THE TOAD

Bufo regularis
Toads and frogs belong to class Amphibia and can therefore live in water and on land.

Habitat: They live in stagnant water, pool of water, freshwater pond. streams, and rivers, under logs of wood, stones or litter and in ditches containing water.


External features of toad The body consists of a head and trunk. Toads and frogs have no neck. The absence of a neck enables water to flow over the body more easily and also allows the toad or frog to hop more efficiently. The body is streamlined and covered with a moist, loose-fitting, warty skin, which is highly vascularised and used for gaseous exchange. The streamlined body shape facilitates easy swimming in water. It is brown or greyish brown in colour for concealment.

Bufo regularis

Head: The head of the toad is wedge-shaped. it bears a terminal wide-gaped mouth, which enables the toad to swallow its prey or food whole. Attached to the front of the floor of the buccal cavity is a long. fleshy tongue with a sticky tip, used for trapping or catching prey It extends backwards into the buccal cavity with its sticky tip lying closing to the throat. It is suddenly nicked out when trapping or catching a prey. Along the edge of the upper jaw and on the roof of the buccal cavity are small, fine vomerine or homodont teeth. The teeth are not used for chewing food but for holding firmly onto preys to prevent them from escaping from the mouth. Above the mouth and anterior to the eyes, is a pair of nostrils also called external nares, which are used for gaseous exchange. The nostrils open in front of the mouth cavity as small openings called internal nares. At the inner end of the external nares are special valves which can be used to close them. Behind the nostrils are two large, spherical, bulging eyes, which are positioned such that they can lie above the surface of the water with the rest of the toad’s body submerged in the water. The eyes have both upper and lower eyelids but the transparent nictitating membrane is present only under the lower eyelid The nictitating membrane is used for clean'ng and moistening the eyes when on land; in water however, they protect the eyes from mechanical injuries. The large bulging eyes are often partially withdrawn Into the buccal cavity to aid in the swallowing of food.

Posterior to the eyes and slightly below them is a pair of circular patch of stretched skin called tympana (singular: tympanum), tympanic membranes or eardrums. The tympana are set into vibrations by sound vibrations and enable the toad to hear. There is no outer or external ear. The ear of the toad therefore consists of middle and inner ears only. Behind the eardrum, on either side of the trunk, is a circular or oval glandular patch of skin called poison gland. The poison glands secrete a thick, milky or whitish, foul-smelling or unpleasant substance when the toad is attacked or threatened. This serves as a protective or defensive mechanism against predators.

Trunk: The trunk bears two pairs of jointed limbs: the fore and hind limbs. The fore limbs are short stout and more angular than the hind limbs. They are distinguished into an upper arm, forearm and hand. The hand ends in four digits which are not webbed. There is an elbow between the upper arm and fore arm and a wrist between the fore arm and hand. The hind limbs are longer, mascular and folded. They stretch to enable the toad to hop. They are also distinguished into a thigh, shank and foot which end in five webbed digits. The joint between the thigh and shank is the knee and that between the shank and foot is the ankle. The hind limbs are powerful and upon stretching enable the toad to hop on land and swim fast in water. The webbed digits provide a large surface area for effective paddling during swimming. On the ventral surface of the trunk and between the hind limbs is the cloaca, which is used for the passage of urine, faeces, sperms and eggs.

The toad has darker dorsal surface and a lighter ventral surface, for concealment from ‘ predators, especially in water. The skin also contains mucus-secreting glands, which secrete mucus, which makes the skin moist and slippery. The slippery skin makes the toad difficult for predators to handle or grasp, thus serving as a protective mechanism.



ADAPTATION OF TOAD TO ITS HABITAT
The toad has features which adapt it to live both on land and in water. The brown or greyish brown colour of the toad blends with the environment on land and is therefore used for concealment on land. The dark dorsal surface and light ventral surface makes the toad or frog difficult to detect in water.


