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In this article we will discuss about Toad:- 1. Introduction to Toad 2. Habit and Habitat of Toad 3. External Structures 4. Locomotion 5. Skeletal Structures 6. Coelom 7. Sense Organs 8. Vocalization 9. Life History 10. Enemy.
Contents:
- Introduction to Toad
- Habit and Habitat of Toad
- External Structures of Toad
- Locomotion in Toad
- Skeletal Structures of Toad
- Coelom in Toad
- Sense Organs of Toad
- Vocalization of Toad
- Life History of Toad
- Enemy of Toad
1. Introduction to Toad:
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Bufo melanostictus is a familiar example of toad. It is found in damp places all over this country. Toad represents a typical example of the class Amphibia. Despite the fact that toad is a highly specialised form amongst the living amphibians, it is studied extensively to gain an elementary knowledge on the anatomy of vertebrates in general and an amphibian in particular.
Systematic position:
According to Duellman and Trueb (1986)
Phylum – Chordata
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Subphylum – Vertebrata (Craniata)
Superclass – Gnathostomata
Class – Amphibia
Subclass – Lissamphibia
Superorder – Salientia
Order – Anura
Family – Bufonidae
Scientific Name Bufo melanostictus Schneider, 1799.
English Name Common Indian Toad:
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Vernacular Name
Bengali – Kuno Beng
Oriya – Katkatia Benga, Luni Benga, Kuji Benga
2. Habit and Habitat of Toad:
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Toad is a cold-blooded or poikilothermous or Exothermic i.e., the temperature of the body depends on the temperature of the external environment) vertebrate. During colder months of the year toads undergo through a phase, called hibernation or winter sleep. An adult toad generally lives on land, but if necessary it can live in water for some time.
It is truly an amphibian both in biological as well as well as literal sense (amphi= both; bios = life). Toad is quite at home in water as well as on land. On land it is obligatory for the toad to remain in dark and moist places, preferably near water. Skin of toad is an accessory respiratory organ and so it is kept moist. It tries to keep itself away from bright light.
Toad is a carnivorous animal and catches insects by its sticky tongue. Toads are nocturnal and usually come out of their hides at dusk. The males can produce croaking sound by the help of the vocal sac, specially during breeding season.
Toad lives on land but breeds in water. All the organ systems of toad are adjusted to land life except the reproductive system. For the purpose of reproduction it had to go back to the primal aquatic abode.
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3. External Structures of Toad:
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Toad has a short bilaterally symmetrical body (Fig. 7.2). There is no exoskeleton over the skin, i.e., the skin is naked. The skin is rough is texture. The dorsal side of the body is blackish-grey while the ventral side is yellowish grey.
Toad can change the colour of the body to match with the hues of its surroundings. Ordinarily, its body colour is same as that of the earth. The body is divisible into head and trunk. Distinct neck is absent. A postanal tail is absent in adult stage. The tail is present and well-developed in the larval condition. The head is semicircular in outline. It is broad, depressed and has a blunt snout.
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The mouth is a wide opening located at the terminal end of the head. At the anterior dorsal side of the head there is a pair of rounded openings, known as nostrils or external nares. The eyes are very large and prominent. These are protruding and are situated one on either side of the head. Each eye is provided with a thick upper eyelid and an ill-developed lower eyelid.
A transparent nictitating membrane (or third eyelid) is present and it is stretched to cover the eye ball. Behind each eye, a circular area, known as eardrum (or tympanum), is present. Just behind each tympanum there is an elongated elevation, called Parotoid gland. These paired glands secrete a whitish, pungent and sticky juice.
It is a poisonous secretion and causes nausea and affects the heart in man, if swallowed. When the secretion falls in the eyes and nose, it causes irritation, but rarely affects the skin. These glands act as the organs of offense and defence. The skin on the floor of the buccal cavity becomes inflated to form vocal sac in males.
The trunk is broad, short and flattened. Numerous small elevations, known as warts, are present on the dorsal side of the body. The cloacal aperture or vent is located on the dorsal side of the posterior end of the body between the two hind limbs. There are two pairs of limbs. These are of unequal size. The forelimbs are smaller than the hind limbs.
Each forelimb consists of brachium (upper arm), and ante- brachium (forearm), a wrist and a manus (hand). The hand is followed by four digits. In male individual, a cushion like thumb pad (or nuptial pad) develops during breeding season at the basal part of inner finger.
