General characteristics of amphibians: internal structure, skeleton, development

Amphibians are quite a few representatives of the animal kingdom, because their species number only about 3 thousand. 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. All of them, of course, can not be listed, but the most interesting to focus on.

Skin covers

They should be mentioned first of all, speaking about the external signs of the class of amphibians. All members of this group have skin naked, without scales. But 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 horny. Take, for example, a toad. 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.

It is interesting that in terrestrial species, keratinization is more pronounced. Scrooge frogs, for example, have claws, as well as clawed newts. They are formed due to keratinization of the thickened skin at the fingertips.

And the legless representatives in the corium (dermis) have bone scales that are the remnants of the amphibian cover 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 the lower layers of the epidermis. The general characteristic of amphibians allows to know that the coloration of these creatures can perform several functions.

It promotes disguise, scaring or warning of rivals / predators, and sometimes helps to distinguish between sex. By the way, in males closer to the beginning of the mating season, the color becomes more saturated. This helps them to meet a sexually mature individual, and also stimulates mating.

And some amphibians are completely capable of changing color, depending on the background. In particular, a tree frog, also called a tree frog. They may differ in a brown-marsh shade or be bright, with contrasting spots.

Skeleton

It is also described in detail by the general characteristics of amphibians. The body of each representative of this class is divided into the head, trunk, tail and five-fingered limbs. It is in connection with the semi-terrestrial way of life that the skeleton of these creatures is divided into departments.

Cervical is represented only by the vertebrae. It consists of two articulated pits. Thanks to them, the skull is articulated with the vertebra.

The trunk vertebrae are larger. But their number varies, depending on the species. In tailless amphibians, they are usually around 7. In legless - about 100.

The sacral vertebra is distinguished by the presence of transverse processes. It is to these that the ileal pelvic bones are attached. True, in the legless sacral vertebra is absent, and the tail section is extremely weak in them. In tailless and even more so. At them it is presented by a small bone, called urostil. It is laid even at the time of embryo development in the form of a number of separate tiny vertebrae, which then fuse.

Brain skull

Telling about the skeleton of amphibians and their internal structure as a whole, it is necessary to tell about the specificity of the brain box. It remains largely cartilaginous throughout life. Basically, due to the poor development of overhead 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 cartilages.

The olfactory bone is located in front of the orbit. In the tail, it is, by the way, a steam room.

Interest is represented by scaly bones that are located on the sides of the back of the skull. They are very pronounced in the legless. The development of the skull base is also influenced by the specificity of the skeleton of amphibians. The fact is that the bottom is lined with a large parasphenoid, and in its superficial formation visceral bones (pterygoid and palatine) participate.

Jaws

About them, too, should be briefly mentioned. The upper jaws are identical in structure to fish. The lower ones are expressed by the mekkel cartilage, covered with angular and dental bones. In connection with this structure, the sublingual arc is not involved in attaching to the skull of the jaw apparatus.

Thus, if we draw conclusions, we can note five features that distinguish the comparative characteristics of amphibians and fish.

In amphibians, the chondral bones are poorly developed, there is autostylia (a combination of palatal-square cartilage with a cerebral skull), a cornification of a cover in some species, reduction of the operculum and modified hyoid arches.

Throat sac

About this amazing anatomical features of representatives of the order of the taillesses, too, it is worth discussing, 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 his help that males during the mating season enhance the sounds issued for the call of a sexually mature individual. So this bag is a kind of resonator-amplifier.

It looks rather elastic. The inner walls of the bag consist of a strong layer of mucous membrane and a leather cover, which in the usual state lies at the base of the oral cavity.

How does the sound form? Quite simply. The frog inhales air, after which it passes through the larynx at the moment of vocal chord oscillation.

Usually these creatures have only one bag. But the males of green frogs have two, and they are on either side of the mouth.

