Features of the internal structure of arachnids table. The external structure of arachnids

excretory system . The excretory system is represented by the Malpighian vessels, which are a neoplasm in Arachnoidea, and the coxal glands, which correspond to the coelomoducts. Malpighian vessels - a pair of branching, blindly closed tubes at the ends, open at the border of the middle and posterior intestines.

They are of endodermal origin, that is, they belong to the middle intestine. Grains of guanine, the main excretory product of arachnids, accumulate in the epithelium and lumen of the Malpighian vessels. The coxal glands are formed by the sac-like part of mesodermal origin, the convoluted duct (labyrinth), the reservoir, and the external excretory duct. They are available in one or two pairs, open at the bases of the legs and rarely function in adult forms.

reproductive system. Arachnids have separate sexes. The sex glands are located in the abdomen and in the initial state of the pair. In some cases, there is a fusion of the right and left gonads. So, in a male scorpion, the testes are paired and each consists of two tubes connected by jumpers; in female scorpions, the ovary is one and consists of three tubes, of which the middle tube is obviously the result of the fusion of two medial tubes, similar to those of the male. In many spiders, harvestmen, and ticks, the paired gonads grow together at the ends into a ring. Paired oviducts and seminal ducts open with an unpaired genital opening, always on the second segment of the abdomen. The structure of the excretory part of the reproductive system and the copulatory adaptations of males are very diverse. Females usually have an extension of the oviducts - the uterus and seminal receptacles. In males, the copulatory organs are either associated with the genital opening, orserve as pedipalps (spiders) or chelicerae (some mites). In some cases, spermatophoric fertilization is carried out with the help of sperm packets.

Development. Most arachnids lay eggs, but there are also viviparous forms (scorpions, some ticks, etc.). Eggs are richyolk, due to which fragmentation is partial, superficial, all segments of the body and limbs are formed in embryonic development, and a small full-segment individual, similar to an adult, hatches from the egg. Post-embryonic development is direct, accompanied mainly by growth. Only in ticks, due to the small size of the eggs, a six-legged larva hatches and metamorphosis takes place. The study of the embryos of primitive arachnids allows us to better understand the structure of adults. So, in the embryo of scorpions, abdominal limbs are laid on all segments of the mesosome, from which the first pair then disappears, the second turns into genital covers, the third into ridge-shaped organs, and the remaining four pairs into lungs.

Representatives of arachnids are eight-legged land arthropods, in which the body is divided into two sections - the cephalothorax and abdomen, connected by a thin constriction or fused. Arachnids do not have antennae. Six pairs of limbs are located on the cephalothorax - two front pairs (mouth organs), which serve to capture and grind food, and four pairs of walking legs. There are no legs on the abdomen. Their respiratory organs are lungs and trachea. The eyes of arachnids are simple. Arachnids are dioecious animals. The class Arachnida includes more than 60 thousand species. The body length of various representatives of this class is from 0.1 mm to 17 cm. They are widely distributed around the globe. Most of them are land animals. Among ticks and spiders there are secondary water forms.

The biology of arachnids can be considered using the example of a spider-cross.

External structure and lifestyle. The cross-spider (so named for the cross-shaped pattern on the dorsal side of the body) can be found in the forest, garden, park, on the window frames of village houses and cottages. Most of the time, the spider sits in the center of its trapping web of sticky thread - cobwebs.

The body of the spider consists of two sections: a small elongated cephalothorax and a larger spherical abdomen (Fig. 90). The abdomen is separated from the cephalothorax by a narrow constriction. At the anterior end of the cephalothorax, there are four pairs of eyes above, and below, a pair of hook-shaped hard jaws - a chelicerae. With them, the spider grabs its prey. There is a canal inside the chelicerae. Through the channel, poison from the poisonous glands located at their base enters the body of the victim. Next to the chelicerae are short, covered with sensitive hairs, the organs of touch - the leg tentacles. Four pairs of walking legs are located on the sides of the cephalothorax. The body is covered with a light, strong and rather elastic chitinous cover. Like crayfish, spiders periodically molt, dropping their chitinous cover. At this time they are growing.

