Meaning and Definition of Amphibians
Amphibians occupy a level located between fish and reptiles , within the great zoological group of vertebrates . Although they depend on water to survive, in adulthood they need to seek land to breathe and hunt insects that constitute their food.
Characteristics of Amphibians
Amphibians form one of the seven classes of which vertebrates are composed. They have four limbs, or legs, that some have lost in the course of their evolution. Their body temperature varies with that of the environment (poikilotherms).
Therefore, when the ambient temperature drops, so does the amphibian, which goes into hibernation in the colder months. The amphibian embryo lacks a protective membrane that, in reptiles, birds, and mammals, forms a fluid-filled cavity.
The life cycle of these animals takes place in two environments, aquatic and terrestrial, and they are distributed throughout the world. Some have an external appearance similar to that of reptiles. Amphibians appeared about 280 million years ago, in the Devonian period.
The first beings that had amphibious characteristics were protected by external breastplates. Its apogee was between the Carboniferous and the Permian: the fossils found show the existence, at this stage, of some large species.
Skin and glands
The skin of these animals has no cover other than the dermal, except in the case of amphibians that lack limbs . It is a moist skin with a very fine texture, a vital characteristic, since amphibians breathe through it (cutaneous respiration).
In addition, it is covered with glands , mostly mucous, which lubricate it and give it its characteristic appearance: slimy and slippery. Amphibians also have wart-like poison glands that produce secretions that are irritating and toxic to other animals. Some species have two of these warts on their heads: they are called parotid glands.
The skin undergoes periodic changes, or seedlings. The color is very variable, from green, with its various shades, to red, passing through yellow, orange, white, etc. The variety of shades is due to the many pigment cells in the epidermis.
Adaptation to life on land caused amphibians to develop limbs with fingers, four at the front and five at the back, and imposed a series of modifications on the spine.
The most important are the strengthening of the pelvis and the appearance of a special vertebra in the neck, which favors the mobility of the head. The rest of the skeleton has several simplifications: the ribs are very rudimentary, and in the skull many bones are fused together and others are cartilaginous.
The need to move in the terrestrial environment caused the development of limb muscles.
As noted, skin respiration is of great importance in amphibians. A high percentage of the gaseous exchange of these animals with the environment takes place through this process. The larvae show gill respiration . In adults, the lungs appear bag-shaped, which have a variable degree of blood supply by blood vessels.
Circulation in adult amphibians is double, as they have a pulmonary circuit of vessels and another that runs through the rest of the body. However, it is incomplete, as there is no complete separation between arterial and venous blood , with some mixing of the two. The heart consists of three chambers: two atria and one ventricle.
Feeding of amphibians
In general, amphibians feed on insects , although larger species such as the American bullfrog even take fish and birds. The sticky tongue protrudes from the mouth to catch prey and retracts. They have small teeth. The rectum, the final part of the intestine, flows into a cloaca to which the bladder also joins. The liquid waste that is generated in the body is expelled by the kidneys and the urinary tract.
Amphibian Nervous System
The nervous system is relatively underdeveloped . The eyes are located on both sides of the head and the field of binocular vision is very limited. The one in which the images of the two eyes are superimposed, accurately determining the distances and reliefs.
The highly expandable pupil in some species appears as a vertical stripe, while in others it is often circular or heart-shaped.
Behind the eyes are the openings to the ears , with the tympanic membrane, through which sound vibrations are captured. Amphibians have, on the palate, a special olfactory organ , called Jacobson’s organ, with which they detect prey, which is highly developed in salamanders.
reproduction of amphibians
Amphibians almost always reproduce in the aquatic environment. In newts and salamanders, fertilization is internal: the male introduces the spermatophore, a kind of sperm sac, into the female’s body.
In frogs it is external. At the moment of the routine, the males of these amphibians emit noisy noises (“croaking”) through their vocal bags and form real choirs in which several individuals sing alternately. During mating they ride on the backs of the females, who are generally larger.
The couple remains united and motionless in a long embrace, which can last for hours, until the female expels the eggs, which are fertilized by the male’s sperm in the water.
The eggs are arranged in long rows or rows, surrounded by a gelatinous sheath, and settle to the bottom of stagnant water. All amphibians undergo metamorphosis. Therefore, the appearance of the larva is not the same as that of the adult, especially in the case of frogs and toads, in which it has a tail and is called a tadpole .
During their active season, amphibians remain hidden on the banks of the water courses they frequent or dive into rivers and streams. At regular intervals, they come out to breathe and remain crouched among the plants on the shore, waiting for their prey to pass by.
In the cold season they hibernate : their activity and metabolism slow down and they hide in holes or in the mud until the winter months are over. Sometimes, as is the case with salamanders, several individuals group together to spend the hibernation phase together.
Most amphibians are diurnal . Only a few species, such as frogs and salamanders, carry out their activities at night.
Ecology and Distribution
Amphibians are distributed throughout the world , except on the Antarctic continent, and live in close association with the aquatic environment. They do not resist salt water and therefore their habitat is limited to continental waters: lakes, swamps and ponds, marshes, rivers, etc.
Newts and salamanders inhabit high-altitude areas. Others, frogs like the toad Bufo alvarius, from the United States, populate arid and even desert regions . Certain frogs, such as tree frogs, are arboreal and have disc-shaped sticky pads on the tips of their fingers.
The class of amphibians is divided into three orders : that of anurans or batrachians, which do not have a tail and to which frogs and toads belong; that of the urodeles, with a tail and with the appearance of reptiles, which includes salamanders and newts; that of the legless, legless ones, in which earthworms and blind snakes are classified as worm-like amphibians.
The order Anurans includes amphibians that, as adults, are tailless and adapted for jumping, thanks to the length and strength of their hind legs. They have vocal sacs that allow them to make different sounds, which become characteristic during the mating season.
The order of urodeles consists of amphibians with tails, and salamanders, newts and proteus belong to it.
The fire salamander (Salamandra salamandra) is widely distributed throughout Eurasia and North Africa. It has a characteristic color, consisting of orange spots on a black background or black stripes on a yellow background.
You nickname them. The order of the apodas, or gymnophiona, is made up of those called caecilians . They are amphibians that lack limbs and have the appearance of small snakes.
Large eggs are deposited in cavities dug in moist soil. Some samples can reach a meter in length, as occurs with certain American caecilians.