareous shell - whole or consisting of several plates. Adjacent to the shell from the inside is a fold of skin surrounding the body - the mantle. In the so-called formed between the mantle and the body. The mantle cavity houses the respiratory organs - gills. The openings of the excretory organs (kidneys), genitals and anus also open here. For movement, mollusks have a leg - a muscular unpaired outgrowth of the abdominal wall of the body. The shell is synthesized by the mantle. It distinguishes between the apex and the mouth through which the head and leg of the mollusk emerge.
The head contains a mouth, tentacles, and eyes. There is a special organ in the mouth - a grater, which allows you to scrape algae from stones. The intestine is usually longer than the body and folded into the body in loops. The nervous system consists of a peripharyngeal nerve ring and several pairs of nerve ganglia. The circulatory system is not closed. The heart consists of a ventricle, one or two atria, and is usually surrounded by a sac called the pericardium.
The phylum of molluscs numbers approx. 130 thousand modern species and is second in number after arthropods. There are several classes distinguished in it, among which the most numerous are gastropods ( snails), cephalopods And bivalves.
Among mollusks there are both dioecious and hermaphrodite species. Fertilization can be external or internal. From the fertilized egg comes either a larva (in marine species), floating for some time in the water and then settling to the bottom, or a formed mollusk (in freshwater and land species), which is occasionally observed live birth. Mollusks live from several months to several decades.
Mollusks serve as food for many invertebrates, fish, and whales. Oysters, scallops, mussels , squid, grape snail and some others are edible, commercially available. Pearls and mollusk shells are used to make jewelry and other items.

81. Systematics, morphology, physiology, reproduction and development. using the example of a grape snail. Ecology and medical significance of gastropods.

General characteristics. Gastropods are molluscs whose body is divided into a head, a trunk and a leg with a wide crawling sole. The shell, if present, is whole and spirally curled. The body is asymmetrical. There are 1-2 pairs of tentacles on the head.

Most have well developed eyes. They breathe with gills or lungs.

Structure and vital functions. The body shape of gastropods is varied, usually asymmetrical due to the twisting of the body in a spiral. On the head there are 1-2 pairs of tentacles capable of retracting and well-developed eyes, located in some species at the tops of the tentacles. The foot is usually wide, with a flat sole. Most gastropods move by sliding along the substrate thanks to the wave-like bends of the sole of the foot.

The shell often has a bizarre shape and bright color. In gastropods that swim in the water column of the seas, the shell is reduced to one degree or another. It is also absent in terrestrial slugs that hide in burrows for the day. When the animal is in a calm state, only its body is placed inside the shell, but in case of danger, the whole body is drawn into it. As a rule, the shell of gastropods is curved in a spiral, but in the limpet mollusk it is conical.

The mantle cavity is located in the lower whorls of the shell. The anus, ureters, and sometimes the genital duct open into it. In aquatic animals, it houses the respiratory organs - gills. In air-breathers, the mantle cavity becomes light, opening outwards with a respiratory opening. In the walls of the cavity there is a dense plexus of blood vessels.

The integument of gastropods is rich in various glands, including mucous glands, which are abundant on the sole of the foot.

The nervous system of these mollusks consists of several pairs of ganglia connected by commissures.

Sense organs. Gastropods have eyes, organs of balance - statocysts located in the leg, organs of touch (tentacles) and chemical senses.

The digestive organs begin with the oral opening located on the underside of the head, which leads into the pharynx. In the pharynx there are one or two jaws and a grater (radula), which looks like a plate with many small teeth arranged in transverse rows. Thanks to it, the mollusk can separate pieces of food and scrape off fouling (micropopulations from underwater plants and objects). The ducts of the salivary glands empty into the pharynx. The pharynx passes into the esophagus, which opens into the stomach, which receives the ducts of the large liver.

From the stomach, food enters the midgut and then the hindgut.

The respiratory organs are gills or lungs. Gills include all marine and some freshwater gastropods. Pulmonary gastropods include all terrestrial and many freshwater species (pond snails, reels, etc.). The latter are forced to periodically rise to the surface of the reservoir in order to draw air into the mantle cavity.

The circulatory system is represented by the heart, vessels and lacunae. The heart lies in the pericardial sac. Arterial vessels depart from it, which pour blood into the lacunae.