These pads facilitate the male’s grip during amplexus. The hind limbs are strongly built and are much longer than the forelimbs. These two limbs are modified for jumping and swimming.
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Each hind limb is composed of a proximal sector, called femur (thigh), which is followed by crus (shank). Distal to the shank lies the pes (foot). The foot consists of a long tarsal region and five elongated slender digits. The digits are united by webs which help in swimming.
Skin:
The skin of toad is kept moist and frequently slimy. Besides its protective function, the skin of toad serves as an additional respiratory organ. During hibernation, toad respires entirely by the skin. The skin is composed of an outer epidermis and an inner dermis (Fig. 7.3).
These two layers are separated by a basement membrane and a fibrous layer containing the chromatophores or pigment cells. The epidermis is a compound structure and is made up of several layers. The outermost layer is called stratum corneum. It is a thin, scaly and cornfield layer.
This layer is dead and is shed periodically. This phenomenon of periodic shedding of the stratum corneum is called ecdysis (or moulting). In other amphibians, the stratum corneum is thin and delicate, while in toad it is thicker and heavily cornfield. The innermost layer of the epidermis is composed of columnar cells with prominent nuclei.
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This layer is called stratum germinativum (or Malpighian layer) which sits on the basement membrane. The cells lying between the stratum germinativum and stratum corneum constitute the transitional layer. These cells form several layers and decrease in size from below upwards.
The dermis is thicker than the epidermis and is divisible into two layers. The outer layer accommodating most of the glands is called stratum spongiosum. This is composed of loose network of connective tissue matrix with blood vessels and lymphatic spaces. The superficial part of this layer contains the pigment cells.
The innermost layer is called stratum compactum which is composed of dense connective tissue, smooth muscle fibers, nerves and blood vessels. Beneath the stratum compactum lies a loose subcutaneous connective-tissue layer containing fatty tissue.
Glands:
The skin of toad is marked by the presence of numerous bumps (or warts) all over the dorsal body surface. These warts may be either due to underlying poison glands or sensory papillae. There are two types of skin glands in toad.
These are:
(a) Mucous glands secreting mucus and
(b) Poison glands producing poison.
The mucous glands are smaller than the poison glands. The poison glands are composed of granular secretary cells. The skin glands usually lie in the stratum spongiosum of the dermis, but the poison glands may be more deeply located. These glands are unsheathed by connective and muscular tissues, which help in squeezing the secretory products. The products of the gland come out through duct to the outside.
Colouration:
The colour of toad depends upon the presence of pigment cells or chromatophores in the dermis. Some chromatophores also invade the epidermis. Depending on the types of contained pigment granules, the chromatophores may be melanphores (containing black pigment), guanophores (containing colourless crystals of guanine) and lipophores (containing yellow pigment).
The melanophores are situated in the deepest layer, the guanophores are intermediate in position while the lipophores are present in the upper layer. The melanophores are responsible for the production of blackish colour and the lipophores cause yellowish effect. The guanine crystals in the guanophores produce diffraction effect.
The colour change in toad is rather a slow process and is controlled under the action of melanophore stimulating hormone (MSH) of the pituitary gland. The colour change in toad is not under nervous control. In fishes, instantaneous colour change is caused through the nervous system while the slower change is under hormonal control.
4. Locomotion in Toad:
In toad, the entire muscular and skeletal systems have become specialised for jumping and swimming. The movements are caused by the thrust of both the hind limbs. The movements are caused by the thrust of both the hind limbs. Besides jumping and swimming, toad is able to walk on land. This is caused by bringing into play a set of proprioceptor reflexes.
In resting state, the anterior part of the body is supported by the hind limbs. The hind limbs are folded in the manner of ‘Z’. From such a sitting or squatting posture, toad jumps by a sudden extension of the hind limbs.
The forelimbs manipulate and adjust the direction before each jump. Swimming is done by the activity of the limbs which act as the propellers. The hind limbs are long and the digits are webbed. These limbs act like oars and enable the animal to swim.
5. Skeletal Structures of Toad:
There is no exoskeleton in toad. The skeleton that supports the soft parts lies internally and is designated as endoskeleton. It is chiefly made up of bones and cartilages. These two structures are associated with one another to form the internal framework.
