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Amphibians are semi‑aquatic vertebrates that show adaptations for life in both of these environments. They occur on all continents except Antarctica. They live on land, e.g. in the soil, on trees or in fresh water. They never occur in seas and oceans.

Amphibians are cold‑blooded animals, which means that their body temperature depends on the temperature of the environment. In our climate, when it's getting colder, amphibians go into a state of numbness. They winter in clusters in different burrows, crevices and in the sludge at the bottom of water reservoirs. They become active again in early spring.

The body structure of amphibians

Due to the presence or absence of a tail, amphibians are divided into the Anura – frogs and toads, and Caudata – newts. In the body structure of Anura we can distinguish the head and trunk. The tail occurs in Caudata as well as larvae of Anura and Caudata. The national representatives of this division have got two pairs of limbs equipped with fingers facilitating moving on land. If they are connected by means of interdigital webbinginterdigital webbinginterdigital webbing, they facilitate swimming. Limbs are widely spaced to the sides, which makes the trunk stay in contact with the ground and gives the movements of most amphibians characteristic slowness and clumsiness. Nevertheless, they are capable of quickly escaping in an emergency.

All amphibians have naked, scaleless, thin skin. It is highly vascularised and moist, which significantly facilitates gas exchange. The mucus layer protects the amphibians from drying on land and makes it easier to move in water, reducing friction. The toad's skin is also equipped with venom glands. Their secretion protects these animals against attacks by predators, causing, for example, irritation of the mouth and hypersalivation. The pigment cells present in the skin allow the amphibians to adopt masking or warning colouring.

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Ilustracja przedstawia budowę wewnętrzną żaby. Na ciemnoniebieskiej sylwetce płaza wrysowano układy narządów. Różowy pęcherzyk w głowie, wydłużony na tułów to mózg i rdzeń kręgowy. Żółtym kolorem oznaczono części kostne: czaszkę (Skull), kręgosłup (Spinal cord) i kości kończyn. Kolor pomarańczowy oznacza układ pokarmowy: otwór gębowy (Mouth cavity), żołądek (Stomach), jelito (intestine). Błękitny pęcherzyk oznacza płuco (lung), a czerwona kulka serce (heart).

Observation 1

Describing the adaptations in the frogs' external body structure for life in water and on land.

You will need

• African dwarf clawed frog (Hymenochirus boettgeri),

• a tank with an area for swimming and being out of the water,

• a net for fishing out a frog,

• magnifying glass.

• a high box.

Instruction

1. Look at the frog. Observe and determine: how it moves in water and on the solid ground, the shape of its body, the location of its eyes, whether there is an interdigital webbing between its fingers.

2. Determine, which features facilitate life on land and which in water.

3. Fish the frog out of the tank, place it in the box. Carefully touch the surface of its skin. Observe its appearance through the magnifying glass. Explain the significance of the skin when the animal is on land and in water.

Summary

When describing the adaptations in the frog's body structure to the environment, we determine the shape of a given structure element, and we think how its shape (or its presence) facilitates life in water. Sometimes, to determine such a relationship, it is necessary to analyse the physical features of water and their effect on the body structure of an amphibian.

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Fotografia przedstawia zbliżenie oliwkowej żaby z boku, pyskiem w prawo. Żaba trawna siedzi na podłożu z brunatnych liści i gałązek. Na głowie duże, wypukłe, okrągłe oczy ze złotawymi tęczówkami. Szpara źrenicy czarna, poziomo owalna. Przez pysk, oko aż do przednich łap brązowa plama. Ciało krępe, kończyny przyciągnięte do tułowia. Brzuch jasny. Na tylnych łapach ciemniejsze, poprzeczne pasy.

Amphibian respiration

Larvae of amphibians take oxygen from water using gills. In adult individuals, the skin and lungslungslungs are involved in the gas exchange process. The air is pressed into the lungs by rhythmical depressing and elevating of the floor of the oral cavity.

