76. General characteristics Annelids
The type of annelids, or ringworms, 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 division internal cavity the body with partitions into separate sections and segmental arrangement of a number of internal organs. Correctly repeated ganglia, ring blood vessels, excretory organs - metanephridia, midgut pouches and genitals. 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 circulatory system absent. 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 organic matter, close in chemical composition to chitin.
Most polychaetes are found mainly in coastal strip 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, the largest representative - an 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 abdominal 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 past 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 in ossia epithelial cells Numerous pigment cells are scattered, 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 organisms open into the pharynx salivary glands.
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. A sucking 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 coastal zone 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 formation 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 the second largest 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 mollusks whose body is divided into a head, torso 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. IN calm state of the animal, 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 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 has the form of 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). Ducts empty into the pharynx salivary glands. 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 (ponds, coils, 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. They move away from it arterial vessels, 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 that, to tell the truth, 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 areas 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 marine “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 harm human economic activity. 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 den 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 playing important role when moving. 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 inside The mantle contains two hard, cartilage-reinforced tubercles, 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. Characteristic features of the organization. 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 different types built differently. 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. In aquatic representatives these are gills, in terrestrial species they are 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 only carries nutrients, but 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 (barnacle, 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, millipedes 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. Highest value for spiders has a sense of touch. 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).
Annelids are a fairly large type of invertebrate (about 9,000 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 species are considered to be archiannelids, which 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 annelid has a body consisting of 3 parts: the body (consists of rings), the head and the anal lobe. The head of the ringlets houses various sensory organs. Most annelids have well-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.
In principle, each ring represents an independent particle of 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. Polychaete worms and some primary ringworms have them. 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 process of transporting nutrients through the body of the rings, secretions of the endocrine glands, oxygen and carbon dioxide occurs, which are directly involved in the respiration 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.
Annelids are the most highly organized type of worms. Includes from 12 thousand (according to old sources) to 18 thousand (according to new) species. According to the traditional classification, annelids include three classes: polychaetes, oligochaetes, and leeches. However, according to another classification, polychaetes are considered in the rank of class, and oligochaetes and leeches are included in the rank of subclasses in the class Zyaskovye; In addition to these groups, other classes and subclasses are also distinguished.
The body length of annelids, depending on the species, varies from a few millimeters to more than 5-6 meters.
During embryonic development, ectoderm, mesoderm and endoderm are formed. Therefore, they are classified as three-layered animals.
In the process of evolution, annelids have a secondary body cavity, i.e. they are secondary cavities. The secondary cavity is called in general. It forms inside the primary cavity, which remains in the form of lumens of blood vessels.
The coelom develops from the mesoderm. Unlike the primary cavity, the secondary cavity is lined with its own epithelium. In annelids, the whole is filled with fluid, which, among other things, performs the function of a hydroskeleton (supporting shape and support during movement). Coelomic fluid also transports nutrients, and metabolic products and germ cells are excreted through it.
The body of annelids consists of repeating segments (rings, segments). In other words, their body is segmented. There can be several or hundreds of segments. The body cavity is not single, but is divided into segments by transverse partitions (septa) of the epithelial lining of the coelom. In addition, two coelomic sacs (right and left) are formed in each ring. Their walls touch above and below the intestine and support the intestines. Between the walls there are also blood vessels and a nerve cord. Each segment has its own nodes nervous system(on the paired abdominal nerve trunk), excretory organs, gonads, external outgrowths.
The head lobe is called the prostomium. The back part of the worm's body is the anal lobe, or pygidium. The segmented body is called the torso.
The segmented body allows annelids to grow easily by forming new rings (this occurs posteriorly in front of the anal lobe).
The appearance of a segmented body is an evolutionary progress. However, annelids are characterized by homonomic segmentation, when all segments are approximately the same. In more highly organized animals, segmentation is heteronomous, when the segments and their functions are different. At the same time, in annelids, the formation of the head section of the body is observed by fusion of the anterior segments with a simultaneous increase in the cerebral ganglion. This is called cephalization.
The body walls, like those of lower worms, are formed by a skin-muscular sac. It consists of skin epithelium, a layer of circular and a layer of longitudinal muscles. Muscles achieve more powerful development.