The long, mascular or powerful hind limbs enable the toad "or frog to hop efficiently on land. ’The absence of a neck and tail in the toad and frog is. an adaptation for efficient jumping or hopping, since their presence would have interfered with the jumping or hopping movements.The long mascular hind limbs with their webbed digits are adaptations for efficient paddling or pushing of water during swimming in water. The short, stout fore limbs enable the toad or frog to absorb the shock of landing when it hops or jumps on land. They are also used for steering when swimming in water.

The warty, loose-fitting and highly vascularised skin is an adaptation of the skin for gaseous exchange in water and on land.

The mucus-secreting glands in the skin secrete mucus which make the toad and frog slippery to touch and thus difficult to grasp. The two poison glands behind the eardrums produce a poisonous, distasteful and foul-smelling or obnoxious milky or whitish and sticky substance that keeps enemies or predators away. The colour of the toad or frog, possession of poison glands and mucussecreting glands in the skin are all adaptations for protection.

The position of the nostrils enables the toad and frog to take in gaseous oxygen, from the atmosphere, when in water. This is an important adaptation for gaseous exchange or respiration due to loss of the gills and possession of lungs in the adult toad or frog.

The large bulging eyes give the toad a wide field of view and a sharp vision. This, together with the possession of all factory organs in the head, enables the toad to detect food, mating partners and avoid enemies.

Sound-production and sound-perception are also important to the amphibian during the breeding season. During this season, the male produces a loud croaky sound, which attracts the female. The female is able to perceive the croaky sound of the male by the possession of eardrums or tympanic membranes, which are sensitive to sound vibrations. The eardrums also enable both sexes to detect the movement of their enemies both in water and on land.

The streamlined body shape of the toad and frog enable these animals to swim in water with reduced resistance, as the water flows smoothly over their bodies.





LIFE PROCESSES OF THE TOAD

Mode of Life: During wet season and in the cool part of the day, i.e. usually early mornings and in the evenings, the toad is commonly seen as it comes out into the open. It however, avoids the hot part of the day by hiding in cool and moist places, such as under leaf litter, logs of trees and under stones. The toad and frog are therefore more active at night to avoid hot sun, they are thus said to be nocturnal.


Respiration: As amphibians, toads and frogs can live both in water and on land. They therefore have respiratory surfaces that adapt them to both environments. The first part of the toad’s life is spent completely in water. The young toad or frog called tadpoles, therefore possess external and internal gills fur gaseous exchange in water.

The adult toad or frog spends most of its life on land. It however, occasionally returns to water. It has three methods of respiration: cutaneous respiration, buccal respiration and lung or pulmonary respiration. Cutaneous respiration, which is respiration involving gaseous exchange through the skin, is the most convenient method of gaseous exchange when in water. When on land, buccal respiration, which is respiration that involves gaseous exchange through the lining of the buccal cavity, is employed, when the toad is not active. Lung or pulmonary respiration is however, employed on land when it is very active, e.g. during hopping or jumping, when it requires a lot of energy for its activities.’ Since the skin of the toad is kept moist with secretions from the mucus glands, cutaneous respiration can also I effectively take place on land.


(i) Cutaneous Respiration: The skin of the toad is thin-walled and has a rich supply of blood capillaries with blood continuously flowing through them, it is therefore said to be highly vascularised, When in water, dissolved oxygen diffuses through the skin into the blood.

The absorbed oxygen is then used to oxidize food to release energy. Carbon (IV) oxide evolved, diffuses out through the skin in the surrounding water. '


(ii) Buccal Respiration: During buccal respiration, the toad closes the mouth tightly, lowers the floor of the buccal cavityand opens the nostrils or external nares. The pressure in the buccal cavity decreases. as the volume of the buccal cavity increases. Air from the atmosphere diffuses into the buccal cavity through the nostrils.

