Frog is an amphibian, living both on land and in water. The amphibians are the first group among the chordates that live outside water. But the amphibian is still chained to water, in water it is born, to water it must periodically return for respiration. They are cold-blooded vertebrates having two pairs of limbs- tetrapods. They have smooth and moist skin. Rana tigrina is studied in detail because of its easy availability and its structural details have very common features to other animals.
Habitat: Frogs lead dual (amphibious) mode of life. They can live on land and in water. They are cosmopolitan in distribution. They are usually found in fresh water ponds, rivers, ditches, under-stones and in damp places except in arid parts of the country. It lives near water probably to carry out the cutaneous respiration by keeping the skin moist as soon as it gets dry and to escape from the enemies by slipping or jumping into the water. In rainy season it may be found far away from its aquatic home. They remain active in spring and rainy seasons. They become inactive both in winter and in summer seasons. Frog is usually known as "indian bull frog" because of its large size and loud voice.
Habits: Frogs characteristically have squat bodies, smooth skin, strong hind legs for leaping and webbed feet. Most reproduce in water, laying eggs that develop into larvae (tadpoles). It shows following habits:
a) Resting: Frogs rest on land in a squatting posture. They keep their short fore limbs upright and the long hind limbs fold inside the body. On any disturbance they suddenly jump in water by extending their hind limbs to escape from enemies.
b) Feeding: Frog is a carnivorous animal. It feeds on earthworms, insects, spiders, snails and tadpoles. However, tadpoles feed on aquatic plants, i.e. they are herbivores. The frog is a cold-blooded or poikilothermic animal which means its body temperature fluctuates with that of the environment. Therefore it cannot bear very cold or very hot temperature. The frog needs hibernation (winter-sleep) and aestivation (summer sleep) to tide over the unfavourable conditions.
c) Hibernation (winter sleep): During winter the frogs bury themselves deep into the mud and take rest. This is called winter-sleep or hibernation. In winter the metabolism is slow and its temperature falls down. Therefore, the frogs become sluggish and inactive. Frog does not respire with the lungs, respiration takes place through skin. The general vital activities of the animal are so low that very little expenditure of the energy is needed to maintain life. Frog does not take any food but consume energy stored in their body in the form of glycogen. They remain inside till the next spring. Similarly frogs needs summer-sleep or aestivation to tide over the drought. They bury themselves in the soft mud and become inactive and sluggish.
d) Locomotion: Locomotion in frog takes place by two methods: (i) leaping on land and (ii) swimming in water.
i) Leaping: While resting on land, the frog keeps its short fore limbs upright and the long hind limbs folded like "Z". The frog jumps by a sudden extension of the hind limbs which act like springs throwing the body up into air. The fore limbs are used to hold up the anterior part of the body and to manipulate the direction, foe which it adjusts its body in a new direction before each leap. A frog may leap a distance of 1.5 to 2 meters in a single jump.
ii) Swimming: The frog swims in water by alternately flexing and extending its hind limbs. As the animal starts to swim the legs are extended quickly. During their backward strokes, the toes are spread apart and the broad webs push against water, moving the body forward. The fore limbs serve two purposes: they help in propelling the animal to some extent and in guiding the direction of the movement.
e) Croaking: The characteristic sound made by frog during breeding season is called croaking. It is a mating call. It is produced by forcing air from lungs over vocal cords into mouth cavity and back again. It is louder in males than in females due to presence of vocal sacs. Male frog croaks to attract the female frogs for copulation.
f) Breeding: Frog breeds during rainy season from the end of June to September. During rainy season male frogs croak to attract females for copulation. It is a mating call. Copulation takes place in shallow water, where they lay down eggs for fertilization. There is no parental care.
Shape and Size: The body of the frog is bilaterally symmetrical. It is spindle shaped, pointed anteriorly and rounded posteriorly. It is some what dorsoventrally flattened and streamlined which offer least resistance during swimming. A light yellow line runs mid dorsally from tip of the snout to the cloacal aperture, called the mid-dorsal line. It measures about 12 to 18 cm in length and 5 to 8 cm in width.
Skin and Colour: The skin forms the outermost layer of the body of frog. It is loose, smooth , moist and slippery without any derivatives like scales, feather and hairs. It is attached to underlying muscles only at intervals by connective tissue septa. Due to loose attachment of skin, spaces are formed between the skin and muscles called subcutaneous spaces filled with lymph. Therefore, frog is very easily skinned. The skin of frog is thick on the dorsal surface and thin on the abdomen or ventral surface. At places the skin is thrown out into a number of folds extending from behind the eyes. These folds are called dermal plicae. On the dorsal side the colour of the skin is dark-green with patches of black colour, while it is pale yellow on the ventral side. The integument with its various derivatives in different group of vertebrates, constitutes the integumentary system. The basic function of integument is to cover and protect the body from various environmental hazards. The skin of frog is different from other vertebrates. In addition to usual functions such as protection and covering, it also takes part in cutaneous respiration. To perform this function efficiently, the skin of frog is kept moist even when the animal is out of water.
Division of Body: The body is divided into two regions: the head and the trunk. The neck and tail are absent. Tail is present only in larval stage. The head and trunk are broadly jointed.
Head: The head is triangular in shape jointed directly to the trunk. It is narrow in front and broad behind. The anterior narrow blunt end is known as snout. It bears mouth, nostrils, eyes and tympanum.
Mouth: A wide opening called mouth is situated at the anterior end of the snout. It is a wide transverse opening, which extends nearly from one ear drum to the other. It is bounded by upper and lower jaws. It has no lips and cheeks.