Paired organs of movement emerged - parapodia. They are found only in polychaete annelids. They are outgrowths of a skin-muscular sac with tufts of bristles. In the more evolutionarily advanced group of oligochaetes, the parapodia disappear, leaving only the setae.
The digestive system consists of the foregut, midgut and hindgut. The walls of the intestine are formed by several layers of cells, they contain muscle cells, thanks to which food moves. The foregut is usually divided into the pharynx, esophagus, crop and gizzard. The mouth is located on the ventral side of the first body segment. The anus is located on the caudal blade. The process of absorption of nutrients into the blood occurs in the midgut, which has a fold on top to increase the absorption surface.
Characterized by a closed circulatory system. Previous types of worms (flat, round) did not have a circulatory system at all. As already mentioned, the lumen of blood vessels is the former primary cavity of the body, whose cavity fluid began to perform the functions of blood. The circulatory system of roundworms consists of a dorsal vessel (in which blood moves from the tail blade to the head), an abdominal vessel (blood moves from the head blade to the tail), half rings connecting the dorsal and abdominal vessels, small vessels extending to various bodies and fabrics. Each segment contains two half rings (on the left and on the right). The closed circulatory system means that blood flows only through the vessels.
Blood moves due to the pulsation of the walls of the spinal vessel. In some oligochaete worms, in addition to the dorsal one, some annular vessels contract.
Blood carries nutrients from their intestines and oxygen supplied through the integument of the body. The respiratory pigment, which reversibly binds oxygen, is found in the blood plasma and is not contained in special cells, as in vertebrates, for example, the pigment hemoglobin is found in red blood cells. Pigments in annelids can be different (hemoglobin, chlorocruarine, etc.), so the color of blood is not always red.
There are representatives of annelids that do not have a circulatory system (leeches), but in them it has been reduced, and a respiratory pigment is present in the tissue fluid.
Although annelids do not have respiratory system and usually breathe over the entire surface of the body, the transport of gases is carried out by the circulatory system, and not by diffusion through tissue fluid. In some marine species, primitive gills are formed on the parapodia, in which there are many small blood vessels located close to the surface.
The excretory organs are represented by metanephridia. These are tubes that have a funnel with cilia at the end located inside the body (in the coelom). On the other side, the tubes open outward through the surface of the body. Each annelid segment contains two metanephridia (right and left).
The nervous system is more developed compared to roundworms. In the head lobe, a pair of fused nodes (ganglia) form something like a brain. The ganglia are located on the peripharyngeal ring, from which the paired abdominal chain extends. It contains paired nerve ganglia in each body segment.
Sense organs of annelids: tactile cells or structures, a number of species have eyes, chemical sense organs (olfactory pits), and an organ of balance.
Most annelids are dioecious, but some are hermaphrodites. Development is direct (a small worm emerges from the egg) or with metamorphosis (a floating trochophore larva emerges; typical for polychaetes).
It is believed that annelids evolved from worms with an undifferentiated body, similar to ciliated worms (phylum flatworms). That is, in the process of evolution, two other groups of worms evolved from flatworms - round and annelid.
The body of which consists of repeating segments, or rings (hence their name - annelids).
General brief characteristics of annelids:
- there is a secondary body cavity (coelom);
- the body is covered on the outside with a cuticle secreted by the ectoderm.
- there is a circulatory system;
- the nervous system is represented by a paired suprapharyngeal node, connected by jumpers to the ventral nerve cord (usually double);
- excretory organs are located in each ring and are formed from ectoderm, they are equipped with cilia;
Structure
The elongated body of annelids seems to be composed of ring segments, the segments are separated by internal partitions; but they are not completely independent, since along the entire body there is a through intestine with oral and anal openings, an abdominal trunk of the nervous system and trunks of a closed circulatory system. These organ systems, piercing the septa one after another, stretch across the entire body of annelids. Each ring segment has a secondary body cavity (coelom). Most of the segments bear on the outside, on the right and on the left, two bunches of setae - organs of movement or attachment in tubes. In leeches, the bristles are lost for the second time.