Due to the fact that the amphibian skin is thin and porous, gas molecules and water can freely penetrate it. Strong blood flow in the skin makes the gas exchange occur on its entire surface. Oxygen taken through the skin into the blood is transferred to all tissues, and the resulting carbon dioxide is transported to the vessels underneath the skin and removed from the body. The mucus covering the body facilitates efficient gas exchange. This ensures constant skin moisture and facilitates the penetration of gas molecules that dissolve in water.

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Ilustracja przedstawia w powiększeniu przekrój przez skórę płaza. Z lewej u góry sylwetka zielonoszarej rzekotki drzewnej i oznaczenie miejsca powiększenia. W kółku pod zieloną warstwą wskazane żółte, kropkowane komórki naskórka. Pod nimi w bladoróżowej skórze wrysowane faliste rurki, obrazujące naczynia krwionośne: różowa tętnica i niebieska żyła. Łączą się one siatką naczyń włosowatych. Strzałka różowa z napisem O dwa pokazuje wnikanie tlenu przez skórę do naczyń włosowatych. Strzałka niebieska i napis CO dwa ilustruje usuwanie dwutlenku węgla przez skórę płaza. Na ilustracji są podpisy: Network of capillaries, Artery ,Vein, Epidermis, Skin,

Reproduction of amphibians

Amphibians are dioecious animals the reproduction and development of which takes place in water. During mating season (occurring in Poland from March to June) most amphibians embark on a journey to the nearest reservoir.

The female amphibian lays eggs, the so‑called spawnspawnspawn, in the water, which are surrounded by jelly‑like substance. The male, joined with the female in a mating position, covers them with sperm. The fertilisation takes place in water. After some time, the larvae, called tadpoletadpoletadpole, hatch from the fertilised eggs. They differ from adult forms: they are herbivorous, they use gills to breathe, they resemble fish in shape – they have a barrel‑shaped trunk with a head barely separated from the body. In the early stages of development, they have got no limbs, however, they have got a long tail with a skin fold acting as a fin. Tadpoles, similarly to fish, are equipped with a lateral line organ. During approximately 16 weeks of life in the water, they undergo a metamorphosismetamorphosismetamorphosis, during which the limbs develop and the tail disappears. Internal organs are also restructured, e.g. the gills disappear and the lungs develop. After transforming into an adult form, most of the anurous amphibians leave the aquatic environment.

Observation 2

Learning about the body structure of a tadpole and indicating embryos in eggs.

You will need

• a tadpole in a tank,

• spawn,

• magnifying glass.

Instruction

1. Using the magnifying glass, look at the body structure of a tadpole. Indicate body structure features, which are evidence of adaptations for life in water.

2. Look at the spawn with the magnifying glass. Look at the embryos in the eggs.

Summary

Tadpoles have got a multitude of adaptations for life in water. During its stay in the aquatic environment many changes in its body structure occur that make it more and more similar to an adult form.

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Fotografia przedstawia w dużym zbliżeniu skrzek na liściu. W sklejonych, przezroczystych kulach znajdują się pomarańczowe zarodki z czarnymi oczami. Prześwitują też czerwone naczynia krwionośne.

Task 5

Analyse the map showing the number of amphibian species in the world and then answer the following questions.

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Ilustracja przedstawia mapę świata. Na sylwetkach kontynentów kolorami oznaczono liczbę gatunków płazów zgodnie z legendą po lewej stronie. Schematyczne „buźki” płazów mają uśmiech tym większy, im więcej gatunków symbolizują. Od góry barwy symboli wskazują liczebność gatunków: od czerwonej (najwięcej) przez pomarańczową i żółtą, zielone, niebieskie, granatową do białej (brak płazów).

Which continent is home to most amphibian species?
Why is there more amphibian species in Brazil than in Poland, Canada or Australia?

Conclusion

• The skin of amphibians is naked, thin, delicate and covered with mucus, that is why, similarly to lungs, it is used to take oxygen and remove carbon dioxide.

• Amphibians are cold‑blooded vertebrates whose adult forms usually live in humid terrestrial habitats and who spend their breeding time in freshwater reservoirs.

• The development of amphibians involves metamorphosis. A larval form known as a tadpole occurs which breaths using gills.

Keywords

frog, amphibians, masking colouring (mimicry)

Glossary