Rice. 90. The external structure of the spider: 1 - leg tentacle; 2 - leg; 3 - eye; 4 - cephalothorax; 5 - abdomen

At the lower end of the abdomen there are three pairs of arachnoid warts that produce cobwebs (Fig. 91) - these are modified abdominal legs.

Rice. 91. Trapping nets of various types of spiders (A) and the structure (with magnification) of the spider web (B)

The liquid released from the spider web warts instantly hardens in the air and turns into a strong spider web. Various parts of arachnoid warts secrete cobwebs different types. gossamer threads vary in thickness, strength, stickiness. The spider uses various types of webs to build a trapping web: at its base, the threads are stronger and not sticky, and the concentric threads are thinner and stickier. Spiders use the web to strengthen the walls of their shelters and to make cocoons for their eggs.

Digestive system the spider consists of a mouth, pharynx, esophagus, stomach, intestines (Fig. 92). In the midgut, long blind outgrowths increase its volume and absorption surface. Undigested residues are brought out through the anus. The cross spider cannot eat solid food. Having caught prey, such as some kind of insect, with the help of a web, he kills him with poison and lets digestive juices into his body. Under their influence, the contents of the caught insect liquefies, and the spider sucks it out. Only an empty chitinous shell remains from the victim. This type of digestion is called extraintestinal.

Rice. 92. The internal structure of the spider-cross: 1 - poison gland; 2 - mouth and esophagus; 3 - stomach; 4 - heart; 5 - lung sac; 6 "- sex gland; 7 - trachea; 8 - spider gland; 9 - intestine; 10 - Malpighian vessels; 11 - outgrowths of the intestine

Respiratory system. The respiratory organs of the spider are the lungs and trachea. Lungs, or lung bags, are located below, in front of the abdomen. These lungs evolved from the gills of distant ancestors of aquatic spiders. The spider-cross has two pairs of non-branching tracheas - long tubes that deliver oxygen to organs and tissues. They are located in the back of the abdomen.

Circulatory system spiders are open. The heart looks like a long tube located on the dorsal side of the abdomen. Blood vessels branch off from the heart.

In a spider, as in crustaceans, the body cavity is of a mixed nature - in the course of development it arises when the primary and secondary cavities of the forehead are connected. Hemolymph circulates in the body.

excretory system It is represented by two long tubes - Malpighian vessels.

With one end, the Malpighian vessels blindly end in the body of the spider, with the other they open into the posterior intestine. Through the walls of the Malpighian vessels, harmful waste products come out, which are then brought out. Water is absorbed in the intestines. In this way, spiders conserve water, so they can live in dry places.

Nervous system The spider consists of the cephalothoracic ganglion and numerous nerves extending from it.

Reproduction. Fertilization in spiders is internal. The male carries the spermatozoa into the female genital opening with the help of special outgrowths located on the front legs. The female, some time after fertilization, lays eggs, braids them with cobwebs and forms a cocoon (Fig. 93).

Rice. 93. Female spider with a cocoon (A) and the resettlement of spiders (B)

The eggs develop into small spiders. In autumn, they release cobwebs, and on them, like on parachutes, they are carried by the wind over long distances - spiders are resettled.

Variety of arachnids. In addition to the cross-spider, about 20 thousand more species belong to the order Spiders (Fig. 94). A significant number of spiders build trapping webs from the web. Y different web spiders differ in shape. So, in a house spider living in a person’s housing, the trapping net resembles a funnel, in a poisonous, deadly for humans karakurt, the trapping net resembles a rare hut. Among the spiders there are also those that do not build trapping webs. For example, side-walker spiders sit in ambush on flowers and wait for small insects arriving there. These spiders are usually brightly colored. Jumping spiders are able to jump and thus catch insects.

Rice. 94. Various spiders: 1 - cross-spider; 2 - karakurt; 3 - spider regiment; 4 - crab spider; 5 - tarantula

Wolf spiders roam everywhere looking for prey. And some spiders sit in minks in ambush and attack insects crawling nearby. These include a large spider that lives in southern Russia - a tarantula. The bites of this spider are painful for humans, but not fatal. The Haymakers include very long-legged arachnids (about 3,500 species) (Fig. 95, 2). Their cephalothorax is indistinctly separated from the abdomen, the chelicerae are weak (therefore, haymakers feed on small prey), the eyes are located in the form of a “turret” on top of the cephalothorax. Harvestmen are capable of self-mutilation: when a predator grabs a haymaker by the leg, he discards this limb, and he runs away. Moreover, the severed leg continues to bend and unbend - “mow”.

