Amphibians are totally aquatic during their larval stage and partially terrestrial animals as adults. Because of this, they are considered intermediate organisms in the evolutionary passage of vertebrates from an aquatic to dry land. Amphibians are also the first tetrapod animals; that is, the first with two pairs of limbs, a typical feature of terrestrial vertebrates. The name “amphibian” comes from the double life (aquatic as larvae and partially terrestrial as adults) of these animals.
Permeable skin, a body subject to dehydration, external fertilization, eggs without shells and a larval stage with branchial respiration are features of amphibians that make them dependent on water to survive.
In amphibians, embryonic development is indirect (there is a larval stage).
Among fish, gas exchange is carried out via the direct contact of water with the branchiae (gills). Gases enter and exit circulation through the gills.
In adult amphibians, gas exchange is carried out through their moist and permeable skin (cutaneous respiration) as well as through the lungs, a set of tiny airway terminations attached to highly vascularized tissue specialized in gas exchange.
The axolotl is an exotic amphibian found in Mexico that lives in water and “breathes” through gills even as an adult.
Amphibian larvae use exclusively branchial respiration. This is one of the reasons why they depend on water to survive.
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The fish heart only has two chambers, an atrium and a ventricle, and the blood that comes to it is purely venous.
In amphibians, the heart contains three chambers (a second atrium is present) and receives arterial blood from the lungs. In these animals, the heart has two atria (one that receives blood from the body and another that receives blood from the lungs) and one ventricle. Arterial blood mixes with venous blood within the ventricle, which in turn pumps the blood to the lungs and to the systemic circulation.
Adult amphibians have kidneys that filter blood. Nitrogen waste is excreted as urea (therefore, amphibians are ureotelic organisms). Their larvae are aquatic and excrete ammonia.
In the majority of species of amphibians, fertilization is external. This is also common among bony fishes and shows that the reproductive system and embryonic development of amphibians have been inherited from osteichthyes.
Curiously, although they use external fertilization, amphibian males and females copulate to stimulate the release of sperm and egg cells. This phenomenon is not classified as internal fertilization, since the gametes unite in water.
Eyelids attached to lacrimal glands protect and keep eyes lubricated against damage from the excessive light of terrestrial environments. Fish do not have eyelids because their eyes are in constant contact with water.
The main problems that vertebrates from an aquatic environment needed to solve to adapt to a terrestrial environment were the following: avoiding dehydration; eliminating waste in an environment where water is less available; protecting themselves against harmful solar radiation; the problem of gamete movement in the fertilization environment; the problem of gas exchange, which was previously carried out via the direct contact of water with gills; and the problem of supporting their body, since water played this role in fish.
Solutions for the dehydration problem: thicker, impermeable skin, allowing them to lose less water, or moist and permeable skin, like in amphibians. Solution for the excretion problem: excretion of urea (also excreted by chondrichthyes) or uric acid, substances that need less water to dissolve. Solutions for the problem of protecting themselves against radiation: skin pigments that filter harmful radiation, feathers, hair or shells. Solution for the gamete movement problem: internal fertilization (except for most amphibians, which have external fertilization). Solution for the gas exchange problem: appearance of airways and lungs. Solution for the body support problem: further development of muscle and bone structures, such as limbs and claws.
Examples of representative species: frogs, toads, salamanders. Basic morphology: two pairs of limbs, eyelids, hydrodynamic larvae. Skin: moist and permeable, mucous glands. Respiration: cutaneous and pulmonary, branchial during the larval stage. Circulation: closed, incomplete, heart with three chambers without an interventricular septum. Nitrogen waste: urea. Thermal control: heterothermic. Types of reproduction: sexual, water-dependent, external fertilization and aquatic larval stage.
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