Amphibians are very few representatives of the animal kingdom, because there are only about 3 thousand of their species. And in Russia there are only 28. This group is considered by biologists as the most primitive terrestrial vertebrates, but nevertheless, their general characteristic is of great interest. Amphibians are distinguished by a mass of amazing physiological and anatomical features. Of course, all of them cannot be listed, but it is worth focusing on the most interesting.
Skin integument
They should be mentioned first of all, speaking about the external characteristics of the amphibian class. All representatives of this group have skin naked, without scales. But on the other hand, the epidermis is saturated with multicellular glands, which ensure the presence of a thin liquid film on the body, which plays an important role in skin respiration and gas exchange.
True, in certain species that represent tailless amphibians, the upper layer is keratinized. Take, for example, toads. Their bodies are approximately 60% covered with a stratum corneum. But its presence does not prevent the penetration of moisture through the skin.
It is important to mention that the secret secreted by the glands of some amphibians often contains signaling, poisonous or irritating substances.
Interestingly, in terrestrial species keratinization is more pronounced. Spur frogs, for example, have claws, just like clawed newts. They are formed due to keratinization of thickened skin at the fingertips.
And the legless representatives in the corium (dermis) even have bone scales, which are the remnants of the cover of amphibians that existed in the Paleozoic era (542 million years ago).
Color
Color is also of interest. It is caused by pigment cells located in the corium and lower layers of the epidermis. The general characteristic of amphibians allows us to know that the coloring of these creatures can perform several functions.
It helps to disguise, scare or prevent rivals / predators, and sometimes helps to distinguish gender. By the way, in males, closer to the beginning of the mating season, the color becomes more saturated. This helps them to meet a mature individual, and also stimulates mating.
And some amphibians are even capable of changing color, depending on the background. In particular, a tree frog, also called tree frog. They may differ in a brown-swamp hue or be bright, with contrasting spots.
Skeleton
The general characteristic of amphibians also details this. The body of each representative of this class is divided into a head, trunk, tail and five-fingered limbs. It is in connection with a half-terrestrial way of life that the skeleton of these creatures is divided into departments.
The cervical is represented by only one vertebra. It consists of two articular fossae. Thanks to them, the skull is articulated with the vertebra.
There are more trunk vertebrae. But their number varies, depending on the species. In tailless amphibians, there are usually about 7. In legless amphibians, there are about 100.
The sacral vertebra is characterized by the presence of transverse processes. It is to them that the iliac pelvic bones are attached. True, the sacral vertebra is absent in the legless, and the caudal region is extremely weak. In tailless and even more so. They have it represented by a small bone called urostyle. It is laid even at the time of embryo development in the form of a whole series of individual tiny vertebrae, which then grow together.
Brain skull
Talking about the skeleton of amphibians and their internal structure as a whole, it is necessary to talk about the specifics of the brain box. It remains largely cartilaginous throughout life. Mainly due to poor development of the false and chondral bones.
The structure as a whole is quite simple. The occipital region is expressed by two lateral bones. Places of the main and upper lobes have approximately the same structure as fish. They are expressed by cartilage.
The olfactory bone is located in front of the orbit. By the tail, by the way, it’s a steam room.
Of interest are scaly bones, which are located on the sides of the back of the skull. They are pronounced in the legless. The specific character of the amphibian skeleton also affects the development of the bottom of the skull. The fact is that the bottom is lined with a large parasphenoid, and visceral bones (pterygoid and palatine) participate in its surface formation.
Jaws
They are also worth a brief mention. The upper jaw is identical in structure to fish. The lower ones are expressed by Meckel cartilage, covered by angular and dental bones. Due to this structure, the sublingual arch is not involved in the attachment of the jaw apparatus to the skull.
Thus, if we draw conclusions, we can note five features that distinguishes the comparative characteristics of amphibians and fish.
Amphibians have poorly developed chondral bones, autostilia (a combination of palatine-square cartilage with a brain skull), keratinization of some species, reduction of the gill cover and altered hyoid arches are observed.
Throat bag
It is also worth telling about this amazing anatomical feature of the representatives of the tailless squad, discussing the internal structure of amphibians.
The throat sac is a leather bubble in the form of a ball or balloon, which is formed at the base of the oral cavity of frogs. It is with its help that males during the mating season amplify the sounds made to call a mature individual. So this bag is a kind of resonator amplifier.
It looks pretty elastic. The inner walls of the bag consist of a strong layer of the mucous membrane and the skin, which in the usual state lies at the base of the oral cavity.