The endoskeleton is described under two broad heads:
(a) The axial skeleton and
(b) The appendicular skeleton.
Axial skeleton:
The axial skeleton comprises of the skull and the vertebral column.
Skull:
The skull of toad is flat and broad. It contains a tubular cranium and is pierced posteriorly by a large aperture, called foramen magnum. Through this aperture the spinal cord passes. On each side of this foramen there is an exoccipital bone which bears a convex occipital condyle. So there are two occipital condyles which fit into the two concavities of the first vertebra.
The occipital condyles are developed from the exoccipitals. The roof of the cranium is made up of two flat bones, called the front parietals. Each frontoparietal is formed by the fusion of two bones, the frontal and parietal (Fig. 7.5A).
The floor of the skull is formed of a dagger-like Para sphenoid. A ring-like sphenethmoid bone is present at the anterior end of the cranium. This bone is completely covered by front parietals on the dorsal side. The nose is covered dorsally by a triangular nasal bone and the floor is provided with the vomer (Fig. 7.5B).
The cartilaginous otic capsules are loosely attached with the cranium. The auditory capsules are situated in front of the exoccipitals. The floor of the auditory capsule is supported by the lateral extension of the Para sphenoid and the dorsal side is covered by prootic bone. A small hammer-like squamosal connects the posterior part of the upper jaw with the otic capsule.
Each half of the upper jaw is made up of small premaxilla, long slender maxilla and quadratojugal. Behind the quadratojugal there is a very small Y-shaped supporting bone, called the quadrate. The other two supporting bones are the pterygoid and the palatine. The palatine is rod-like and connects the maxilla with the sphenethmoid bone.
The lower jaw is composed of two halves and the halves are united anteriorly by ligament (Fig. 7.6A). Each half is developed from a Meckel’s cartilage and consists of three bones, the dentary, angulosplenial and mentomeckelian. The posterior part of the angulosplenial articulates with the upper jaw. Both the upper and lower jaws are toothless in toad. In frogs the upper jaw besets small conical teeth.
Hyoid apparatus:
The hyoid apparatus is essentially a cartilaginous structure which supports the floor of the buccal cavity (Fig. 7.6B). It also forms the supporting frame-work for the attachment of the tongue. The body of the hyoid constitutes the main bulk of the apparatus.
There are two pairs of prolongations (or horns) from the body of hyoid apparatus. The anterior pair are longer and extend up to the auditory capsules. These are called anterior cornua.
Similar pair on the posterior side are known as posterior cornua which enclose the laryngotracheal chamber.
Vertebral column:
The vertebral column (Fig. 7.7A) is composed of nine vertebrae and a terminal rod-like structure, called urostyle (oura=tail and style=rod).
Typical vertebra:
A typical vertebra has a solid cylindrical part, known as centrum. The centrum is procoelous, i.e., the centrum is concave anteriorly and convex posteriorly (Figs. 7.7 C1, C2). On the dorsal side, the centrum bears a ring-like neural arch which encloses the neural canal. The roof of the neural arch possesses a median elevation known as neural spine. In the lateral side the neural arch carries transverse processes. The articulating processes are present on the neural arch.
The anterior pair of processes are known as prezygapophyses and the posterior pair are called postzygapophyses (singular—postzygapophysis). All the vertebrae excepting the first and the last one have typical structural construction. But in frog, the eighth vertebra is provided with amphicoelous central, i.e., both the ends of a centrum are concave.
First vertebra or atlas:
It articulates with the occipital condyles of the skull. It is ring-like in appearance (Fig. 7.7B). The transverse processes and the prezygapophyses are absent. Anteriorly it possesses two concave facets which fit with the paired occipital condyles of the skull. The centrum is greatly reduced.
Ninth vertebra:
This vertebra is peculiar and has two-rounded condyles on the posterior side of the centrum for articulation with the anterior paired concavities of the urostyle. The transverse processes are stout, fan-shaped (Fig. 7.7 D1, D2) and articulate with the pelvic girdle. The postzygapophyses are absent. The ninth vertebra is also known as sacral vertebra. In frog, the transverse processes are not fan shaped but are cylindrical.
Urostyle:
The urostyle is a long slender rod-like structure (Fig. 7.7 E1, E2) and is formed by the fusion of a number of vertebrae.