The excretory organs are the kidneys, the funnels of which open into the pericardial sac. The ureters end in the mantle cavity.

The reproductive organs of gastropods have different structures. Marine forms are usually dioecious, while terrestrial and many freshwater forms are hermaphrodites. Fertilization of eggs occurs inside the mother's body.

Development occurs without transformations or with the presence of a larval stage. There are viviparous species.

Practical significance gastropods are quite large. They play a significant role in the cycle of substances in water bodies. Living at the bottom and consuming various organic sediments, they accelerate their decomposition. Many serve as food for commercial fish, whales and pinnipeds. Sea whelks are a source of chain black and pink pearls; purple snails have special glands, from the secretion of which purple dye is obtained. Gastropods are of great importance as crop pests.

The most well studied of all gastropods is the grape snail. Grape snail (Helix)- a large mollusk with a striped shell. Grape snails live not only in vineyards, but also in parks, gardens, and on the edges of forests. The snail digs a hole in the loose earth - a nest, places eggs in it, covers it with earth and crawls several times over its clutch - its surface is now smoothed and indistinguishable from the immediate surroundings. After 25 days, the cubs emerge from the eggs almost simultaneously - tiny snails, but with a shell (although, to tell the truth, it can only formally be considered their protective house - it is translucent and breaks at the slightest touch). Only after a few years will they acquire a sufficiently strong shell.

By eating the leaves and buds of the grapevine, snails harm the vineyards. People have no idea how toothy the peaceful grape snails are. Their teeth are located on the tongue. The American garden snail, for example, has more than 14 thousand of them! With the help of this “grater,” snails devastate gardens and vegetable gardens. In a number of European countries they are valued as an excellent delicacy, and in some regions they are common food for the population and are eaten in huge quantities.

82.Bivalve. Systematics, morphology, physiology, reproduction and development. Using the example of the common toothless one. Ecology and significance of bivalves.

The class Bivalve (shells) unites sedentary marine and freshwater mollusks. Their body is enclosed in a shell consisting of two valves connected to each other on the dorsal side by means of a ligament and teeth. The shell valves open passively due to the elasticity of the ligament; they close with the help of two closing muscles located across the body of the mollusk and attached at their ends to the two shell valves. By contracting the muscles, the valves are attracted to each other. Shells are usually even less mobile than snails, although they are not particularly fast. Most often, bivalves lie motionless, attached to underwater objects with special threads called byssal threads. In ancient times, these threads were even used to make expensive fabric. But the larvae of shells can be very mobile. They swim, flapping their doors, looking for fish in order to tightly cling to it with sharp hooks on the edges of the shell. Soon the larva finds itself inside a small tumor on the fish's body. It grows there, feeding on fish juices. Then the tumor bursts, and the young shell falls to the bottom. This is how sedentary shells settle.

The main feature of bivalves is the absence of a head section of the body, and, consequently, a pharynx with a grater. The body is bilaterally symmetrical, laterally flattened; consists of a torso and a leg, most often in the shape of a wedge and, when the shell valves are open, extending between them. Attached mollusks (oysters) have no legs.

The body is covered with a mantle that covers it, hanging from the sides in the form of folds. On the dorsal side it fuses with the body of the mollusk. Often, the free edges of the mantle at the posterior end of the body grow together in places, leaving siphon openings for the passage and exit of water from the mantle cavity.

There are two plate-like gills on either side of the leg. The gills, as well as the inner side of the mantle, are covered with cilia, the beating of which creates a flow of water through the inlet (lower) siphon into the mantle cavity. Water with a suspension of algae and bacteria is filtered through the gills and through the outlet (upper), the siphon is removed outside. From the surface of the gills, with the help of special cilia, food particles are directed into the mouth opening. Thus, the gills of bivalves are not only a respiratory organ, but also a filtering device for filtering out food particles suspended in water. This method of obtaining food is typical for sedentary organisms and is called filtration.

The excretory system is represented by paired kidneys.

Due to a sedentary lifestyle, the absence of a head and a passive method of nutrition in the nervous system, the number of ganglia has decreased to three pairs. Sense organs are poorly developed.

Bivalves are dioecious animals. Fertilization is most often external.