Secondary body cavity (coelom)
The secondary body cavity (coelom) is of mesodermal origin. It is surrounded by a mesodermal membrane and filled with fluid. The cavity occupies the space between the walls of the body and the intestinal tube. The main part of the mesoderm lining the secondary cavity is the muscles that make up the body wall. They ensure the movement of the animal. In addition, the muscles of the intestinal wall, alternately contracting, push food through.
The secondary body cavity performs the following functions:
The secondary body cavity, to one degree or another, is a characteristic feature of all types of multicellular animals that follow in the evolutionary development, starting with annelids.
Classification
Annelids are a numerous type of worms that have a more complex body structure compared to flat and protocavitary worms. It is divided into three classes: Polychaetes, Girdles (including the subclasses Oligochaetes and Leeches), Mysostomidae.
Origin
According to a comparative study of the structure of worms, annelids evolved from primitive whole worms, similar to flat ciliated worms. Important evolutionary acquisitions of annelids are the secondary body cavity (coelom), the circulatory system, and the division of the body into separate rings (segments). Polychaete annelids are the ancestral group for other annelids. During the transition to freshwater and terrestrial lifestyles, oligochaete worms separated from them. Leeches evolved from oligochaete worms.
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Annelids– bilaterally symmetrical segmented animals.
Taxonomy. The phylum includes 5 classes, of which the most famous classes are Polychaeta - 13,000 species, Olygochaeta - 3,500 species and Leeches (Hirudinea) - about 400 species.
Body shape and size. The body of ringlets is overwhelmingly worm-shaped, round or oval in cross section. The body has pronounced both external and internal segmentation. In this case they talk about true metamerism. In this case, metamerism also extends to the internal structure of worms. In leeches, external segmentation does not correspond to internal segmentation.
The sizes of annelids range from a few millimeters to 2 m (terrestrial forms) and even up to 3 m (marine species).
External body structure. Polychaetes have a well-defined head section, bearing organs for various purposes: tentacles, ocelli, palps. In some species, the palps grow into a complex trapping apparatus. The last segment contains one or more pairs of sensory antennae. Each body segment bears parapodia on the sides - complex outgrowths of the body. The main function of these outgrowths is the movement of the worm. Each parapodia consists of two lobes, inside of which there are numerous setae. Of these, several are larger, they are called aciculi. A pair of sensitive antennae are attached to the blades. The parapodia often includes the gill apparatus. Parapodia have a fairly diverse structure.
In oligochaete worms, the head section is weakly expressed, and there are no lateral projections (parapodia). There are only relatively few setae. A “belt” consisting of thickened segments is clearly visible on the body.
Leeches have powerful suckers at the front and rear ends of their bodies. Few species have gill projections on the sides.
Skin-muscle bag. On the outside, the body of annelids is covered with a thin cuticle, under which lie skin epithelial cells. The skin of worms is rich in glandular cells. The secretion of these cells has a protective value. In a number of species, skin secretions are used to build unique houses. Worm bristles are derivatives of the epithelium. Under the skin lies a layer of circular muscles, which allows the animal to change the transverse size of the body. Below are the longitudinal muscles, which serve to change the length of the body. In leeches, between the layers of circular and longitudinal muscles there is a layer of diagonal muscles. The ringlets have special muscles that move parapodia, palps, suckers, etc.
Body cavity. The space between the body wall and the internal organs of the rings represents the coelom - the secondary body cavity. It differs from the primary one by the presence of its own epithelial walls, called coelomic epithelium (coelothelium). The coelothelium covers the longitudinal muscles of the body wall, intestines, muscle cords and other internal organs. On the walls of the intestine, the coelothelium is transformed into chloragogenic cells that perform an excretory function. In this case, the coelomic sac of each body segment is isolated from neighboring ones by partitions - dessepiments. Inside, the coelomic sac is filled with fluid containing various cellular elements. In general, it performs different functions - supporting, trophic, excretory, protective and others. In leeches, the coelom has undergone a strong reduction and the space between the body wall and the internal organs is filled with a special tissue - mesenchyme, in which the coelom is preserved only in the form of narrow canals.
The midgut is shaped like a simple tube that can become more complex. Thus, in leeches and some polychaetes, the intestines have lateral projections. In oligochaetes, on the dorsal side of the intestine there is a longitudinal fold that protrudes deeply into the intestinal cavity - typhlosol. These devices significantly increase the internal surface of the midgut, which allows for the most complete absorption of digested substances. The midgut is of endodermic origin. In oligochaete worms, at the border of the foregut and midgut there is an extension - the stomach. It can be either ectodermal or endodermal.