It has a pair of concave cavities at its anterior end for articulating with the paired posterior convexities of the ninth vertebra. It has a mid-dorsal neural crest. A very narrow hole is present throughout the urostyle which represents the reduced neural canal. Through this canal passes the filum terminale of the spinal cord.
Appendicular skeleton:
The skeletal frame of the paired limbs and the girdles constitute the appendicular skeleton (Fig. 7.8).
Pectoral girdle:
The pectoral girdle consists of two symmetrical halves. These halves are united at the mid-ventral line but are free dorsally (Fig. 7.8B). This girdle forms a bony framework for encircling the anterior part of the trunk. Each half of the pectoral girdle is made up of a broad dorsally placed and partly cartilaginous plate like structure, known as suprascapula.
Attached to it, there is a strong bone, called scapula. Two rod-like bones are connected with the scapula. The anterior one is called clavicle, while the posterior one is known as coracoid. The clavicle encloses the precoracoid cartilage which is hardly visible. The clavicle and the coracoid are joined with partly overlapping pieces, the epicoracoids.
The space present between these three bones is designated as coracoid fontanella. Just at the junction of the clavicle, coracoid and scapula there is a cup shaped depression, called glenoid cavity, into which the head of humerus fits. Projecting posteriorly from the united posterior end of the epicoracoid there lies a sternum which has a terminal flattened cartilaginous xiphisternum.
Pelvic girdle:
The pelvic girdle is a V- shaped bony structure having a disc-like posterior end. The disc is formed by the union of three bony units on each side, the ilium, ischium and pubis (Fig. 7.8D). A cavity known as acetabulum is present on each side of the disc. The head of femur fits into this cavity.
The ilia are elongated curved rod-like structures which are attached with the transverse processes of ninth vertebra. They form the anterior and dorsal sectors of the disc. Two pubes (Singular—pubis) are represented by a triangular cartilage on the ventral side of the disc. The posterior sectors of the disc are formed by the ischium.
Forelimbs:
There are two forelimbs and each forelimb is composed of several long bones arranged end to end (Fig. 7.8A). The first one in the series is called humerus whose middle region is slightly curved. The proximal end is termed as the head of the humerus which fits with the glenoid cavity of the pectoral girdle. The distal end carries a pulley-like trochlea.
A prominent crest, called deltoid ridge, is extended from the head to the middle region of the humerus. The second in the series is the radio- ulna formed by the fusion of two separate bones, radius and ulna, the anterior end of the radio-ulna is concave and fits with the trochlea of the humerus. The proximal end is drawn out into an olecranon process.
The distal end is flattened to give attachment of six carpal bones which are arranged in two rows. Four slender rod-like bones, called metacarpals, are connected with the digits. There are four digits. The third and fourth digits have three phalanges and the first and second have two phalanges.
Hind limbs:
The hind limbs, like the forelimbs, are also paired structures. Each is made up of series of long bones (Fig. 7.8C), The proximal one is the femur which is gently curved. The anterior end of the femur is rounded to form the head of the femur and the posterior end is slightly flattened to form the condyle. The next part is known as tibiofibula formed by the fusion of two bones, the tibia and fibula.
Both the ends of the tibiofibula are expanded to give articulation with the condyle of the femur anteriorly and with the tarsal bones distally. The tarsal bones are arranged in two rows. The proximal tarsals are the astragalus and calcaneum. These are elongated structures and are slightly curved outward. Both the astraglaus and calcaneum are joined with one another at both ends.
The distal tarsals are composed of two or three small bones. The foot is constituted of five metatarsals. There are five digits having variable number of phalanges.
The first and second digits have two phalanges, the third and fourth have four phalanges and in the fifth there are three phalanges. A bony projection made up usually of two small bony nodules is present on the outer side of the hallux. This structure is known as the pre-hallux or calcar.
6. Coelom in Toad:
The coelom is a large and undivided cavity. The internal organs or viscera are lodged in the coelom. The coelom is lined by the coelomic epithelium, called peritoneum. The peritoneum also encircles the alimentary canal and other organs.
These visceral organs are suspended to the body wall by fan-shaped folds, called mesenteries. The pericardial cavity housing the heart is a coelomic derivative which becomes cut off from the main coelom and exists as a separate cavity to house the heart.
7. Sense Organs of Toad:
The sense organs are the receptors for external stimuli. These are the avenues through which the central nervous system is kept informed of the outside world. Each receptor organ can respond to a particular type of stimulus and produces its own specific sensation.