Toothless - a typical representative of bivalve mollusks - lives in fresh water bodies with weak currents. It has a wide thin shell with a poorly developed mother-of-pearl layer. The shell valves are connected only by an elastic ligament; there are no teeth, hence the name - toothless. It feeds on microscopic plant and animal organisms and bacteria suspended in water.

The toothless genitals are located at the base of the leg. The eggs laid by the female fall onto their own gills, where they are fertilized by the male's sperm, carried along with the water into the mantle cavity. The developed larvae are carried into the water. With the help of denticles on the edges of the shell, they attach to the gills or skin of the fish after they are released from the body of the female mollusk into the water. For some time they feed on the inflamed tissues of the fish, grow and, falling out of the tumor to the bottom of the reservoir, continue to grow and develop. Free larvae in life cycle sedentary animals ensure their settlement.

The importance of bivalves in nature and agriculture is great and varied. Oysters, mussels, and scallops are used as food. Since the reserves of these mollusks are depleted, they are bred in large quantities on sea “farms” (specially prepared shallows and small bays and artificial reservoirs protected from predators).

Freshwater bivalves purify the water of ponds and lakes in which they live. Meat of freshwater species (barley, toothless) is used to feed pigs and ducks, and ground shellfish shells are used for mineral nutrition of young animals. Mother-of-pearl items such as buttons, brooches, etc. are made from mollusk shells.

The largest of the bivalve mollusks is the giant tridacna. Zoologists found mollusks with a shell 137 cm long and weighing 340 kg (of which the shell weighed 331 kg). Among bivalves there are many species that cause harm economic activity person. Thus, the shipworm sharpens long, winding passages in wood, which is why port facilities and the bottoms of ships fail. All they have left of the shell is a rudiment - this rudiment serves as a drill for them. A worm-eaten board can be pierced with your finger. The length of the worm is about 2 meters. The zebra mussel mollusk attaches to various hydraulic structures (water intake pipes, protective gratings), which makes it difficult for water to pass through and requires constant cleaning of fouling.

In natural reservoirs, the role of bivalves is great as biological filters that promote self-purification of water. They are the favorite food of stray fish.

83. Cephalopods. Systematics, anatomy, physiology, reproduction and development. Ecology and practical use of cephalopods.

Cephalopods, a class of the most highly organized marine mollusks. The length with tentacles ranges from 1 cm to 5 m, and in the giant squid it reaches 18 m. The body shape is very diverse and depends on the lifestyle of the mollusks. Inhabitants of the water column, which include most squids, have an elongated, torpedo-shaped body. Benthic species, among which octopuses predominate, are characterized by a sac-like body. In cuttlefish living in the bottom layer of water, the body is flattened in the dorsoventral direction. Narrow, spherical or jellyfish-like planktonic species of cephalopods are distinguished by their small size and gelatinous body.

The other part of the leg turns into a funnel, which plays an important role in movement. It grows to the ventral side of the body, opening at one end into the mantle cavity, and at the other into the external environment. The mantle cavity in cephalopods is located on the ventral side of the body. At the junction of the body and the head, it communicates with external environment transverse abdominal foramen. To close it, in most cephalopods, paired semilunar fossae are formed on the ventral side of the body. Opposite them, on the inner side of the mantle, lie two solid tubercles reinforced with cartilage, the so-called. cufflinks As a result of muscle contraction, the cufflinks fit into the semilunar recesses, tightly fastening the robe to the body. When the abdominal opening is open, water freely penetrates into the mantle cavity, washing the gills lying in it. After this, the mantle cavity closes and its muscles contract. The water is forcefully pushed out of the funnel lying between the two cufflinks, and the mollusk, receiving a reverse push, moves forward with the rear end of the body. This method of movement is called reactive.

In the vast majority of modern cephalopods, the shell is vestigial and hidden under the skin. Only nautiluses retain an external, spirally twisted shell, divided into internal chambers. In cuttlefish, the shell, as a rule, has the appearance of a large porous calcareous plate. Only Spirula retains a spirally twisted shell hidden under the skin. In squids, only a thin horny plate is retained from the shell, stretching along the dorsal side of the body. In octopuses, the shell is almost completely reduced and only small crystals of lime carbonate remain. Female argonauts (one of the species of octopuses) develop a special brood chamber, shaped very much like an external shell. However, this is only an apparent similarity, since it is secreted by the epithelium of the tentacles and is intended only to protect developing eggs.

