Gymnosperms got their name because... Why are gymnosperms more widespread than higher spore plants?

Let us consider the systems of internal organs of amphibians also using the example of the lake frog (Fig. 98). The digestive system of amphibians is more complex than the digestive system of fish. It begins with the oral cavity, at the bottom

To which the tongue is attached at the front end. (Consider how the way a frog's tongue is attached differs from the way a human's tongue is attached.) The main function of the tongue is to catch insects, not to push food through. The ducts of the salivary glands open into the oral cavity, producing saliva intended for moistening food. (Remember which other animals have salivary glands and what their purpose is.) The oral cavity opens into the esophagus, which opens into the stomach, where food digestion begins. The duodenum lies under the stomach and receives the bile duct of the liver and the pancreatic duct. The intestine forms several loops, passes into the rectum and ends in the cloaca. Water is absorbed in the intestines.

Oddly enough, some amphibians are helped to swallow food... by their eyes. Perhaps you have watched a frog roll its eyes in pain as it eats a fly. This is by no means a sign of sympathy for the victim, but the process of swallowing food.

The excretory system of amphibians is similar to the excretory system of fish. The bare skin of the lake frog does not prevent water from penetrating into the body, so excess water constantly accumulates in it. This excess water is removed by large body buds (fish also have similar ones). From the kidneys, urine is discharged through the ureters into the cloaca.

Amphibians living in the desert know how to save water. That is why urine is not produced in the South American quakia - phylomedusa. Harmful substances in it accumulate in the bladder in the form of solid crystals and dissolve when the animal enters the water.

Respiratory system. The frog's respiratory organs - the lungs - look like bags divided into cells. The lungs are penetrated by a branched network of blood vessels.

The inhalation mechanism is as follows: when the bottom of the oropharyngeal cavity drops, a rarefied space is created in it, and air enters through the open nostrils. Next, the nostrils are closed with special valves, and the bottom of the cavity rises, pushing air into the lungs.

Due to the insufficient development of the lungs in amphibians, skin respiration becomes of great importance. The amount of oxygen entering the body through the skin, and carbon dioxide also released through the skin, is half the total volume of gases necessary for the animal to breathe.

A pond frog, which was placed in water and thus deprived of the ability to breathe with its lungs, can live for three weeks, a grass frog - for a month. An ordinary newt once stayed underwater for seven months! He felt great and would have sat there for who knows how long, but the scientists were tired of watching him. A whole family of amphibians - bezlegenevi salamanders - do without lungs at all, and breathe only through the skin and using the surface of the oral cavity.

The circulatory system of amphibians is adapted to the pulmonary type of breathing. (Remember the structure of the perch’s heart.) This means that the lake frog has a three-chambered heart, consisting of two atria and a ventricle, and two circles of blood circulation allow partial separation of arterial and venous blood and better saturation of body tissues with oxygen. Mixed blood collects in the right atrium: venous, rich in carbon dioxide, from the internal organs, and arterial, rich in oxygen, from the skin. Only arterial blood enters the left atrium and is enriched with oxygen in the lungs. Both atria contract simultaneously, and blood from them enters the ventricle. The blood in the ventricle is mixed. Thanks to the presence of the heart valve, blood is distributed: arterial blood goes to the brain, venous blood goes to the lungs and skin, where it is saturated with oxygen and turns into arterial blood, and mixed blood goes to all organs. Due to the fact that in amphibians arterial and venous blood mix and oxygen saturation of tissues does not occur quickly enough, metabolism is slowed down. (Remember what metabolism is.) Therefore, the body temperature of amphibians, like fish, does not differ from the ambient temperature.

The nervous system of the lake frog is more complex than the nervous system of fish. The frog's more developed forebrain is divided into two large hemispheres, thanks to which the frog is a smarter creature than, for example, a perch.

Sense organs. Complications of the nervous system are directly related to the development of the sensory organs.

