External Features and Body Wall of Toad (With Diagram)

In this article we will discuss about the external features and body wall of toad.

External Features of a Toad:

The body of a toad is divisible into head, trunk and limbs. Head and trunk are not separated by a flexible neck; this condition is very suitable for aquatic (aqua = water) life. The adult has no tail.

The toad is a bilaterally symmetrical animal; that is, if the animal is cut through the middle of its length, the parts on the right half will be similar to those on the left half. For the purpose of description, the back of the animal is spoken of as the dorsal and the underneath as the ventral surface; the part of the body which is foremost during locomotion is the anterior end and the rear is the posterior end.

The part of a limb which lies nearest to the trunk is proximal and the opposite is distal. Each side of the body is said to be lateral and structures on the middle line of the body are medial.

1. Head:

The head is nearly semicircular in outline with the bluntly rounded snout in front. The mouth is a wide gaping slit which extends from side to side along the anterior end of the snout; it is meant for the ingestion of food.

On the dorsal surface of the head, there are two small openings near the anterior end. These are the nostrils or external nares which lead into the nasal passages and serve as the inlet or outlet of air. On either side of the head, there is a large protruding eye which can be closed by means of eyelids.

Each eye is provided with three eyelids:

(i) A thick and prominent upper eyelid;

(ii) A rudimentary lower eyelid; and

(iii) A transparent third eyelid or nictitating membrane which is attached below to the lower eyelid and can be stretched up to cover the eyeball. There are no eyelashes. The bulging eyes enable the animal to see all rounds, and the nictitating membrane protects the eyeball from dirt. Behind each eye there is a circular patch of tightly stretched greyish membrane. This is the eardrum or tympanum for the purpose of hearing.

There is no pinna of the ear in the toad. A little behind the eardrum, there is a round ele­vated structure called parotid or parotid gland. These extend behind into the trunk and when the toad apprehends any danger, the glands secrete a pungent sticky juice for repelling the enemy. Paro­toid glands, therefore, are organs of offence and defence.

The ventral surface of the head forms the floor of the mouth cavity. Its rise and fall along with breathing can be observed in a living specimen. The skin in this part of the body is loose and may, in male toads, be inflated by a dark-coloured vocal sac. The female has no vocal sac.

2. Trunk:

The trunk is short and flattened from above down wards. There is nothing to indicate the boundary between the head and the trunk. The dorsal surface is covered with in numerable rough warts which indicate the position of the poison glands.

The ventral surface is comparatively smooth and free from warts. The posterior end of the trunk is broadly rounded. The vent cloaca aperture lies dorsally at the posterior end, between the bases of the two hind legs; it serves for the exit of faeces, urine and germ cells.

A toad, when disturbed, often passes water through the vent. There is a widely spread rumour that handling of toads causes warts on human hands; this is a false superstition.

3. Limbs:

Attached to the trunk are a pair of forelimbs and a pair of hind limbs. The hind limbs are kept in a folded state when the animal is at rest.

i. Fore Limbs:

The forelimbs are short and divisible into segments by movable joints. The proximal part, by which it joins the trunk, is known as the arm or brachium. To this is joined the forearm or ante-brachium.

Distal to the forearm is an insignificant wrist or carpus followed by a hand or manus. There are four digits or fingers corresponding to those on our own hand; but the thumb is absent. The male toad develops a cushion-like swelling, called thumb pad, at the bases of the index and middle fingers.

ii. Hind Limbs:

The hind limbs are longer and stronger than the forelimbs. Each hind limb consists of a proximal segment called thigh or femur which is followed by the shank or crus. Distal to the shank is a long ankle or tarsus, a foot or pes, and five slender digits or toes of which the fourth toe is the longest. The toes are united by a thin fold of skin called web.

The limbs are very suitable for the purpose of hopping on land as well as for swimming in water. A toad, when disturbed, jerks off its strong hind limbs, leaps into the air, and finally lands on the ground at a considerable distance.

This peculiar method of locomotion is known as hopping. When taking the hop, the long ankles and feet are pressed against the ground and used as levers. The animal swims by alternately bending and straightening its powerful hind limbs, the webbed feet being used as paddles.

4. Integument:

The skin is rough but moist. It does not bear scales, feathers, hairs or claws and is, therefore, said to be naked. Toads are almost of the same colour as the earth; the dorsal surface of the animal is greyish-black and the ventral surface is yellowish-grey. They are thus very difficult to locate when not moving.

Toads have the power of changing the colour of their skin, so that they may match with the background and thus become invisible to the enemy. This phenomenon is known as protective colouration. The tree frog Hyla is better than toad in this respect and can change its colour very quickly.

The skin forms a cover for the entire body. It protects the underlying soft parts from injury and excludes disease germs. It serves as an additional respiratory organ by absorbing oxygen and giving out carbon dioxide. Toads never drink water; they freely absorb water by their skin.

The skin is composed of two distinct layers:

(i) An outer epidermis, and

(ii) An inner dermis.

The epidermis consists of several layers of epithelial cells. The superficial epidermal cells are thin, scale-like and devoid of nuclei. They form a horny layer called stratum corneum which is shed from time to time as one piece. This process is known as ecdysis or moulting. The deeper epidermal cells are tall and polygonal. They form the stratum germinativum which produces new cells to replace the old horny layer.

The dermis is much thicker than the epidermis. It is composed of glands, pigment cells, involuntary muscle cells, blood vessels and nerves, all enclosed and held together by connective tissue.

Scattered in the superficial part of the dermis arc spider-like pigment cells or chromatophores which can change the colour of the animal by simply shifting their position. The chromatophores arc either black or yellow, but never green. The glands produce the rough warts on the outer surface.

There are two kinds of glands:

(i) Poison glands and

(ii) Mucous glands.

They manufacture useful fluids or secretions which pass out by narrow ducts and moisten the surface of the skin. The poison glands are larger and more; numerous. They secrete a thick whitish fluid with burning taste which repels the enemy. The mucous glands secrete a thin watery fluid which keeps the skin moist and shiny. Body Wall and Body Cavity of Toad:

The toad’s body is essentially a double tube. The outer tube is formed by the body wall and the inner tube by the digestive canal. The two tubes are separated from one another by the body cavity or coelom.

The body wall is composed of the following structures:

(i) Skin,

(ii) Subcutaneous connective tissue,

(iii) Muscle layers, and

(iv) Perito­neum.

The skin is loosely attached to the underlying muscles by a thin layer of yellowish white connective tissue. There are large spaces enclosed between the skin and the muscular layer. As these are filled with a colourless fluid called lymph, they are known as the subcutaneous lymph spaces. The peritoneum lies beneath the muscular layers.

It is a thin glistening membrane which forms an internal lining for the whole of the body cavity or coelom. Most of the internal organs of toad lie in the coelom and are exposed by cutting down the different layers of the body wall through an incision in the mid-ventral line.

The peritoneum not only lines the body cavity but it also encircles the digestive tube and other organs, suspending them to the body wall by fan-shaped folds called mesentery. The relative position of the various internal organs or viscera is shown in Fig. 7.

The integumentary system, including the skin and its various functions.