All cephalopods are predators and feed on a variety of crustaceans and fish. They use tentacles to capture prey, and powerful horny jaws to kill. They are located in the muscular pharynx and resemble the beak of a parrot. 1 or 2 pairs of salivary glands open into the pharynx. Their secretion contains hydrolytic enzymes that break down polysaccharides and proteins. Often the secretions of the second pair of salivary glands are poisonous. The venom also helps immobilize and kill large prey. Immediately before the anus, the duct of the ink gland opens into the lumen of the hindgut. It secretes a dark secretion, a small amount of which can cloud a large amount of water. Cephalopods use it to escape from predators.

One of distinctive features cephalopods is the presence of an internal cartilaginous skeleton. Cartilage, similar in structure to the cartilage of vertebrates, surrounds the head cluster of ganglia, forming a cartilaginous capsule. Branches extend from it, reinforcing the eye openings and balance organs. In addition, supporting cartilages develop in the cufflinks, base of the tentacles and fins. All cephalopods are dioecious animals; Some of them have well-defined sexual dimorphism. Fertilization is external-internal and occurs not in the female’s reproductive tract, but in her mantle cavity. Some species take care of their offspring by carrying and protecting developing eggs. Development is direct.

About 650 modern species belonging to 2 subclasses: nautiloids and coleoids. There are much more extinct species - about 11 thousand. They belong to 3 subclasses: ammonites, belemnites and bactrites. Modern cephalopods are widespread in all seas (except desalinated ones). They live in the water column and at the bottom. Despite the fact that they are all predators, they often themselves serve as food for many fish and marine mammals. Some cephalopods are edible and are subject to commercial fishing.

84. Arthropods. Classification. Character traits organizations. Medical significance.

Aromorphoses of the Arthropod type:

1) exoskeleton;

2) jointed limbs;

3) striated muscles;

4) isolation and specialization of muscles.

The phylum Arthropods includes the subtypes Gill-breathing (the class Crustaceans is of medical importance), Cheliceraceae (Arachnida class) and Tracheine-breathing (Insect class).

In the Arachnida class, representatives of the orders Scorpions (Scorpiones), Spiders (Arachnei) and Ticks (Acari) are of medical importance.

Morphology

Arthropods are characterized by a three-layered body, that is, development from three germ layers. There is bilateral symmetry and heteronomous body segmentation (body segments have different structures and functions). The presence of metamerically arranged jointed limbs is characteristic. The body consists of segments that form three sections - head, chest and abdomen. Some species have a single cephalothorax, while in others all three sections merge. Jointed limbs work on the principle of a lever. There is an outer chitinous cover, which plays a protective role and is intended for muscle attachment (exoskeleton). Due to the inextensibility of the chitinized cuticle, the growth of arthropods is associated with molting. In higher crustaceans, chitin is impregnated with calcium salts, in insects - with proteins. The body cavity, myxocoel, is formed as a result of the fusion of the primary and secondary embryonic cavities.

Characterized by the presence of digestive, excretory, respiratory, circulatory, nervous, endocrine and reproductive systems.

The digestive system has three sections - anterior, middle and posterior. Ends with the anus. In the middle section there are complex digestive glands. The anterior and posterior sections have a cuticular lining. Characterized by the presence of a complex oral apparatus.

The excretory system is structured differently in different species. It is represented by modified metanephridia (green or coxal glands) or Malpighian vessels.

The structure of the respiratory organs depends on the environment where the animal lives. U water representatives- these are gills, in terrestrial species - sac-like lungs or tracheas. The gills and lungs are modified limbs, the tracheas are invaginations of the integument.

The circulatory system is not closed. On the dorsal side of the body there is a pulsating heart. Blood carries only nutrients, not oxygen.

The nervous system is built from the cephalic ganglion, peripharyngeal commissures and the ventral nerve cord of partially fused nerve ganglia. The largest ganglia - subpharyngeal and suprapharyngeal - are located at the anterior end of the body. The sense organs are well developed - smell, touch, taste, vision, hearing, balance organs.

There are endocrine glands, which, like the nervous system, play a regulatory role.

Most representatives of the type are dioecious. Sexual dimorphism is pronounced. Reproduction is only sexual. Development is direct or indirect, in the latter case - with complete or incomplete metamorphosis.

