Amphibians are aquatic-terrestrial vertebrates i.e. they live as well on land as in water. They undergo metamorphosis and the larvae called tadpoles breath using gills; the adults use lungs.

Amphibians are different from fish because they have paired limbs with digits.  Their limbs do not support the body above the ground.

Amphibians are different from fish and reptiles in that they have no covering scales on their skin.

The skin serves the following functions:

Examples of amphibians include frogs and toads.

A toad showing typical characteristics of leathery and warty skin and brown coloration.

 

A toad

Toads are usually nocturnal.

Their body consists of a head and a trunk with no neck or tail which would interfere with the hopping.

They usually enter water only to breed.

In the dry season they enter small holes in the ground or under stones. 

Features on a toad:

The head bears a mouth with a very wide gape, a pair of nostrils and protruding eyes.

Only the lower eyelid can move slightly and it has a transparent margin called the nictitating membrane which can slide rapidly over the eye cleaning and moistening it without reducing vision.

The nostrils can be closed or opened by valves.

A toad has a tympanic membrane or ear drum which is a circular skin tightly stretched.  This is the sense organ for hearing.

There is a poison gland which offers protection against enemies.

The skin is dry and rough (warty) with irregular patches.  The toad has the ability to change colour according to the surrounding environment; so it can camouflage.

The hind limbs which are for hopping are about twice as long as the fore limbs which are for reducing shock when the toad lands after a hop.

There are five toes on the hind limb and they have a slight web between the bases of the toes.  This web provides a surface area for swimming.

The fore limbs have four toes and no web.

At the end of the body there is a cloaca for discharge of urine, faeces and gametes (egg and sperms).

A frog on a tree

Characteristics of Frogs

Adult frogs are characterized by long hind legs, a short body, webbed finger-like parts, and the lack of a tail. They also have a three-chambered heart, as do all tetrapods except birds and mammals. Most frogs live part of the time in water and part of the time on land, and move easily on land by jumping or climbing.

To become great jumpers, frogs evolved long hind legs and long ankle bones. They also have a short backbone with only 10 vertebrae. Frog and toad skin hangs loosely on the body, and skin texture can be smooth, warty, or folded.

In order to live on land and in water, frogs have three eyelid membranes: one is see-through to protect the eyes underwater, and two other ones let them see on land. Frogs also have a tympanum, which acts like a simple ear.

They are found on each side of the head. In some species, the tympanum is covered by skin.

Differences between frogs and toads:

The skin of frogs is slimy and smooth whereas that of toads is rough and dry.

The hind limbs of frogs are comparatively much longer than those of toads.

The web between the toes in the hind limbs is more extensive in the frog than in the toad.

So the frog is more effective in swimming than the toad.

 Frogs' eggs are laid in a mass whereas toads' eggs are laid in strands.

The frog can use the skin to breathe whereas the toad cannot.

Feeding In Amphibians:

Toads are carnivorous feeding on insects and worms which they may pick using the mouth.

Flying insects are caught by the tongue which is shot out at the insect.  The tip of the tongue is sticky so that it prevents escape of insects.

The prey is swallowed whole.

Reproduction in Amphibians:

Breeding occurs usually in the wet season.

Croaking of the male toad attracts and excites the female which is swollen with eggs.

The female carries the male for over two days; the female laying eggs and the male shedding sperms over them.  So the eggs are fertilized externaly.

The eggs are laid in a jelly which:

a)         Protects the eggs from injury.

b)        Prevents small water fleas from eating the eggs; and

c)         Separates the eggs from each other so that each has enough oxygen.

Pairing of toads ensures that fertilization takes place before eggs reach water where the jelly would become too thick.

Development:

The fertilized egg divides repeatedly forming an embryo, then the tadpole hatches out.

External gills for breathing appear and persist for four days.

The external gills are then covered by an operculum forming internal gills.

The tadpole then feeds actively and grows.

The hind limbs appear first as buds after 25 days and the fore limbs after 40 days.

The tail then disappears and the toad is fully developed after 45 days.

But all development for amphibians depends on temperature.  So in cold places a toad is fully formed after 70 days (10 weeks).  In warm places like in the tropics development takes 45 days only!

Aestivation:

In the dry season a toad finds a place, be it a hole, under a stone or some shelter with humid conditions, and it stays there without feeding.  This inactive state during the dry season is called aestivation and its aim is to reduce evaporation.

Aestivation is different from hibernation which takes place in the cold season with the aim of reducing the search for food required to keep temperature up when there is scarcity of food.  Hibernation takes place in the temperate regions especially in small mammals and birds.

BREATHING IN AMPHIBIANS:

Tadpoles:

These use external gills for the first five days.

After that an operculum covers the gills forming internal gills.

Water enters through the mouth and out through an opercular spout.

The gills extract oxygen from the water and carbon dioxide diffuses out of the gills into the water.

Toads and frogs:

These have two common methods of breathing i.e. the mouth (buccal method) and the lungs (pulmonary method).

The frog has a third method of using the skin (cutaneous method).

The Buccal Method:

To take in air into the mouth cavity:

So air rushes into the mouth cavity from the atmosphere through the nostrils.

Oxygen in the air diffuse into the many capillaries lining the mouth cavity.

Carbon dioxide diffuses out of blood capillaries into the air in the mouth cavity.

To send air out of the mouth cavity: 

• The mouth is closed and the nostrils are open.
• The glottis closes the entrance to the lungs.
• The floor of the mouth cavity is raised.
• The volume of the mouth cavity decreases.
• Pressure in the mouth cavity increases.
• So air rushes out of the mouth cavity to the atmosphere through the nostrils.

The Pulmonary Method:

To take air from the mouth cavity to the lungs:

• The mouth and the nostrils are closed.
• The floor of the mouth cavity is raised.
• The volume of the mouth cavity decreases.
• Pressure in the mouth cavity increases.
• The glottis opens the entrance to the lungs.
• Air rushes into the lungs from the mouth cavity.
• Oxygen in the air diffuses into the blood capillaries of the lungs.
• Carbon dioxide diffuses out of blood capillaries into the air in the lungs.

 

To take air out of the lungs into the mouth cavity:

• The mouth and the nostrils are closed.
• The glottis is open.
• The floor of the mouth cavity is lowered.
• The volume of the mouth cavity increases.
• Pressure in the mouth cavity decreases.
• So air rushes out of the lungs into the mouth cavity.
• Later the nostrils will open and the floor of the mouth cavity will be raised to push air out into the atmosphere.

The Cutaneous Method:

The skin of the toad is not moist and so it cannot use it for gaseous exchange.

The skin of the frog is moist and smooth and has many blood capillaries near the skin surface.

Oxygen in air or in water dissolves in the moisture lining the skin.

It then diffuses into the numerous capillaries beneath the skin.

Carbon dioxide in the blood capillaries diffuses out.

It then dissolves in the moisture lining and into the atmosphere or water.