Receptors for cold, heat, pain and touch are present beneath the epidermis of the skin. These are microscopic in structure and are usually regarded as cutaneous receptors.
Receptors for smell:
These receptors are scattered in the nasal passage. The mucous membrane lining the nasal passage contains peculiar olfactory cells and slender supporting cells. The olfactory cells are connected with nerve fibres from the olfactory nerve and are the actual receptors for smell.
Receptors for taste:
The taste buds present in the tongue and mouth cavity are the receptors for taste. Each taste bud is made up of two types of cells, the taste cells and the supporting cells. The taste buds are located in the tongue in association with minute elevations called papillae.
Receptors for vision:
The eyes are the two very compact photosensitive organs. These organs are lodged in the orbits. Each eye has a spherical body and is usually called eye ball. The eye ball can be rotated inside the orbit within its limit by six extrinsic muscles.
These are:
(i) Superior rectus,
(ii) Inferior rectus,
(iii) External rectus,
(iv) Internal rectus,
(v) Superior oblique and
(vi) Inferior oblique.
The eye can be protruded by the levator bulbi and can be withdrawn by the retractor bulbi to a certain limit.
The eye ball is composed of three layers arranged in a regular sequence (Fig. 7.21 A).
These are:
(a) Sclerotic layer:
This is the outermost layer of the eye ball. It is very tough and thick. It consists of two parts: An anterior transparent circular portion, called cornea and an opaque posterior portion, called white of the eye ball. The cornea permits the entry of light rays inside the eye.
The outer surface of the cornea is covered by a thin transparent membrane, called conjunctiva. The sclera is composed of cartilage and fibrous tissue and gives protection to the delicate parts of the eye.
(b) Uvea or middle layer:
This layer is divided into three parts:
(i) Choroid,
(ii) Iris and
(iii) Suspensory ligament.
The choroid part is a highly vascular pigmented layer. At the anterior end and just behind the cornea, the choroid is modified to form a circular pigmented disc, called iris. The iris contains an aperture at the centre which is known as pupil.
The iris acts as a diaphragm and contains circular and radial muscle fibres which help the pupil to contract or dilate. Excepting the pupil, the eye ball is totally light-proof. The iris regulates the entry of light into the eye ball. Just behind the iris, there are suspensory ligaments which keep the lens in position.
(c) Retinal layer:
The retinal layer forms the innermost and the light sensitive screen of the eye. It contains two types of photosensitive cells, called rods and cones. The cones are primarily concerned with the colour vision in bright light and the rods are chiefly useful in colourless vision at low light intensities. The photosensitive cells are connected with the nerve fibres of the optic nerve.
A crystalline lens is situated just behind the pupil and is kept in position by suspensory ligament. The lens divides the cavity of the eye into two chambers. The anterior chamber between the lens and the cornea is filled up with a watery fluid, the aqueous humor and the posterior one behind the lens is filled up with a transparent jelly-like substance, the vitreous humor.
Just at the point of entry of the optic nerve into the retinal layer there is a depression, known as blind spot, where no image is formed. Two muscles, one dorsal and the other ventral, are connected with the suspensory ligament of the lens and with the cornea.
These muscles are known as protractor lentis. Contraction of the protractor lentis draws the lens closer to the cornea and when relaxed the lens is pushed away from the cornea.
Mechanism of vision:
Light rays, on the way through cornea, pupil and the lens, are converged to the retinal layer. In the retinal layer an inverted and reduced image is formed which is transmitted to the brain via the optic nerves. This inverted image is translated into a corrected one by the brain. Adjustment of the image (accommodation) on the retinal layer is done by the forward and backward movements of the lens by the protractor lentis.
The photochemical basis of image for motion relates that the vision depends upon the photosensitive pigments present in the photosensitive cells. Visual purple (rhodopsin) is abundant in the rods and visual violet (iodopsin) is present in the cones.
The synthesis of both these pigments depends upon the presence of vitamin A. The pigments are decomposed by the light and various products are produced which can create impulses in the photoreceptor cells of the retina.
Two types of vision are encountered in animals one is the binocular vision which means that the two eyes can be focused on the same object and the other is the monocular vision when each eye has a different visual field. In toad, the vision is monocular, because eyes laterally placed and cover different visual areas.