85. Crustaceans. Classification. Characteristic features of the organization. Medical significance

Structure

Dimensions - from 1 mm (planktonic forms) to 80 cm in length (leg span - up to 2 meters). The king crab (Alaskan king crab, or Paralithodes camtschatica) reaches a weight of 10 kg, and the Tasmanian giant crab (Pseudocarcinus gigas) - up to 14 kg.

The outer shell of crustaceans - the cuticle - contains chitin and calcium, which makes it incredibly durable, serves as an external skeleton and protects the body from external mechanical influences. As crustaceans grow, they are forced to change their shell, molt, replacing it with another, larger one.

The body of crustaceans consists of many segments, which are usually grouped into three tagmas: the head, thorax and abdomen.

Crustaceans differ from other arthropods by the presence of two pairs of head antennae, the so-called antenulla and antennae, which are sensory organs.

Distinctive features from other arthropods:

The respiratory organs are gills that develop on special outgrowths of the legs. Small representatives breathe across the entire surface.
On the head there are two pairs of antennae, simple and compound eyes, three pairs of jaws used for capturing and crushing food.
The body is segmented and divided into head, thorax and abdomen. Often the head merges with the chest, forming the cephalothorax.
The thoracic limbs are varied and perform various functions: motor, respiratory and others. Not all crustaceans have abdominal legs.
The limbs retain their primitive structure - they are usually bibranched. During the process of evolution, one of the branches is reduced and the limbs become single-branched. Around the world, about 1 million tons of crayfish and sea-dwelling shrimp and crabs are caught annually.

Lifestyle

Crustaceans mainly lead an aquatic lifestyle, with the exception of a number of amphibiotic forms from among decapod crustaceans and land woodlice (a specialized group of isopod crustaceans).

Reproduction

Crayfish are dioecious. Some crustaceans exhibit parthenogenetic (virgin, without fertilization) reproduction. Unfertilized eggs hatch into females, who in turn lay unfertilized eggs.

In many freshwater representatives of the cyprid family Cypridae, the males are generally unknown. In aquarium conditions, individuals of the species Herpetocypris reptans were kept for 30 years, during which time parthenogenetic reproduction occurred, and males never appeared. There are species that in the northern part of the distribution area reproduce parthenogenetically, and in the southern part both males and females are found and the process of fertilization is observed.

Classification

Shrimp Macrobrachium formosense Bate, 1868 from the class of higher crayfish

The crustacean subphylum is divided into 800 families (Martin, 1999) and six classes:

Branchiopoda

Cephalocarida

Higher crayfish (Malacostraca)

Jawfish (Maxillopoda) - includes barnacles (sea acorn, etc.)

Shellaceae (Ostracoda)

Remipedia

86. Arachnids. Classification. Characteristic features of the organization. Medical significance

Arachnids (lat. Arachnoidea or Arachnida) are a class of invertebrate animals such as arthropods. Typical representatives: spiders, scorpions, ticks.

Features Edit

There are four pairs of walking legs, which immediately distinguishes them from insects. Characteristic feature arachnids have a tendency to merge body segments to form the cephalothorax and abdomen.

The body in most cases consists of two sections, the cephalothorax and abdomen, less often it is not dissected at all (some mites, tardigrades).

The cephalothorax (Cephalothorax) is usually solid, less often divided into two segments, the head and thorax itself (in bichorks), sometimes it merges with the abdomen (in ticks), and is equipped with six pairs of limbs.

The abdomen is devoid of limbs and consists of rings clearly separated from each other or fused.

Veils Edit

In arachnids, they bear a relatively thin chitinous cuticle, under which lies the hypodermis and basement membrane. The cuticle protects the body from loss of moisture through evaporation, which is why arachnids inhabited the driest areas of the globe. The strength of the cuticle is given by proteins encrusting chitin.

Respiratory organs Edit

File:Arthro characters.jpeg

The main differences between arachnids and insects (on the left is a bed bug; on the right is a brown recluse spider): limbs (1), segmented body (2), cover (3)

The respiratory organs are the trachea (in bichorchs, false scorpions, harvestmen and some ticks) or the so-called pulmonary sacs (in scorpions and flagellates), sometimes both together (in spiders); lower arachnids do not have separate respiratory organs; these organs open outward on the underside of the abdomen, less often the cephalothorax, with one or several pairs of respiratory openings (stigma).