Circulation

Without a description of this topic, the general characteristics of amphibians can not be dispensed with either. The circulatory system of these creatures is closed, and the heart is three-chambered. Blood is mixed in the ventricle. The exception is made only by salamanders, who do not have 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. Skin and pulmonary promote the transport of venous blood to the skin and lungs. Through the arcs of the aorta, it is delivered to the organs of the body. And the sleepy aortas supply the head organs with arterial blood.

Talking about the anatomical specificity of these creatures, I would also like to mention that amphibians have an extremely low level of metabolism. All because the amphibian organs are supplied with mixed blood. This same feature causes their coolness.

The visual organs

Everything that concerns them, also considers biology. Amphibians with the help of sight receive the overwhelming majority of information about the external world. It plays a crucial role in obtaining food. Many amphibians have a pupil vertical, resembling a cat. They hunt at night. In the daytime, due to the specific structure of the pupils, they turn into a very narrow crack. So they are protected from exposure to sunlight.

But there are also day hunters. In such amphibians, the pupil is horizontal, and they catch the prey while they are above the water.

In general, the visual organs of these creatures enable them to recognize a variety of objects, analyze the hazard and respond to it. And protective reflexes, by the way, are as simple as possible. Seeing the predator, which in the future may attack, the frog immediately jumps to where it's darker, hoping to become less noticeable.

Even amphibians have a memory. It stores previously acquired knowledge about the signs of the surrounding world (about predators, food, etc.). That is why amphibious objects are distinguished, especially dangerous ones. The scientists managed to find out that the toads after a sting of a bee, a wasp or a bumblebee for a very long time are wary of all insects, somehow resembling their early "offender".

Smell

It is impossible not to leave this topic without attention, telling about the internal structure of amphibians. The olfactory organ is specific for them. The outer nares are closed and opened under the influence of special muscles, and the internal nostrils communicate with the oral cavity.

Also included in this system are olfactory bags, which give off a special secret that moistens them. The volume of each species is different. The largest bags of legless (cecilia) and tailless (frogs, toads).

It is interesting that the olfactory organ functions only in the air. If the amphibian is immersed in water, then its nostrils are closed.

Particularly interesting is the sense of smell of burrowing worms. They are able to recognize the smell of food, representatives of their own or someone else's species, as well as habitats. Amphibians have a highly sensitive sense of smell, which is exacerbated in the spring.

Life cycle

Now it is possible to consider in more detail such a theme as the development of amphibians. In the life cycle, biologists have identified four stages - the egg, the larva, the stage of metamorphosis, growing up.

So, the eggs of the amphibians do not have a shell, just like the fish. Because they require constant moisturization for development, adult individuals deposit them in freshwater reservoirs or on land, but near the source. Also there are amphibians who carry eggs on themselves. Copepods frogs attach them to the abdomen, for example, and the male of the Surinamese pip and at all press the fertilized caviar into the back of the female.

After a certain time, the larvae hatch. In structure, they resemble fish. Then a deep transformation takes place and the larva turns into an adult. This process for all proceeds in different ways. The tailless amphibians develop very fast, but the legless amphibians and salamanders grow up slowly.

Care for the offspring

Finally I would like to touch on this topic, talking about the development of amphibians. In some species, caring for the offspring is very interesting.

Take, for example, a bullfrog (Latin Lithobates catesbeianus). All the time, until the caviar ripens, the female guards it. While hatched larvae grow up - too. And the male at this time controls the water level in the drying puddles, where the offspring develop. If there is a need, it deepens them or digs a ditch into the next, where the tadpoles then overtake.

And the notorious tree frogs can even build a nest on a plant convenient for them, which will replace the larvae with reservoirs. Some and separate "sources" dig. All because these species of frogs live in the crowns of tropical forests, and there are big problems with finding water for breeding offspring. Therefore, leafworms (which are among the most poisonous creatures on the planet, by the way) lay their eggs on the leaves of trees. They guard it, and when the larvae are born, they carry them on the wet back to the microvagosomes, located in the axils of the leaves.

About amphibians, in fact, you can tell a lot more interesting things. However, more detailed information can be accessed individually, as there are a lot of it, despite the fact that the class, as it was said before, is considered small.