The frog's visual organs - the eyes - are well adapted to terrestrial conditions. She is able to see moving objects at a considerable distance, and also follow prey without turning her head and body.

The frog's hearing organs consist not only of the internal (like in fish), but also of the middle ear. The outer opening of the middle ear cavity is covered with the eardrum - a special elastic membrane. The middle ear includes the auditory ossicle - the stapes, which transmits signals from the eardrum to the inner ear.

The olfactory organs are represented by paired external and internal nostrils and are quite well developed. With their help, amphibians find their way to their native reservoir, covering a distance of more than a kilometer.

The organ of taste is the taste buds, which are located on the tongue. You can observe how the frog first grabs an inedible object with its tongue, and then gets rid of it by pushing it out of its mouth with its front paw.

Unlike fish, amphibians have a more complex structure of the nervous, digestive and circulatory systems, lungs, and better developed sense organs. However, the structure of the excretory system is similar to the excretory system of fish.

Terms and concepts: duodenum, bile duct, pancreas, rectum, oropharyngeal cavity, cutaneous respiration, cerebral hemispheres, middle ear, stirrup.

Check yourself. 1 . What is the structure of the lake frog's digestive system? 2. How does a frog breathe?

3. Why does a frog have pulmonary respiration and not suffocate under water?

4 . How does the circulatory system of a frog differ from the circulatory system of a fish, what is this difference? 5 . What blood goes to the left atrium and what blood goes to the right? 6. How is the perfection of the nervous system and sensory organs of amphibians manifested in comparison with the nervous system and sensory organs of fish?

How do you think? Why is the presence of two circulation circles and a three-chambered heart a progressive sign of the structure of the animal?

Class amphibians or amphibians

general characteristics

Amphibians or amphibians (lat. Amphibia) are a class of vertebrate four-legged animals, including, among others, newts, salamanders, frogs and caecilians - in total there are about 4,500 modern species, which makes this class relatively small.

The group of amphibians are among the most primitive terrestrial vertebrates, occupying an intermediate position between terrestrial and aquatic vertebrates: reproduction and development occur in the aquatic environment, and adult individuals live on land.

Skin

All amphibians have smooth, thin skin that is relatively easily permeable to liquids and gases. The structure of the skin is characteristic of vertebrates: a multilayered epidermis and the skin itself (corium) are distinguished. The skin is rich in cutaneous glands that secrete mucus. For some, mucus may be toxic or facilitate gas exchange. The skin is an additional organ of gas exchange and is equipped with a dense network of capillaries.

Horny formations are very rare, and ossifications of the skin are also rare: Ephippiger aurantiacus and the horned toad of the species Ceratophrys dorsata have a bony plate in the skin of the back, and legless amphibians have scales; Toads sometimes develop lime deposits in their skin when they get old.

Skeleton

The body is divided into a head, torso, tail (in caudates) and five-fingered limbs. The head is movable and connected to the body. The skeleton is divided into sections:

axial skeleton (spine);

head skeleton (skull);

skeleton of paired limbs.

The spine is divided into 4 sections: cervical, trunk, sacral and caudal. The number of vertebrae ranges from 10 in tailless amphibians to 200 in legless amphibians.

The cervical vertebra is movably attached to the occipital part of the skull (provides head mobility). Ribs are attached to the trunk vertebrae (except in tailless animals, which lack them). The only sacral vertebra is connected to the pelvic girdle. In tailless animals, the vertebrae of the caudal region are fused into one bone.

The flat and wide skull articulates with the spine using 2 condyles formed by the occipital bones.

The skeleton of the limbs is formed by the skeleton of the limb girdle and the skeleton of the free limbs. The shoulder girdle lies in the thickness of the muscles and includes paired shoulder blades, collarbones and crow bones connected to the sternum. The skeleton of the forelimb consists of the shoulder (humerus), forearm (radius and ulna) and hand (bones of the wrist, metacarpus and phalanges). The pelvic girdle consists of paired iliac ischial and pubic bones fused together. It is attached to the sacral vertebra through the ilia. The skeleton of the hind limb includes the thigh, tibia (tibia and fibula) and foot. Bones of the tarsus, metatarsus and phalanges of the fingers. In anurans, the bones of the forearm and tibia are fused. All bones of the hind limb are greatly elongated, forming powerful levers for mobile jumping.