Scorpions are well represented in the subtropics and deserts by small animals 4-6 cm long (Fig. 95, 3). Large scorpions up to 15 cm long live in the tropics. The body of a scorpion, like that of a spider, consists of a cephalothorax and abdomen. The abdomen has a fixed and wide anterior part and a narrow, long movable posterior part. At the end of the abdomen there is a swelling (the poisonous gland is located there) with a sharp hook. With it, the scorpion kills its prey and defends itself from enemies. For a person, the injection of a large scorpion with a poisonous sting is very painful, and can lead to death. The chelicerae and tentacles of scorpions are claw-shaped. However, chelicerae claws are small, while leg tentacle claws are very large and resemble those of crayfish and crabs. In total, there are about 750 species of scorpions.

Rice. 95. Various representatives of arachnids: 1 - tick; 2 - haymaker; 3 - scorpion; 4 - phalanx

Ticks. There are more than 20 thousand species of ticks. The length of their body usually does not exceed 1 mm, very rarely - up to 5 mm (Fig. 95, 1 and 96).

Unlike other arachnids, ticks do not have a body divided into cephalothorax and abdomen. Ticks that feed on solid food (microscopic fungi, algae, etc.) have gnawing jaws, while those that feed on liquid food form a piercing-sucking proboscis. Ticks live in the soil, among fallen leaves, on plants, in water, and even in human homes. They feed on rotting plant debris, small fungi, algae, invertebrates, suck plant sap; in human living quarters, microscopic mites feed on dry organic residues contained in dust.

Rice. 96. Ixodid tick

The meaning of arachnids. Arachnids play a big role in nature. Known among them are both herbivores and predators that eat other animals. Arachnids, in turn, feed on many animals: predatory insects, birds, animals. Soil mites are involved in soil formation. Some ticks are carriers of serious diseases of animals and humans.

Arachnids are the first terrestrial arthropods that have mastered almost all habitat conditions. Their body consists of the cephalothorax and abdomen. They are well adapted to life in the ground-air environment: they have dense chitinous covers, they have pulmonary and tracheal breathing; save water, play important role in biocenoses, are important for humans.

Lesson learned exercises

What are the signs of the external structure of arachnids that distinguish them from other representatives of arthropods
Using the example of a spider-cross, tell about the methods of obtaining and digesting food. How are these processes related to internal organization animal?
Give a description of the structure and activity of the main organ systems, confirming the more complex organization of arachnids compared to annelids.
What is the importance of arachnids (spiders, ticks, scorpions) in nature and human life?

The arachnid class unites over 36,000 species of terrestrial chelicerae belonging to more than 10 orders.

Arachnida- higher chelicerate arthropods with 6 pairs of cephalothoracic limbs. They breathe through the lungs or trachea and, in addition to the coxal glands, have an excretory apparatus in the form of Malpighian vessels lying in the abdomen.

Structure and physiology. external morphology. The body of arachnids most often consists of a cephalothorax and abdomen. The acron and 7 segments are involved in the formation of the cephalothorax (the 7th segment is underdeveloped). Solpugs and some others lower forms only the segments of the 4 anterior pairs of limbs are soldered together, while the posterior 2 segments of the cephalothorax are free, followed by clearly demarcated segments of the abdomen. Thus, the salpugs have: the anterior part of the body, according to the segmental composition corresponding to the head of trilobites (acron + 4 segments), the so-called propeltidia; two free thoracic segments with legs and a segmented abdomen. Salpugs, therefore, belong to the arachnids with the most richly dissected body.

The next most dismembered detachment is scorpions, in which the cephalothorax is fused, but it is followed by a long 12-segment, like in Gigantostraca, the abdomen, subdivided into a wider anterior abdomen (of 7 segments) and a narrow posterior abdomen (of 5 segments). The body ends in a telson carrying a twisted poisonous needle. This is the same character of segmentation (only without dividing the abdomen into two sections) in representatives of the orders of flagellates, pseudo-scorpions, haymakers, in some ticks and in primitive arthropod spiders.