The lung sacs are more primitive structures. It is believed that they occurred as a result of modification of the abdominal limbs in the process of mastering the terrestrial lifestyle by the ancestors of arachnids, while the limb was pushed into the abdomen. The pulmonary sac in modern arachnids is a depression in the body; its walls form numerous leaf-shaped plates with large lacunae filled with hemolymph. Through the thin walls of the plates, gas exchange occurs between the hemolymph and air entering the pulmonary sac through the openings of the spiracles located on the abdomen. Pulmonary respiration is present in scorpions (four pairs of pulmonary sacs), flagipes (one or two pairs) and low-order spiders (one pair).

In false scorpions, harvestmen, salpugs and some ticks, the trachea serves as the respiratory organs, and in most spiders (except the most primitive) there are both lungs (one is preserved - the anterior pair) and trachea. Tracheas are thin branching (in harvestmen) or non-branching (in false scorpions and ticks) tubes. They penetrate the inside of the animal’s body and open outward with the openings of the stigmata on the first segments of the abdomen (in most forms) or on the first segment of the chest (in salpugs). The trachea is better adapted to air gas exchange than the lungs.

Some small ticks do not have specialized respiratory organs; in them, gas exchange occurs, like in primitive invertebrates, through the entire surface of the body.

Nervous system and sensory organsEdit

The nervous system of arachnids is characterized by a variety of structures. The general plan of its organization corresponds to the ventral nerve chain, but there are a number of features. There is no deuterocerebrum in the brain, which is associated with the reduction of acron appendages - antennules, which are innervated by this part of the brain in crustaceans, centipedes and insects. The anterior and posterior parts of the brain are preserved - the protocerebrum (innervates the eyes) and the tritocerebrum (innervates the chelicerae).

The ganglia of the ventral nerve cord are often concentrated, forming a more or less pronounced ganglion mass. In harvestmen and ticks, all the ganglia merge, forming a ring around the esophagus, but in scorpions a pronounced abdominal chain of ganglia is retained.

The sense organs of arachnids are developed differently. The sense of touch is of greatest importance to spiders. Numerous tactile hairs - trichobothria - are scattered in large numbers over the surface of the body, especially on the pedipalps and walking legs. Each hair is movably attached to the bottom of a special pit in the integument and connected to a group of sensitive cells that are located at its base. The hair perceives the slightest vibrations in the air or web, sensitively reacting to what is happening, while the spider is able to distinguish the nature of the irritating factor by the intensity of the vibrations.

The organs of the chemical sense are the lyre-shaped organs, which are 50-160 µm long slits in the integument, leading to a recess on the surface of the body where sensitive cells are located. Lyre-shaped organs are scattered throughout the body.

The visual organs of arachnids are simple eyes, the number of which varies from 2 to 12 in different species. In spiders, they are located on the cephalothorax shield in the form of two arcs, and in scorpions, one pair of eyes is located in front and several more pairs on the sides. Despite the significant number of eyes, arachnids have poor vision. At best, they are able to more or less clearly distinguish objects at a distance of no more than 30 cm, and most species - even less (for example, scorpions see only at a distance of several cm). For some vagrant species (for example, jumping spiders), vision is more important, since with its help the spider looks out for prey and distinguishes between individuals of the opposite sex.

Digestive and excretory systems Edit

The digestive system is adapted to feeding on semi-liquid foods.

The intestine consists of a narrow esophagus receiving the salivary glands, a stomach equipped with paired and unpaired processes, and a hindgut, usually with an enlarged cloaca, in front of which the excretory, so-called Malpighian glands, flow.

There are other excretory organs, the so-called coxal glands.

Genitals

Except for slow-moving insects, all arachnids are dioecious and in most cases exhibit pronounced sexual dimorphism.

The genital opening in both sexes is located at the base of the abdomen; most lay eggs, but some orders are viviparous (scorpions, bichorchi, flageopods).

Special authorities Edit

Some units have special bodies.

venom-carrying apparatus - scorpions and spiders

spinning apparatus - spiders and false scorpions.

Habitat

Arachnids are almost exclusively predators; only some mites (Oribatidae) feed on plant matter.

Arachnids play an important role in nature and human life, destroying flies, mosquitoes and other insects. At the same time, many cause harm to animals and humans (poisonous arachnids, ticks).