Musculature

Musculature is divided into the muscles of the trunk and limbs. The trunk muscles are segmented. Groups of special muscles provide complex movements of lever limbs. The levator and depressor muscles are located on the head.

In a frog, for example, the muscles are best developed in the jaw area and the muscles of the limbs. Tailed Amphibians (fire salamanders) also have highly developed tail muscles.

Respiratory system

The respiratory organ of amphibians is:

lungs (special air breathing organs);

skin and mucous lining of the oropharyngeal cavity (additional respiratory organs);

gills (in some aquatic inhabitants and in tadpoles).

Most species (except for lungless salamanders) have small lungs, in the form of thin-walled sacs entwined with a dense network of blood vessels. Each lung opens with an independent opening into the laryngeal-tracheal cavity (the vocal cords are located here, opening a slit into the oropharyngeal cavity). Air is forced into the lungs by changing the volume of the oropharyngeal cavity: air enters the oropharyngeal cavity through the nostrils when its bottom is lowered. When the bottom rises, air is pushed into the lungs. In toads, adapted to living in more arid environments, the skin becomes keratinized, and respiration is carried out primarily through the lungs.

Circulatory organs

The circulatory system is closed, the heart is three-chambered with blood mixed in the ventricle (except for lungless salamanders, which have a two-chambered heart). Body temperature depends on the ambient temperature.

The circulatory system consists of the systemic and pulmonary circulations. The appearance of the second circle is associated with the acquisition of pulmonary breathing. The heart consists of two atria (in the right atrium the blood is mixed, mainly venous, and in the left - arterial) and one ventricle. Inside the walls of the ventricle, folds form that prevent the mixing of arterial and venous blood. An arterial cone, equipped with a spiral valve, emerges from the ventricle.

Arteries:

cutaneous pulmonary arteries (carry venous blood to the lungs and skin)

carotid arteries (the organs of the head are supplied with arterial blood)

The aortic arches carry mixed blood to the rest of the body.

The small circle is pulmonary, begins with the cutaneous pulmonary arteries, carrying blood to the respiratory organs (lungs and skin); From the lungs, oxygenated blood is collected in paired pulmonary veins, which flow into the left atrium.

The systemic circulation begins with the aortic arches and carotid arteries, which branch into organs and tissues. Venous blood enters the right atrium through the paired anterior vena cava and the unpaired posterior vena cava. In addition, oxidized blood from the skin enters the anterior vena cava and therefore the blood in the right atrium is mixed.

Due to the fact that the body's organs are supplied with mixed blood, amphibians have a low metabolic rate and therefore are cold-blooded animals.

Digestive organs

All amphibians feed only on mobile prey. The tongue is located at the bottom of the oropharyngeal cavity. In tailless animals, its front end is attached to the lower jaws; when catching insects, the tongue is thrown out of the mouth and the prey is attached to it. The jaws have teeth that serve only to hold prey. In frogs they are located only on the upper jaw.

The ducts of the salivary glands open into the oropharyngeal cavity, the secretion of which does not contain digestive enzymes. From the oropharyngeal cavity, food enters the stomach through the esophagus, and from there into the duodenum. The ducts of the liver and pancreas open here. Digestion of food occurs in the stomach and duodenum. The small intestine passes into the large intestine, ending in the rectum, which forms an extension - the cloaca.

Excretory organs

The excretory organs are paired trunk kidneys, from which ureters depart, opening into the cloaca. In the wall of the cloaca there is an opening of the bladder into which urine that enters the cloaca from the ureters flows. There is no reabsorption of water in the trunk kidneys. After filling the bladder and contracting the muscles of its walls, concentrated urine is discharged into the cloaca and thrown out. Some metabolic products and a large amount of moisture are released through the skin.