The hindgut, which is a derivative of the ectoderm, is usually short and opens into the anus.
The circulatory system of annelids is closed, that is, blood moves everywhere through the vessels. The main vessels are longitudinal - dorsal and abdominal, connected by circular ones. The spinal vessel has the ability to pulsate and performs the function of the heart. In oligochaetes, this function is also performed by the annular vessels of the anterior part of the body. Blood moves from back to front through the spinal vessel. Through the annular vessels located in each segment, the blood passes into the abdominal vessel and moves in it from front to back. Smaller vessels depart from the main vessels, and they in turn branch into tiny capillaries that carry blood to all the tissues of the worms. In leeches, the blood vessel system is significantly reduced. Blood moves through the system of sinuses - remnants of the coelom.
The blood of most annelids contains hemoglobin. This allows them to exist in conditions with little oxygen.
There are usually no special respiratory organs, so gas exchange occurs through the skin by diffusion. Polychaete worms and some leeches have well-developed gills.
The excretory system is most often represented by metanephridia, which are located metamerically, that is, in pairs in each segment. A typical metanephridium is represented by a long convoluted tube. This tube begins as a funnel, which opens into the whole (secondary body cavity) of the segment, then it penetrates the septum between the segments (dissepiment) and enters the glandular metanephridial body located in the next segment. In this gland, the tube twists strongly and then opens with an excretory pore on the lateral surface of the body. The funnel and tube are covered with cilia, with the help of which the cavity fluid is driven into the metanephridium. As it moves through the tube through the gland, water and various salts are absorbed from the liquid, and only products that need to be removed from the body (urine) remain in the cavity of the tube. These products are excreted through the excretory pore. In many species, in the posterior part of the metanephridial tube there is an extension - the bladder, in which urine temporarily accumulates.
In primitive annelids, the excretory organs, like flatworms, are structured like protonephridia.
The nervous system consists of the peripharyngeal ring and the ventral nerve cord. Above the pharynx lies a powerfully developed paired complex of ganglia, representing a kind of brain. A pair of ganglia also lies under the pharynx. The brain is connected to the subpharyngeal ganglia by nerve cords that cover the pharynx from the sides. This entire formation is called the peripharyngeal ring. Further, in each segment under the intestine there is a pair of nerve ganglia that are connected both to each other and to the ganglia of neighboring segments. This system is called the ventral nerve cord. Nerves extend from all ganglia to various organs.
Sense organs. On the head section of polychaete worms there are well-developed sense organs: antennae and palps (organs of touch), eyes (sometimes quite complex), olfactory pits. Some forms have developed balance organs - statocysts. On the lateral outgrowths of the body (parapodia) there are antennae that perform a tactile function.
In polychaete worms, the sensory organs are much less developed than in polychaete worms. There are chemical sense organs, sometimes tentacles, statocysts, and poorly developed eyes. The skin contains a large number of light-sensitive and tactile cells. Some tactile cells have a pin.
Leeches have many sensitive cells scattered throughout their skin; they also always have eyes and chemical sense organs (taste buds).
Reproductive system. Among annelids there are both hermaphroditic and dioecious forms.
Polychaete worms are mostly dioecious. Sometimes sexual dimorphism occurs. The sex glands (gonads) are formed in the coelomic epithelium. This process usually occurs in the posterior segments of the worm.
In oligochaete worms, hermaphroditism is more common. The gonads are usually located in certain segments of the anterior part of the worm. Relatively small male gonads (testes) have excretory ducts, which are either modified metanephridia or canals separated from them. The larger female gonads (ovaries) have ducts that are modified metanephridia. For example, when the ovary is located in the 13th segment, the female genital openings open on the 14th. There are also seminal receptacles, which are filled during mating with the sperm of another worm. Leeches are mostly hermaphrodites. The testes are located metamerically, there is one pair of ovaries. Fertilization in leeches occurs through the exchange of spermatophores between partners.
Reproduction. Annelids have a wide variety of forms of reproduction.