It then closes the nostrils or external nares with their valves and raises the floor of the buccal cavity. This presses the air against the lining of the buccal cavity, which is thin and highly vascularised. The Oxygen diffuses into the blood in the capillaries. It is used to oxidize food to release energy.

The toad then opens the nostrils and raises the floor of the buccal cavity. This increases the pressure in the buccal cavity which pushes the air out into the atmosphere through the nostrils.


(iii) Pulmonary or Lung Respiration:
This is also known as ventilation of the lungs. The two sac-like lungs are the main respiratory organs in the toad. They are made up of hundreds of air sacs called alveoli. The alveoli are thin-walled and highly vascularised. The bronchi (singular: bronchus) connect the lungs to the larynx or voice box, which is connected to the buccal cavity by a slit-like opening at the posterior end of the buccal cavity called glottis.


During pulmonary respiration, atmospheric air is drawn into the buccal cavity through the nostrils, as described in buccal respiration above. The nostrils are closed, the glottis is opened and floor of the buccal cavity is raised. This forces the air into the lungs through the glottis. Gaseous exchange occurs in the highly vascularised, thin-walled alveoli of the lungs. Oxygen diffuses into the blood and combines with haemoglobin to form oxy-haemoglobin. The oxygen is carried to the tissues, where it is used to oxidise food to liberate energy. Carbon (IV) oxide evolved, diffuses into the lungs from the blood. The expired air diffuses out of the body through the nostrils. Pulmonary respiration is employed by the toad when the energy requirement is very high.


Nutrition:
The toad is carnivorous and feeds on worms and insects. It suddenly flicks out the long tongue and catches or traps its prey with the sticky tip of the tongue. The prey or food is carried into the buccal cavity as the tongue is rolled back into the mouth. The vomerine or homodont teeth are used to hold onto the prey to prevent it from escaping. The wide-gaped mouth, enables the toad to swallow its prey whole. Swallowing is aided by raising of the floor of buccal cavity and depression of the large bulging eyes into buccal cavity.

The food passes into the stomach from the oesophagus when the cardiac sphincter relaxes. The protein food is converted into peptones by pepsin in the stomach. The food then passes into the duodenum.

In the duodenum, the peptones are converted into amino acids by trypsin and erepsin. Amylases also convert carbohydrates into glucose and fructose. Lipase also converts fats and oils into fatty acids and glycerol. The end products of digestion are absorbed into the blood in the ileum. Re-absorption of water, from the indigestible food materials, occurs in the rectum while undigested food materials or faeces are egested through the cloaca.


Movement:
Toad moves by hopping on land and swimming in water.

Hopping on land: When at rest, the fore limbs are extended and the long, muscular and powerful hind limbs are folded in the shape of

the letter “Z" in order to hop. The long, muscular and powerful hind limbs are suddenly extended or stretched. This produces a thrust which lifts the toad from the ground and pushes it forward and upwards at an angle. On landing, the short, stout fore limbs are used to absorb the shock of landing. It lands on its palms while the hind limbs are folded under the body, which touches the ground with the hind limbs still folded beneath it.


Swimming in Water: When in water, the toad moves about by swimming. In swimming, the webbed digits on the hind limbs are spread, to provide a large surface area for paddling in water. The long, mascular and powerful hind limbs are used to forcefully push the water backwards. This moves the toad forward. The short, stout fore limbs with their webless digits are moved to steer the toad when swimming.

The first part of the toad’s life is spent wholly in water. The young tadpole uses its long tail to swim in water by heating it from side to side. This propels the tadpole forward. The tail is also used for steering. In older tadpoles however, paddling and steering are provided by the tail and the pair of hind limbs with their webless digits.

Excretion: The nitrogenous waste substance produced by the toad is urea. This together with excess water and salts are excreted from the blood as urine by the wolflian ducts in the kidneys. The urine is removed from the body through the cloaca. The tadpole excretes ammonia as its nitrogenous waste substance. The ammonia is removed from the body rapidly by diffusion over the entire body surface into the surrounding water by the tadpole.