These features did not allow amphibians to completely transition to a terrestrial lifestyle.

Nervous system

Compared to fish, the brain weight of amphibians is greater. The weight of the brain as a percentage of body weight is 0.06-0.44% in modern cartilaginous fish, 0.02-0.94 in bony fish, 0.29-0.36 in tailed amphibians, and 0.50-0.5 in anurans. 0.73%

The brain consists of 5 sections:

the forebrain is relatively large; divided into 2 hemispheres; has large olfactory lobes;

the diencephalon is well developed;

the cerebellum is poorly developed;

the medulla oblongata is the center of the respiratory, circulatory and digestive systems;

the midbrain is relatively small.

Sense organs

The eyes are similar to the eyes of fish, but do not have silvery and reflective membranes, as well as a sickle-shaped process. Only Proteas have underdeveloped eyes. There are adaptations for functioning in the air. Higher amphibians have upper (leathery) and lower (transparent) movable eyelids. The nictitating membrane (instead of the lower eyelid in most anurans) performs a protective function. There are no lacrimal glands, but there is a Harderian gland, the secretion of which moistens the cornea and protects it from drying out. The cornea is convex. The lens has the shape of a biconvex lens, the diameter of which varies depending on the lighting; accommodation occurs due to changes in the distance of the lens to the retina. Many people have developed color vision.

The olfactory organs function only in the air and are represented by paired olfactory sacs. Their walls are lined with olfactory epithelium. They open outward with the nostrils, and into the oropharyngeal cavity with the choanae.

There is a new section in the organ of hearing - the middle ear. The external auditory opening is closed by the eardrum, connected to the auditory ossicle - the stapes. The stapes rests against the oval window, which leads into the cavity of the inner ear, transmitting vibrations of the eardrum to it. To equalize the pressure on both sides of the eardrum, the middle ear cavity is connected to the oropharyngeal cavity by the auditory tube.

The organ of touch is the skin, which contains tactile nerve endings. Aquatic representatives and tadpoles have lateral line organs.

Genitals

All amphibians are dioecious. In most amphibians, fertilization occurs externally (in water).

During the breeding season, the ovaries filled with mature eggs fill almost the entire abdominal cavity of females. Ripe eggs fall into the abdominal cavity of the body, enter the funnel of the oviduct and, after passing through it, are brought out through the cloaca.

Males have paired testes. The seminiferous tubules extending from them enter the ureters, which at the same time serve as vas deferens for the males. They also open into the cloaca.

Life cycle

In the life cycle of amphibians, four stages of development are clearly distinguished: egg, larva, metamorphosis, imago.

Amphibian eggs (eggs), like fish eggs, do not have a waterproof shell. For the egg to develop, it needs constant moisture. The vast majority of amphibians lay their eggs in fresh water bodies, but exceptions are known: caecilians, the amphium frog, giant salamanders, Alleghamian cryptobranchs and some other amphibians lay eggs on land. Even in these cases, the eggs need high environmental humidity, the provision of which falls on the parent. Species are known that carry eggs on their bodies: the female reticulated copefrog attaches them to her stomach, and the male midwife toads wrap the cord-like clutch around their hind legs. The care of the offspring of the Suriname pipa looks especially unusual - the fertilized eggs are pressed by the male into the back of the female and the latter carries it on herself until young pipas hatch from the eggs.

The eggs hatch into larvae that lead an aquatic lifestyle. In their structure, the larvae resemble fish: they lack paired limbs, breathe with gills (external, then internal); have a two-chamber heart and one circle of blood circulation, lateral line organs.

Undergoing metamorphosis, the larvae turn into adults leading a terrestrial lifestyle. The process of metamorphosis in tailless amphibians occurs rapidly, while in primitive salamanders and legless amphibians it is greatly extended over time.