The next stage of fusion of the trunk segments is found by most spiders and some mites. They have not only the cephalothorax, but also the abdomen, which are continuous undivided parts of the body, but the spiders have a short and narrow stalk between them, formed by the 7th segment of the body. The maximum degree of fusion of body segments is observed in a number of representatives of the order of ticks, in which the whole body is whole, without borders between segments and without constrictions.

As already mentioned, the cephalothorax carries 6 pairs of limbs. The two front pairs are involved in the capture and crushing of food - these are chelicerae and pedipalps. Chelicerae are located in front of the mouth, most often in arachnids they are in the form of short claws (solpugs, scorpions, false scorpions, haymakers, some ticks, etc.). They usually consist of three segments, the terminal segment plays the role of a movable claw finger. More rarely, chelicerae end in a movable claw-like segment or have the appearance of two-segmented appendages with a pointed and serrated edge, with which ticks pierce the integument of animals.

The limbs of the second pair, pedipalps, consist of several segments. With the help of a chewing outgrowth on the main segment of the pedipalp, food is crushed and kneaded, while the other segments make up the genus of tentacles. In representatives of some orders (scorpions, false scorpions), pedipalps are turned into powerful long claws, in others they look like walking legs. The remaining 4 pairs of cephalothoracic limbs consist of 6-7 segments and play the role of walking legs. They end in claws.

In adult arachnids, the abdomen is devoid of typical limbs, although they undoubtedly descended from ancestors with well-developed legs on the anterior abdominal segments. In the embryos of many arachnids (scorpions, spiders), the rudiments of legs are laid on the abdomen, which only subsequently undergo regression. However, in the adult state, the abdominal legs are sometimes preserved, but in a modified form. So, in scorpions on the first segment of the abdomen there is a pair of genital opercula, under which the genital opening opens, on the second - a pair of comb organs, which are equipped with numerous nerve endings and play the role of tactile appendages. Both those and others represent modified limbs. The nature of the lung sacs located on the segments of the abdomen in scorpions, some spiders and pseudoscorpions is the same.

Spider web warts also originate from the limbs. On the lower surface of the abdomen in front of the powder, they have 2-3 pairs of tubercles, seated with hairs and carrying tube-like ducts of numerous arachnoid glands. The homology of these spider warts to the abdominal limbs is proved not only by their embryonic development, but also by their structure in some tropical spiders, in which the warts are especially strongly developed, they consist of several segments and even resemble legs in appearance.

Integuments of chelicerae They consist of the cuticle and the underlying layers: the hypodermal epithelium (hypoderm) and the basement membrane. The cuticle itself is a complex three-layer formation. Outside, there is a lipoprotein layer, which reliably protects the body from moisture loss during evaporation. This allowed the chelicerae to become a real land group and populate the most arid regions. the globe. The strength of the cuticle is given by proteins, tanned with phenols and encrusting chitin.

Derivatives of the skin epithelium are some glandular formations, including poisonous and spider glands. The first are characteristic of spiders, flagellates and scorpions; the second - to spiders, false scorpions and some ticks.

Digestive system in representatives of different orders of chelicerates varies greatly. The foregut usually forms an extension - a pharynx equipped with strong muscles, which serves as a pump that draws in semi-liquid food, since arachnids do not take solid food in pieces. A pair of small " salivary glands"In spiders, the secret of these glands and the liver is able to vigorously break down proteins. It is introduced into the body of the killed prey and brings its contents into a state of liquid slurry, which is then absorbed by the spider. The so-called extraintestinal digestion takes place here.