How is the sound made? Pretty simple. The frog inhales air, after which it passes through the larynx at the moment of vibration of the vocal cords.
Usually these creatures have only one bag. But the male green frogs have two of them, and they are on both sides of the mouth.
Circulation
A general description of amphibians cannot do without a description of this topic. The circulatory system of these creatures is closed, and the heart is three-chambered. Mixing of blood occurs in the ventricle. The only exception is salamanders, who have no lungs, and the heart consists of two chambers.
The circulation of amphibians is of particular interest, because it affects the temperature of the body, which, in turn, depends on what it is outside. These are cold-blooded creatures.
Arteries are divided into three types. Dermo-pulmonary contribute to the transport of venous blood to the skin and lungs. Through the arches of the aorta, it is delivered to the organs of the body. And the carotid aorta supplies the organs of the head with arterial blood.
Speaking about the anatomical specificity of these creatures, I would also like to mention that amphibians have an extremely low metabolic rate. That's because amphibian organs are supplied with mixed blood. The same feature determines their composure.
Visual organs
Everything that concerns them is also considered by biology. Amphibians with the help of vision receive the vast majority of information about the outside world. It plays a crucial role in obtaining food. In many amphibians, the pupil is vertical, resembling a cat. They hunt at night. In the daytime, due to the specific structure, the pupils turn into a very narrow crack. So they are protected from exposure to sunlight.
But there are day hunters. In such amphibians, the pupil is horizontal, and they watch the prey, being above the water.
In general, the visual organs of these creatures enable them to recognize a variety of objects, analyze the danger and respond to it. And protective reflexes, by the way, are as simple as possible. Seeing a predator that could potentially attack, the frog immediately jumps to where it is darker, hoping to become less noticeable.
Amphibians also have a memory. It stores previously acquired knowledge about the signs of the world (about predators, food, etc.). That is why amphibians distinguish objects, especially dangerous ones. Scientists managed to find out that toads after a bite of a bee, wasp or bumblebee for a very long time beware of all insects, at least somehow resembling their early “offender”.
Sense of smell
We can not ignore this topic, talking about the internal structure of amphibians. Their olfactory organ is specific. The external nostrils close and open under the influence of special muscles, and the internal communicate with the oral cavity.
Also included in this system are olfactory bags, which secrete a special secret that moistens them. The volume of them is different for each species. The largest bags are in the legless (cecilia) and tailless (tree frogs, toads).
Interestingly, the olfactory organ functions only in the air. If the amphibian is immersed in water, then its nostrils are closed.
Of particular interest is the organ of smell in digging worms. They are able to recognize the smell of food, representatives of their own or another's species, as well as habitat. Amphibians have a highly sensitive sense of smell, which is exacerbated in the spring.
Life cycle
Now you can consider in more detail such a topic as the development of amphibians. In the life cycle, biologists have identified four stages - the egg, the larva, the stage of metamorphosis, and maturation.
So, amphibian eggs do not have a shell, just like fish. Since they require constant hydration for development, adult individuals lay them in fresh water or on land, but close to the source. There are also amphibians carrying eggs on themselves. Their copepods are attached to the stomach, for example, and the male Pipa of Surinamese completely press the fertilized eggs into the back of the female.
After a certain time, larvae hatch. In structure, they resemble fish. Then there is a deep transformation of the body and the larva turns into an adult. This process is different for everyone. Tailless amphibians develop extremely quickly, but legless amphibians and salamanders grow up slowly.
Offspring Care
In the end, I would like to touch on this topic, telling about the development of amphibians. In some species, care for offspring is very interesting.
Take, for example, a bull frog (lat. Lithobates catesbeianus). All the time, while the caviar matures, the female protects her. While hatched larvae grow up - too. And the male at this time controls the water level in the drying puddles, where the offspring develops. If the need arises, he deepens them or digs a ditch in the adjacent one, where he then drives the tadpoles.
And the notorious tree frogs can even build a nest on a plant convenient for them, which will replace the larvae of water bodies. Some and individual "sources" are dug up. This is because these tree frog species live in the crowns of tropical forests, and there are big problems with finding water to breed offspring. Therefore, leaf climbers (which are one of the most poisonous creatures on the planet, by the way) lay eggs on the leaves of trees. They protect it, and when the larvae are born, they transfer them on a wet back to the microcurrents located in the axils of the leaves.
In fact, a lot of interesting things can be said about amphibians. However, more detailed information can be found on an individual basis, since there really is a lot of it, despite the fact that the class, as mentioned earlier, is considered small.