Receptors for hearing and balancing:
The ears sub-serve dual functions, hearing and balancing. The ear of toad consists of three parts: the external ear, the middle ear and the internal ear. The external ear is represented by a tightly stretched membrane, called tympanum. The middle ear is a tube like cavity. The cavity of the middle ear is in communication with the buccal cavity by Eustachian tube.
Presence of the eustachian tube equalised the atmospheric pressure on the two sides of the tympanum. A bony rod, the columella connects the tympanum with the membranous partition separating the middle and the internal ear. The internal ear is represented by the membranous partition separating the middle and the internal ear.
The internal ear is represented by the membranous labyrinth which is enclosed by the auditory capsule (Fig. 7.21 B). The membranous labyrinth floats in a fluid, known as perilymph and the cavity of the labyrinth is filled with another fluid, known as endolymph. The auditory capsule is sealed from all sides by a membranous partition. Membranous labyrinth is made up of two chambers. The upper chamber is called utriculus and the lower one is the sacculus.
The utriculus gives out narrow tubular semicircular canals (Fig. 7.21C). There are three semicircular canals, one is horizontal in position and the other two are vertically disposed. All the three semicircular canals are arranged at right angles with one another. Both the ends of the semicircular canals open into the utriculus. Each canal bears an ampulla at its one end only.
The sacculus produces a short projection, called lagena. Patches of sensory receptors are present in the inner wall of the membranous labyrinth. Each patch is composed of sensory cells and supporting cells. Receptor cells are connected with the nerve fibres from the auditory nerve.
Mechanism of hearing:
Sound waves directly impinge upon the tympanum and the vibrations are conveyed by the columella to the perilymph. From the perilymph, the vibrations are carried to the endolymph and thus stimulate the sensory cells of the sacculus and lagena. The impulses are transmitted to the brain through the auditory nerves and are perceived as sound in the brain.
Mechanism of balancing:
The semicircular canals maintain the equilibrium of the body. Calcareous particles (otoliths) present inside the semicircular canals strike upon the bristles of receptor as the animals lose the balance. Besides, the semicircular canals are arranged in such a fashion, that these can easily detect changes of the centre of gravity during movement.
8. Vocalization of Toad:
According to Noble (1931), the voice of toads and frogs is to attract mates. Wellman (1917) observes that in case of American Toad, the voice of male has a strong influences in attracting of the females. The vocalization of male Bufo melanostictus can be classified into 2 types.
a. Advertisement call:
(i) Premating call:
Before amplexus, one of the males begins to croak with low-pitch which is quickly followed by some other males in the breeding site area. This call continues until the females approach closely. The call is made up of a series of identical notes.
(ii) Mating call:
During amplexus, males often produce a guttural croak with high pitch whose duration is very short.
(iii) Territorial call:
Territorial calls function as spacing mechanisms among males and are most common among species that have prolonged breeding seasons. In B. melanostictus (Fig. 7.24), one of the males begins a single note call, is followed quickly by other males in the same breeding ground area, to form a chorus.
If one of the croakers stops, others will gradually stop their croaking. This call is used as advertisement to attract the females for mating.
b. Distress call:
Common toads (B. melanostictus) when escape from their enemies, rarely croak or chirp. But when their predators like snakes try to swallow by seizing their hind legs or middle part of body, they produce a screaming sound.
9. Life History of Toad:
Many amphibians maintain a typical biphasic life-history pattern. Adults visit to breeding ponds, deposit eggs, and return to terrestrial habitats. The life-history of toad is very much complicated and may be discussed in following points.
Breeding habits:
Breeding of Bufo melanostictus takes place from December to September coinciding with the pre-monsoons, monsoons and post-monsoons. Heyer (1973) mentioned that the highest reproductive activity takes place at the beginning of the rainy season.
After a heavy shower of rainfall, the males call can be heard at the bank of ponds, ditches and near a stagnant rain water pool of the land. Mating call could be defined as a short peep.
On hearing the mating call, the female approaches, several males scramble around her and begin to ride on the back of the female. But most of them become unsuccessful due to their joint struggling to ride. Lastly, one of them becomes successful and holds-firmly by his fore and hind limbs (Fig. 7.25).
When axillary amplexus begins, the female moves in the water for egg laying. During the breeding season sexual dimorphism is prevalent. Males possess black vocal sac and nuptial thumb-pad at each ironer most finger of the hand. McCann (1928) reported one instance of amplexus in captivity which was continued for 21 days. Daniel (1963) observes that the species is a prolific breeder.