In most arachnids, the midgut forms long lateral protrusions that increase the capacity and absorptive surface of the intestine. So, in spiders, 5 pairs of blind glandular sacs go from the cephalothoracic part of the middle intestine to the bases of the limbs; similar protrusions are found in ticks, harvestmen and other arachnids. In the abdominal part of the middle intestine, the ducts of the paired digestive gland - the liver - open; it secretes digestive enzymes and serves for absorption nutrients. Intracellular digestion takes place in the liver cells.

excretory system arachnids compared to horseshoe crabs has a completely different character. At the border between the midgut and hindgut, steam opens into the alimentary canal for the most part branching malpighian vessels. Unlike Tracheata they are of endodermal origin, that is, they are formed at the expense of the midgut. Both in the cells and in the lumen of the Malpighian vessels there are numerous grains of guanine, the main excretory product of arachnids. Guanine, like uric acid excreted by insects, has low solubility and is removed from the body in the form of crystals. At the same time, moisture loss is minimal, which is important for animals that have switched to life on land.

In addition to the Malpighian vessels, arachnids also have typical coxal glands - paired sac-like formations of a mesodermal nature, lying in two (rarely in one) segments of the cephalothorax. They are well developed in embryos and in young age, but in adult animals they more or less atrophy. Fully formed coxal glands consist of a terminal epithelial sac, a looped convoluted canal, and a more direct excretory duct with a bladder and external opening. The terminal sac corresponds to the ciliary infundibulum of the coelomoduct, the opening of which is closed by the remainder of the coelomic epithelium. The coxal glands open at the base of the 3rd or 5th pair of limbs.

Nervous systemArachnida varied. Being connected in origin with the ventral nerve cord annelids, in arachnids it exhibits a distinct tendency towards concentration.

The brain has a complex structure. It consists of two sections: the anterior, which innervates the eyes, is the protocerebrum and the posterior is the tritocerebrum, which sends nerves to the first pair of limbs - the chelicerae. The intermediate part of the brain, the deutocerebrum, characteristic of other arthropods (crustaceans, insects), is absent in arachnids. This is due to the disappearance in them, like in the rest of the chelicerae, of the appendages of the acron - antennules, or antennae, which are innervated precisely from the deutocerebrum.

The metamerism of the ventral nerve cord is preserved most clearly in scorpions. In addition to the brain and near-pharyngeal connectives, they have a large ganglionic mass in the cephalothorax on the ventral side, giving nerves to the 2-6th pairs of limbs and 7 ganglia, throughout the abdominal part of the nerve chain. In salpugs, in addition to the complex cephalothoracic ganglion, one more node remains on the nerve chain, and in spiders, the entire chain has already merged into the cephalothoracic ganglion.

Finally, among harvestmen and ticks there is not even a clear distinction between the brain and the cephalothoracic ganglion, so that nervous system forms a continuous ganglionic ring around the esophagus.

sense organsArachnida varied. Mechanical, tactile stimuli, which are very important for arachnids, are perceived by differently arranged sensory hairs, which are especially numerous on the pedipalps. Special hairs - trichobothria, located on the pedipalps, legs and surface of the body, register air vibrations. The so-called lyre-shaped organs, which are small gaps in the cuticle, to the membranous bottom of which sensitive processes of nerve cells fit, are organs of chemical sense and serve for smell. The organs of vision are represented by simple eyes, which most arachnids have. They are located on the dorsal surface of the cephalothorax and usually there are several of them: 12, 8, 6, less often 2. Scorpions, for example, have a pair of median larger eyes and 2-5 pairs of lateral ones. Spiders most often have 8 eyes, usually arranged in two arcs, with the middle eyes of the anterior arc being larger than the others.

Scorpions recognize their own kind only at a distance of 2-3 cm, and some spiders - for 20-30 cm. In jumping spiders (family. Salticidae) vision plays a particularly important role: if males cover their eyes with opaque asphalt varnish, then they cease to distinguish between females and produce the “love dance” characteristic of the mating period.

Respiratory system Arachnids are varied. Some have lung sacs, others have tracheae, and others have both at the same time.

Only lung sacs are found in scorpions, flagellates, and primitive spiders. In scorpions, on the abdominal surface of the 3rd-6th segments of the anterior abdomen, there are 4 pairs of narrow slits - spiracles that lead to the lung sacs. Numerous leaf-like folds parallel to each other protrude into the cavity of the sac, between which narrow slit-like spaces remain, air penetrates into the latter through the respiratory gap, and hemolymph circulates in the lung leaflets. The flagellated and lower spiders have only two pairs of lung sacs.

