Amphibians, or amphibians, belong to the most primitive of all currently existing terrestrial vertebrates. They occupy an intermediate place between water and land animals for the reason that their reproduction and development proceeds in one environment (aquatic), and the residence of adults in another (on land). These are unusual and sometimes amazing creatures.
The article provides information on the origin of the Amphibian class, the general characteristics of the class, structure and systematics.
Background
The conditions necessary for the mass development of land by vertebrate animals formed in the middle of the Devonian period, approximately 385 million years ago. This happened due to the establishment of a warm and humid climate, the presence of a good food base (fauna from invertebrates). In addition, during this period, a lot of organic matter got into water bodies on Earth as a result of leaching, and this led to the oxidation of oxygen in water and, consequently, to a decrease in its concentration. Fish adapted to this by breathing atmospheric air.
Evolution: Briefly
The progenitors of the modern class Amphibians, the general characteristics of which are given later in the text, appeared at the end of the Devonian in fresh water bodies, they are called ichthyostegides. They were a transitional form between cysterae fish and real amphibians.
The transition from an aquatic lifestyle to a terrestrial one was accompanied by two crucial adaptive changes: movement along a solid substrate and breathing of atmospheric air. In other words, gradually gill breathing was replaced by pulmonary, and fins transformed into five-fingered limbs. At the same time, there was a transformation of all other organs: the nervous system and blood circulation, the sensory organs.
A second group of primitive amphibians, lepospondyls, appeared in Carboniferous. They were smaller and perfectly adapted to life in the aquatic environment, and some species lost limbs for the second time. Modern amphibians took shape only at the end of the Mesozoic period.
Taxonomy
Modern taxonomy has divided amphibians in a broad sense into three subclasses: Labyrinthodonts, Thin vertebrates, and Shellless. The first two groups of animals became extinct in the Early Mesozoic and Paleozoic, respectively. All modern amphibians, and this is more than 6,700 species, belong to the subcategory Unsanitary. It, in turn, is divided into three units that appeared in the Jurassic period.
- Tailless. Now 5602 species are represented, united in 48 families. All known frogs and toads belong to this order.
- Legless or worms (pictured above). The smallest squad, represented by 190 species, united in 10 families.
- Tailed amphibians. This order includes salamanders and newts, in total about 570 species (10 families).
Class Amphibians: general characteristics and structural features
A distinctive feature of amphibians is smooth and thin skin, which has a relatively good permeability to gases and liquids. Its structure has features characteristic of vertebrate animals. The skin itself (corium) and the superficial multilayer epidermis stand out. It is abundantly equipped with mucus-releasing glands. The secret can be of two types: poisonous and improves gas exchange. Horny formations or ossifications on the skin of amphibians are rare.
The body has a tail (in caudate), a trunk and a head (movable joint), as well as five-fingered limbs. The spine is divided into four sections: trunk, sacral, caudal and cervical. The number of vertebrae can be different: from 7 to 200.
Giving a brief general description to the amphibian class, one cannot fail to mention the muscles of these vertebrates: trunk and limbs. The first is segmented. Special muscles provide complex movements with lever limbs. On the head are the lever and lowering muscles. For example, the fire salamander, like other caudate representatives, has well developed tail muscles.
Respiratory system
Many probably remember some information about the structure of the respiratory system from the school biology course (they study the general characteristics of amphibians in grade 7).
The main respiratory organ in amphibians is the lungs. Most species have them, with the exception of lungless salamanders and the frog Kalimantan barbula. Lungs of small volume, have the appearance of thin-walled sacs, wrapped in a dense mesh of blood vessels. Each of them opens into the laryngeal-tracheal cavity with an independent opening. By increasing and decreasing the volume of the oropharyngeal cavity, breathing is carried out.
Additional respiratory organs are the mucous membrane and skin lining the oropharyngeal cavity. Some aquatic species as well as tadpoles breathe through the gills.
Circulatory system
Of course, the greatest attention when studying the general characteristics of amphibians in the 7th grade of the school was given to the circulatory system. Studying various groups of vertebrates, its evolution was traced from the most primitive to highly developed, characteristic of mammals.
Amphibians have a closed circulatory system with a three-chambered heart, mixing of blood occurs in the ventricle. The body temperature of amphibians depends on the environment, they belong to cold-blooded animals.
Amphibian circulatory system is characterized by two circles of blood circulation: small and large. The appearance of the first is due to the "acquisition" of breathing through the lungs. The heart is divided into two atria and one ventricle. Venous blood flows to the lungs and skin through the cutaneous pulmonary artery, and arterial blood goes to the head through the carotid artery. Mixed blood is brought to the rest of the body by aortic arches.
Digestive system
All representatives of the amphibian class, a general characteristic of which is given in the article, eat only mobile prey. The tongue is located at the bottom of the oropharyngeal cavity. In tailless species, it is attached to the lower jaw. The tongue is used to catch insects, it is thrown from the mouth, and prey sticks to its surface. The teeth on the jaws serve only to hold food.
The secret of the salivary glands, whose ducts open into the oropharyngeal cavity, does not contain digestive enzymes. Food enters the duodenum through the stomach. It is in it that the ducts of the pancreas and liver are open. The small intestine enters the rectum, forming an expansion called the cloaca.