A single female may lay over a thousand eggs in any convenient patch of water. Several workers have reported that amphibian breeding season in India and other tropical countries is normally between May and August (during the monsoon period). But the author of 6th edition has observed that the breeding of Bufo melanostictus takes place in winter months (December-January) in Southern Bengal if rain happens.
Structure of germ cells:
The eggs are spherical cells. Each egg has a blackish animal pole and a whitish vegetal pole. The animal pole is full of protoplasm and the vegetal pole is full of yolk. Such a type of egg is called telolecithal type. Each egg is surrounded by vitelline membrane.
The egg gets a coating of jelly-like albumen while passing through the convoluted part of the oviduct. The spermatozoa are highly specialised cells with an oval head containing nucleus, a short neck having centrosome and a long wavy protoplasmic tail.
Fertilization:
Union of male and female gametes takes place externally, i.e., fertilization is external. The female toads lay their pigmented eggs in quiet water inside the weeds or around the stem, leaves within a translucent slimy tube, and the male discharge their spermatozoa or milt over the eggs as they are expelled.
Daniel (1963) reported that in the absence of plants the eggs are laid in long strings at the bottom of the pond or stream.
The outer membrane of the egg gives an impasse to one sperm after which the outer membrane becomes impervious to other sperms. Only the head portion of the spermatozoa enters into the cell-body of ovum and the tail is left out.
The sperm nucleus is called male pro-nucleus, and the egg nucleus is known as female pro-nucleus. During fertilization, the male and female pronuclear fuse to form a single nucleus. The egg, thus fertilized, is known as Zygote.
Embryonic development:
The zygote undergoes rapid divisions known as cleavage (or segmentation) and results in the production of a large number of blastomeres (Fig. 7.26). The cells now arrange to form a cellular ball, known as blastula.
Now the blastula enters into a complicated stage, known as gastrula and the process is known as gastrulation. The gastrulation is essentially a process of cell movement when the different cells take up their respective position for future differentiation.
During this process three primary germinal layers:
(a) Ectoderm,
(b) Mesoderm and
(c) Endoderm are differentiated.
All the structures of the adult are developed out of these three primary germinal layers. After about two weeks a small embryo is seen moving and wriggling. The developing embryo gets nourishment from the yolk and eventually hatches as the tadpole larva.
The larvae spend in aquatic medium for various amounts of time depending on hydro period, thermal range, predation, competition and other related factors. Fig. 7.27 illustrates the larval development and metamorphosis of toad.
10. Enemy of Toad:
Their main enemies are birds, and the snakes – the greatest enemies in India are striped keelback (Amphiesma stolata), checkered keelback water snake (Xenochropis piscator), rat snake (Ptyas mucosus), Indian spectacled cobra (Naja naja naja), saw-scaled viper (Echis carinatus), Indian monoceled cobra (Naja naja kaouthia) and flying snake (Chrysopelea ornata).
The wheels of bicycles, rickshaws and automobiles may be considered a great enemy of the toads. The recent but most dangerous – the effluents of industries are destroying their breeding grounds, their larvae and the abodes of their adults.
Status:
The population is gradually declining, though not alarmingly in the country. Due to heavy use of insecticides, filling up of the breeding grounds for human habitation, sewage pollutants, conversion of low land plots and small water bodies for housing developments, their breeding sites are on the way of destruction.
Another major factor is the use of B. melanostictus in the school and college laboratories for dissection purposes. In this connection, thousands and thousands of toads are killed every year in India.
These causal factors are also responsible for the decline of population in Rana tigerina and R. hexadactyla. It was estimated that in 1979 India earned 4.7 million dollar from the export of frog’s legs (Rana tigerina and R. hexadactyla), despite the ban on the export of frog’s legs in 1977 by the Indian Government.
As an adult frog and toad consume mostly insects, approximately its own weight every day, it can be estimated that 5,000 tonnes of toads and frogs would eat about 4,50,000 tonnes of food, mainly insects in over 90 days period.
So it can be concluded safely that the killing of vast number of toads and frogs must lead to an innumerable increase in- the pest population. The insecticides are then used to the insect infected crops, which in turn pollute environment and upset the ecological balance.