In most other arachnids (solpugs, haymakers, false scorpions, some ticks), the respiratory organs are represented by tracheae. There are paired respiratory openings, or stigmata, on the 1st or 2nd segments of the abdomen (on the 1st thoracic segment in the salpugs). From each stigma, a bundle of long, thin air tubes of ectodermal origin, blindly closed at the ends, extends into the body (they form as deep protrusions of the outer epithelium). In false scorpions and ticks, these tubes, or tracheas, are simple and do not branch; in haymakers, they form side branches.

Finally, in the order of spiders, both types of respiratory organs are found together. The lower spiders, as already noted, have only lungs; among 2 pairs they are located on the lower side of the abdomen. The rest of the spiders retain only one anterior pair of lungs, and behind the latter there is a pair of tracheal bundles that open outwards with two stigmas. Finally, in one family of spiders ( Caponiidae) there are no lungs at all, and the only respiratory organs are 2 pairs of tracheas.

The lungs and trachea of arachnids arose independently of each other. The lung sacs are undoubtedly more ancient organs. It is believed that the development of the lungs in the process of evolution was associated with a modification of the abdominal gill limbs, which the aquatic ancestors of arachnids possessed and which were similar to the gill-bearing abdominal legs of horseshoe crabs. Each of these limbs retracted into the body. This created a cavity for the lung leaflets. The lateral edges of the stalk adhered to the body almost along its entire length, except for the area where the respiratory gap was preserved. The abdominal wall of the lung sac, therefore, corresponds to the former limb itself, the anterior section of this wall corresponds to the base of the leg, and the lung leaflets originated from the gill plates located on the back of the abdominal legs of the ancestors. This interpretation is confirmed by the development of lung sacs. The first folded rudiments of the lung plates appear on the posterior wall of the corresponding rudimentary legs before the limb deepens and turns into the lower wall of the lung.

The tracheae arose independently of them and later as organs more adapted to air breathing.

Some small arachnids, including some mites, have no respiratory organs, and breathing takes place through thin covers.

Circulatory system. In forms with clearly expressed metamerism (scorpions), the heart is a long tube lying in the anterior abdomen above the intestines and equipped with 7 pairs of slit-like awns on the sides. Other's arachnid structure the heart is more or less simplified: for example, in spiders it is somewhat shortened and carries only 3-4 pairs of ostia, while in haymakers the number of the latter is reduced to 2-1 pairs. Finally, ticks have a heart in best case turns into a short pouch with one pair of awns. In most ticks, due to their small size, the heart completely disappears.

From the anterior and posterior ends of the heart (scorpions) or only from the anterior (spiders) departs through the vessel - the anterior and posterior aorta. In addition, in a number of forms, a pair of lateral arteries departs from each chamber of the heart. The terminal branches of the arteries pour out the hemolymph into the system of lacunae, i.e., in the intervals between internal organs, from where it enters the pericardial portion of the body cavity, and then through the ostia to the heart. The hemolymph of arachnids contains a respiratory pigment, hemocyanin.

Sexual system. Arachnids have separate sexes. The sex glands lie in the abdomen and in the most primitive cases are paired. Very often, however, there is a partial fusion of the right and left gonads. Sometimes, in one sex, the gonads are still paired, while in the other, the fusion has already occurred. So, male scorpions have two testes (each of two tubes connected by jumpers), and females have one whole ovary, consisting of three longitudinal tubes connected by transverse adhesions. In spiders, in some cases, the gonads remain separate in both sexes, while in others, in the female, the posterior ends of the ovaries grow together, and a whole gonad is obtained. Paired genital ducts always depart from the gonads, which merge together at the anterior end of the abdomen and open outward through the genital opening, the latter in all arachnids lies on the first segment of the abdomen. Males have various additional glands, females often develop spermatheca.