Asexual reproduction is characteristic of some polychaete and oligochaete worms. In this case, either strobilation or lateral budding occurs. This is a rare example of asexual reproduction among highly organized animals in general.
During sexual reproduction of polychaetes, individuals containing mature gonads (epitocenes) switch from a crawling or sessile lifestyle to a swimming one. And in some species, the sexual segments, when the gametes mature, can even tear off from the body of the worm and lead an independent swimming lifestyle. Gametes enter the water through breaks in the body wall. Fertilization occurs either in water or in the epitocine segments of the female.
Reproduction of oligochaetes begins with cross-fertilization. At this time, the two partners touch each other with their ventral sides and exchange sperm, which enters the seminal receptacles. After which the partners separate.
Subsequently, abundant mucus is secreted on the girdle, forming a muff around the girdle. The worm lays eggs in this muff. When the coupling is moved forward, it passes past the openings of the seminal receptacles; At this moment, fertilization of the eggs occurs. When the sleeve with fertilized eggs slides off the head end of the worm, its edges close, and a cocoon is obtained in which further development occurs. An earthworm cocoon usually contains 1-3 eggs.
In leeches, reproduction occurs in approximately the same way as in oligochaete worms. Leech cocoons are large, reaching 2 cm in length in some species. Different species have from 1 to 200 eggs in the cocoon.
Development. The zygote of annelids undergoes complete, usually uneven, fragmentation. Gastrulation occurs by intussusception or epiboly.
In polychaete worms, a larva called a trochophore is subsequently formed from the embryo. She has eyelashes and is quite mobile. From this larva the adult worm develops. Thus, in most polychaete worms, development occurs with metamorphosis. Species with direct development are also known.
Oligochaete worms have direct development without a larval phase. Fully formed young worms emerge from the eggs.
In leeches, the eggs in the cocoon form peculiar larvae that swim in the cocoon liquid using the ciliary apparatus. Thus, an adult leech is formed by metamorphosis.
Regeneration. Many annelids are characterized by a developed ability to regenerate lost body parts. In some species, an entire organism can regenerate from just a few segments. However, in leeches regeneration is very weakly expressed.
Nutrition. Among polychaete worms there are both predators and herbivorous species. There are also known facts of cannibalism. Some species feed on organic debris (detritivores). Oligochaete worms are primarily detritivores, but predators are also found.
Oligochaete worms are mostly soil dwellers. In soils rich in humus, the number of, for example, enchytraeid worms reaches 100-200 thousand per square meter. They also live in fresh, brackish and salt water bodies. Aquatic inhabitants inhabit mainly surface layers of soil and vegetation. Some species are cosmopolitan, but there are also endemics.
Leeches inhabit fresh water bodies. Few species live in the seas. Some switched to a terrestrial lifestyle. These worms either lead an ambush lifestyle or actively seek out their hosts. A single blood sucking provides leeches with food for many months. There are no cosmopolitans among leeches; they are confined to certain geographical areas.
Paleontological finds of annelids are very few. Polychaetes represent greater diversity in this regard. Not only prints have been preserved from them, but also, in many cases, remains of pipes. On this basis, it is assumed that all the main groups of this class were already represented in the Paleozoic. To date, no reliable remains of oligochaete worms and leeches have been found.
Origin. Currently, the most plausible hypothesis is the origin of annelids from parenchymal ancestors (ciliated worms). Polychaetes are considered to be the most primitive group. It is from this group that the oligochaetes most likely originate, and from the latter the group of leeches emerged.
Meaning: In nature, annelids have great value. Inhabiting various biotopes, these worms are included in numerous food chains, serving as food for a huge number of animals. Land worms play a leading role in soil formation. By processing plant residues, they enrich the soil with mineral and organic substances. Their passages help improve soil gas exchange and drainage.
In practical terms, a number of species of earthworms are used as vermicompost producers. The worm, enchytraeus, is used as food for aquarium fish. Enchitraeans are propagated in huge quantities. For the same purposes, the tubifex worm is harvested from nature. Medicinal leeches are currently used to treat certain diseases. In some tropical countries, palolo is eaten as food - the sexual (epitocene) segments of worms that have separated from the front of the animal and floated to the surface of the water.