Carbon (IV) oxide, evolved during respiration.is excreted from the body by diffusion through the skin, lining of the buccal cavity and lungs.

Reproduction: The breeding season in the toad is usually the rainy season, as an external source of water is required for fertilization. During the breeding, the male produces loud croaky noise by the vibrations of the vocal cords in the voice box. The croaky noise by the male wards away other males but attracts females to the male for mating. During this treason. the male develops thickened, horny pads called nuptial pads on the underside of the first digit on the fore limb. The female is larger than the male at this time, since its body is filled with eggs.

During mating, the male mounts the female and grasps the armpit of the female with its nuptial pads. The female carries the male on her back for several days until it lays her eggs. The male then sheds sperms over the eggs to effect fertilizition exernally in the water. The eggs of the toad are laid in long strands 'or chains 0f strings called toad spawn. 'They are black above and white below for, concealment. The eggs are covered or surrounded by a jelly-like substance made up of albumen -with pure protein. The jelly-like substance makes the eggs distasteful to predators and protects the eggs from mechanical injuries and infection by bacteria. It also makes the eggs buoyant or enables the eggs to float. The long Strands or chains enable the eggs to be entangled in vegetation in the breeding grounds to prevent them from being washed away. Eggs of frogs are laid in a mass.


External Gill Stage: A fertilized egg hatches into a comma-shaped tadpole within a day or two. A sucker or cement gland attaches the tadpole to the vegetation or water weed.
The tadpole at this stage possesses a long tail, external feather-like gills and a long coiled intestine, which adapts the tadpole to herbivorous life. It also possesses horny jaws for scraping vegetation. The long tail is used for swimming in water while the featherlike external gills enable the tadpole to absorb dissolved oxygen for respiration. The external gill stage lasts for about four days.


Internal Gill Stage:
About four days after hatching, the external gills begin to shrink and disappear. Four pairs of internal gills also develop, covered a fold of skin on either side of the head called operculum (plural: opercula). The opercula grow over the degenerating external gills. Four gill slits also develop on both sides of the head. The internal gills and gill slits are employed in gaseous exchange by the tadpole.

The tadpole’s tail at this stage develops powerful tail muscles. The tail is used for swimming in water. The tadpole still possesses a long coiled intestine, used for digestion of plant materials or vegetation, as it continues to feed on water weeds or vegetation in water. It still possesses horny jaws for scraping or nibbling vegetation. This stage of the tadpole lasts for about two weeks.



Limbs and Lungs Stage:
Between twenty (20) to thirty five (35) days after hatching, the gills of the tadpole begin to degenerate and are replaced by two lungs. The lungs are used to absorb atmospheric or gaseous oxygen, which is obtained when the tadpole uses its tail to swim to the surface of the water occasionally to gulp in air. Hind limb buds appear. These later develop into hind ‘ limbs followed by the fore limbs. The tail shortens and is gradually absorbed or disappears.

About thirty five (35) days after hatching, the tadpole stops feeding on vegetation as its long coiled intestine is replaced by a much shorter alimentary canal adapted for carnivorous feeding. The horny jaws are also replaced by a terminal mouth.

After about forty live (45) day after hatching, the trunk shortens in length, the mouth widens and the tail is completely absorbed. The tadpole now changes to become a young toad.

The young toad then leaves the water to begin a carnivorous life on land. The series of gradual changes by which the tadpole develops into an adult toad is known as metamorphosis.


Life processes in the frog is the same as that in the toad. described above. Structurally, the frog has similar features as that of the toad but the frog has a more streamlined body and is lighter in weight than the toad. The skin of the frog is slimier, softer and less warty than that of the toad. These features make the frog more adapted to aquatic life than the toad.


The limbs of the frog are longer, more mascular and stronger than that of the toad. The digits on the hind limbs of the frog are webbed swimmer in water and hops higher and fart her on land than. the toad.

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