Development. Instead of external fertilization, which was characteristic of the distant aquatic ancestors of arachnids, they developed internal fertilization, accompanied in primitive cases by spermatophoric insemination or, in more advanced forms, by copulation. The spermatophore is a sac secreted by the male, which contains a portion of seminal fluid, thus protected from drying out during exposure to air. In false scorpions and in many ticks, the male leaves the spermatophore on the ground, and the female captures it with the external genitalia. At the same time, both individuals perform a "nuptial dance" consisting of characteristic postures and movements. The males of many arachnids carry the spermatophore into the female genital opening with the help of chelicerae. Finally, some forms have copulatory organs, but no spermatophores. In some cases, parts of the body that are not directly connected with the reproductive system serve for copulation, for example, the modified end segments of the pedipalps in male spiders.

Most arachnids lay eggs. However, many scorpions, false scorpions, and some ticks have live births. Eggs are mostly large, rich in yolk.

Found in arachnids Various types crushing, however, in most cases surface crushing takes place. Later, due to the differentiation of the blastoderm, the germinal streak is formed. Its surface layer is formed by the ectoderm, the deeper layers are the mesoderm, and the deepest layer adjacent to the yolk is the endoderm. The rest of the embryo is dressed only in ectoderm. The formation of the body of the embryo occurs mainly due to the embryonic streak.

In further development, it should be noted that segmentation is more pronounced in embryos, and the body consists of more segments than in adult animals. So, in the embryos of spiders, the abdomen consists of 12 segments, similar to adult scorpions and scorpions, and there are rudiments of legs on 4-5 anterior segments. With further development, all abdominal segments merge, forming a whole abdomen. In scorpions, the limbs are laid on 6 segments of the anterior abdomen. The anterior pair of them gives genital caps, the second - comb organs, and the development of other pairs is associated with the formation of lungs. All this indicates that the class Arachnida descended from ancestors with rich segmentation and with limbs developed not only on the cephalothorax, but also on the abdomen (prone belly). Almost all arachnids have direct development, but mites have metamorphosis.

Literature: A. Dogel. Zoology of invertebrates. Edition 7, revised and enlarged. Moscow " graduate School", 1981

Respiratory system. The respiratory organs of the cross are a pair of leaf-folded lungs and tubular tracheae. The lungs are located at the base of the abdomen on the sides of the genital opening, where there are two transverse slits - stigmas of the lungs.

The stigma leads to the lung cavity, on the wall of which there are a number of flat pockets that diverge in a fan-like fashion. The pockets are connected with jumpers and do not fall off, so that air freely penetrates between them. Blood circulates in the cavities of the pockets, the exchange of gases occurs through their thin cuticular walls.

The tracheal system consists of two non-branching tubes, which are directed forward from a common pocket, which opens with an inconspicuous transverse slit in front of the arachnoid warts.

excretory system. There are two types of excretory organs: Malpighian vessels and coxal glands. In addition, the excretory function is performed by special cells (nephrocytes and guanocytes) lying in the body cavity. The Malpighian vessels are represented by four branching tubes blindly closed at the ends, which flow into the rectal bladder along its sides at the border of the middle and posterior intestines. Malpighian vessels are lined with squamous epithelium, in the cells of which grains of guanine, the main excretion product, are formed. The coxal glands, which in arachnids are the remains of the coelomoduct system, are located at the base of the first pair of legs. In an adult spider, they do not function.

poison glands. Poisonous glands are located in the anterior part of the cephalothorax at the base of the chelicerae. This is a pair of rather large cylindrical glands that enter the cavity of the main segments of the chelicerae. The outer lining of the gland is formed by a spirally curled ribbon-like muscle, during the contraction of which the poison is poured out through a thin duct that opens at the end of the claw-like segment of the chelicerae.

Spinning apparatus. The spinning apparatus is represented by three pairs of arachnoid warts and spider glands. At rest, spider warts, together with the anal tubercle, form a common closed group. At the tops of the warts there are numerous arachnoid tubes through which a secret is secreted - a web that hardens when it comes into contact with air. Spider glands fill the lower part of the female's abdominal cavity.

Their structure and size are not the same; distinguish tubular, ampulloidal, dendritic and pear-shaped glands. The latter are especially numerous and connected in bundles according to the number of warts (Plate X). The role of various glands and warts is different, the tubular glands secrete a web for the egg cocoon, the ampulloidal glands for building a network, the pear-shaped glands for braiding prey; arboreal secrete a sticky secret that covers the network.