Amphibians of some species take care of their offspring (toads, tree frogs).

Lifestyle

Most amphibians live in damp places, alternating between land and water, but there are some purely aquatic species, as well as species that live exclusively in trees. The insufficient adaptability of amphibians to live in the terrestrial environment causes sudden changes in their lifestyle due to seasonal changes in living conditions. Amphibians are capable of hibernating for a long time under unfavorable conditions (cold, drought, etc.). In some species, activity may change from nocturnal to diurnal as temperatures drop at night. Amphibians are active only in warm conditions. At a temperature of +7 - +8°C, most species fall into torpor, and at −2°C they die. But some amphibians are able to withstand prolonged freezing, drying out, and also regenerate significant lost parts of the body.

Amphibians cannot live in salt water, which is due to the hypotonicity of tissue solutions to sea water, as well as the high permeability of the skin. Therefore, they are absent from most oceanic islands, where conditions are generally favorable for them.

Nutrition

All modern amphibians in the imago stage are predators, feed on small animals (mainly insects and invertebrates), and are prone to cannibalism. There are no herbivores among amphibians due to their extremely sluggish metabolism. The diet of aquatic species may include juvenile fish, and the largest ones may prey on chicks of waterfowl and small rodents caught in the water.

The feeding pattern of the larvae of tailed amphibians is almost similar to the feeding of adult animals. The larvae of anurans are fundamentally different, feeding on plant food and detritus (a set of small (from several microns to several cm) undecomposed particles of plant and animal organisms or their secretions), switching to predation only at the end of the larval stage.

Reproduction

A common feature of the reproduction of almost all amphibians is their attachment during this period to water, where they lay eggs and where the larvae develop.

Amphibian poison

The most poisonous vertebrates on Earth belong to the order of amphibians - dart frogs. The poison, which is secreted by the skin glands of amphibians, contains substances that kill bacteria (bactericides). Most amphibians in Russia have poison that is completely harmless to humans. However, many tropical frogs are not so safe. The absolute “champion” in terms of toxicity among all creatures, including snakes, should be recognized as a resident of the tropical forests of Colombia - a tiny, only 2-3 cm in size, cocoa frog. Her skin mucus is terribly poisonous (contains batrachotoxin). Indians use the skin of cocoa to make poison for arrows. One frog is enough to poison 50 arrows. 2 mg of purified poison from another South American frog, the terrible frog, is enough to kill a person. Despite the terrible weapon, this frog has a mortal enemy - the small snake Leimadophis epinephelus, which feasts on young leaf climbers.

Amphibians and humans

Due to their vitality, amphibians are often used as laboratory animals.

The healing properties of amphibian venom are known. Powder from dried toad skins is used in China and Japan for dropsy, to improve heart function, for toothache and for bleeding gums. Relatively recently, in the tropical forests of South America, a tree frog was discovered that secretes substances that are 200 times more effective than morphine.

Classification

Modern representatives are represented by three groups:

Anurans (frogs, toads, tree frogs, etc.) - about 2100 species.

Tailed animals (salamanders, newts, etc.) - about 280 species.

Legless, the only family of caecilians - about 60 species.

Evolution

In evolutionary terms, amphibians descended from ancient lobe-finned fish and gave rise to representatives of the class reptiles. The most primitive order of amphibians are the tailed ones. Tailed amphibians are most similar to the most ancient representatives of the class. More specialized groups are the tailless amphibians and the legless amphibians.

There is still debate about the origin of amphibians, and according to the latest data, amphibians descend from ancient lobe-finned fish, specifically from the order Rhipidistia. In terms of the structure of the limbs and skull, these fish are close to fossil amphibians (stegocephalians), which are considered the ancestors of modern amphibians. The most archaic group is considered to be ichthyostegids, which retain a number of features characteristic of fish - a caudal fin, rudiments of gill covers, organs corresponding to the organs of the lateral line of fish.