Excretory system
The general characteristic of the class of amphibians, or amphibians, includes, among other things, information about the excretory system. It is represented by paired kidneys, departing from them and opening the ureters in the cloaca. There is a bladder in the cesspool, it is there that urine that gets into it accumulates. A very specific mechanism of fluid excretion. As soon as the bladder fills, its walls contract and the concentrated urine is discharged into the cesspool, which is then thrown out. This complexity is due to the need for amphibians to retain a lot of moisture. Part of the metabolic products and a large amount of water is released through the skin. These features of the body did not allow the amphibians to completely switch to a terrestrial way of life.
Nervous system
A progressive evolutionary trait is a greater brain weight compared to fish in all representatives of the amphibian class. General characteristics:
- the brain is characterized by the presence of five sections: middle, oblong, intermediate, anterior (with two hemispheres) and poorly developed cerebellum;
- brain weight as a percentage of total body weight is 0.29-0.36 in caudate amphibians and 0.50-0.73 in tailless amphibians versus 0.06-0.44% in cartilaginous fish;
- 10 pairs of brain nerves leave the brain in amphibians;
- sympathetic NS was represented by rather good development, represented mainly by two nerve trunks located on the sides of the spine;
- spinal nerves form well-defined lumbar and brachial plexus.
Amphibian sensory organs
The general characteristic of amphibians presented in the school course (grade 7, biology) provides superficial information about the sensory organs characteristic of them. The hearing organ received a new section in the process of evolution - the middle ear. The eardrum is connected to the stapes (auditory ossicle) and closes the external auditory opening. On both sides of the cavity, the middle ear cavity is connected using the auditory tube with the oropharyngeal cavity.
The structure of the eye is adapted to life in the air. And although the visual apparatus resembles a fish one, it is nevertheless distinct and does not have a reflective and silvery membrane, a crescent process. Higher amphibians have an upper and lower mobile eyelid. Blinking membrane is characteristic of lower amphibians. It performs a protective function instead of the lower eyelid. The shape of the cornea is convex, the lens is in the form of a biconvex lens, the diameter of which varies depending on the lighting. Many amphibians have color vision.
The skin of all representatives of the amphibian class, the general characteristics of which are presented in the text, performs the function of touch, since it contains numerous nerve endings. Aquatic species and tadpoles have a sideline.
The paired olfactory bags "work" exclusively in the air. They are lined from the inside by the olfactory epithelium and open outward by the nostrils, and by the choans - into the oropharyngeal cavity. The latter also has taste organs.
Reproductive system
Giving a general characterization to the amphibian class, reproduction and the life cycle of development cannot be overlooked either. All amphibians are dioecious animals, and fertilization in most representatives is carried out in the aquatic environment. Gonads are paired.
In females, granular ovaries filling almost the entire body cavity by spring are suspended on the mesentery. Next to it are fat bodies that accumulate nutrients and thereby ensure the formation of sexual products during hibernation. Oviducts are long and thin Müller channels. Each of them opens into the body cavity. Ripened eggs by rupture of the walls of the ovary enter it and through the oviducts, gradually, being covered by the mucous membrane of the protein membrane, enter the cloaca, from where they are released.
The males have paired testes of a rounded shape and adjoining fat bodies. Both of them are attached to the mesentery near the front edges of the kidneys. From the testicles depart the vas deferens, opening into the ureters, serving simultaneously as the vas deferens. The latter are displayed in the cesspool.
Life cycle
The life cycle of almost all amphibians has a clear division into four stages.
- Egg. In amphibians, as in fish, eggs do not have a waterproof shell and need constant hydration. The vast majority of species do masonry in fresh water, some amphibians (worms, giant salamanders, etc.) - on land, others - carry them on their bodies (midwife, Pipa Surinamese).
- Larva. She hatches from an egg and leads an aquatic lifestyle. In structure, the larvae are very similar to fish: the absence of paired limbs, gill breathing, a two-chamber heart and a single circle of blood circulation, the presence of lateral line organs. And only some representatives of amphibians skip this stage and the next, being born already in the form of small tailless frogs.
- Metamorphosis. Depending on the species, the transformation can occur rapidly, as in most tailless amphibians, or drag out in time, as for example, in primitive salamanders.
- Adult
Especially amphibian lifestyle
We present the final block of information about the amphibian class - the general description and characteristics of the habitat, lifestyle.
The vast majority of species are inhabitants of humid places, which alternate their stay on land with life in water. In addition, there are exclusively wood and water amphibians. Lack of good adaptability to life on land gives rise to drastic changes associated with the change of seasons. Amphibians under adverse environmental conditions hibernate for a long time. They are active only under warm conditions, most species die at -1 ° C. The life rhythm of some species changes throughout the day.
Most amphibians are inhabitants of fresh water bodies and only a small part can live in salt water, for example, sea toad (pictured below).
In the adult stage, all amphibians are predators. Their diet is made up of small animals, mainly invertebrates and insects, juvenile fish, in exceptional cases, chicks of waterfowl and even small rodents.