Nostrils: There is a pair of small openings called nostrils or external nares on the anterior tip of the snout. Each narial opening is guarded by a membranous valve and leads into the buccal cavity through internal nares. They allow only air to pass in or out during breathing.
Eyes: There are two large prominent eyes on each side of the head. The eyes are round, protuberant and dorso laterally placed, on the top of head. As the eyes are large, protuberant and laterally situated, they fulfill the absence of neck and frog can see all around without moving the body. Each eye is protected by two eyelids, the upper eyelid is thick pigmented and immovable, but the lower eyelid is thin, semi-transparent and capable of free movement. A third eyelid or nictitating membrane is present as an outgrowth of the lower lid. It is transparent and freely movable. It covers and protects the eye during swimming and keep it moist in air. In the middle of the head, just in front of the eyes, there is a light coloured patch - the brown spot which represents third eye of the ancestral frogs.
Tympanum: Behind and below the eye, on either side, there is a circular obliquely placed membrane, the eardrum or the tympanum. There is no external ear or pinna. The tympanum receives sound vibrations from the environment. In male frog, under the mouth, there is a pair of bluish patches of skin called vocal sacs. The vocal sacs help in croaking. The male croaks more loudly than the female. The croaking of frogs appear to be a call for mating during rainy season.
Trunk: The posterior portion of the body consists of somewhat flattened trunk. Its back is raised in the middle as a characteristic sacral prominence or hump. At the posterior end, the trunk contains an opening, the cloacal aperture between the two legs. It is a common aperture through which faeces, urine and reproductive products (sperms and ova) are discharged.
Limbs: The limbs are lateral in position. The trunk bears two pairs of limbs. The fore limbs are shorter and arise anteriorly from the trunk just behind the head. It consists of three distinct parts: the upper arm (brachium), the forearm (antebrachium) and the hand or the manus. The hand consists of a palm and four fingers without web. The base of first inner finger in male frog has a dark rough swelling known as nuptial or copulatory pad. Specially in breeding season the copulatory pad becomes very thick and sticky. They are used for grasping the female in amplexus. The hind limbs are longer and much powerful than the forelimb. It arises close together posteriorly from trunk. It consists of upper thigh, middle shank and the terminal foot or pes. The foot is further differentiated into ankle, metatarsus and five webbed toes. The greatly elongated, powerful and webbed legs of the frog are admirably adapted for jumping and swimming. The fore limbs are meant to hold and support the front part of the body at the time of jumping but the hind limbs assist in swimming as webs are present in between the toes.
Sexual Dimorphism: Frog shows sexual dimorphism i.e. the male and female can be distinguished by their morphological characters:
i) The males are usually smaller and darker in colour than the female.
ii) Males are slimmer while the females are stouter.
iii) The male frog has vocal sacs which help in croaking loudly. The female has no vocal sac.
iv) The forelimbs in male frog possess swollen copulatory pad on the first inner finger. In breeding season the copulatory pad becomes very thick and sticky. It helps in grasping the female. It is not found in female.
Histology of Skin: Like other vertebrates, the skin of frog consists of two layers: the outer epidermis and inner dermis.
Epidermis: Epidermis is the outermost later of skin. It is thin, non-vascularized and stratified layer. It is composed of several layers of epithelial cells, which are ectodermal in origin. Epidermis can be distinguished into stratum corneum and stratum malpighii layer. The stratum corneum is the outermost layer. It is made of flattened, squamous epithelial cells arranged in four to five layers. The outer layer of the stratum corneum consists of keratinized cells (dead cells). The keratinized cells are shed off periodically and replaced by cells from the underlying layer. The shed off layer is eaten by the frog. The stratum malphighii is the innermost layer of the epidermis. It is composed of active columnar cells, resting on a thin basement membrane. The columnar cells get nourishment from the blood vessels of underlying dermis. They grow and divide mitotically to produce new cells of the stratum corneum, which are pushed towards the surface. During their journey to the surface, the cells become flattened and then scale like. During flattening the proteins of their protoplasm change into keratohyaline which appears as minute refractile granules. The keratohyaline soon gets dissolved in the protoplasm and converts into eledin. Finally, when the cells reach to the outer surface of the skin, they lose water and their protoplasm changes into a hard, tough, insoluble, fibrous horny protein called keratin. With the formation of keratin, the cells become dead and are said to be keratinized. The keratinized cells shed off periodically. The keratinization makes the epidermis impervious to fluids other than water. It also checks excessive intake and loss of water from the body. The periodical shedding off of the stratum corneum checks entry of bacterias and harmful substances into the body. As the keratin is formed from the unwanted protein, the process of keratinization helps in thhe removal of such proteins.
Dermis: Dermis is the inner layer of the skin. It is thick, vascularised and consists of the connective tissue, muscle fibres, nerve fibres, blood vessels, capillaries, chromatophores and cutaneous glands. The dermis is mesodermal in origin. It consists of two layers - stratum spongoiosum and stratum compactum. The stratum spongiosum is a loose spongy layer lying just under the basement membrane. It consists of connective tissue, fibres, lymph spaces, blood capillaries, blood vessels, nerve fibres, cutaneous glands and chromatophores. The cutaneous glands are of two types: the mucous glands and the poison glands. These glands are the derivatives of the epidermis but they lie in the dermis. they open to the exterior by narrow ducts lined with a layer of small flattened epithelial cells. the mucous glands are small, flask-shaped, more numerous, uniformly distributed over the entire surface of the body. The poison glands are large and fewer in number. They are less evenly distributed. The mucous glands secrete a colourless watery fluid that keeps the skin moist and sticky. The poison glands secrete a milky white and probably poisonous substance which serves to protect it from enemies. There are three types of pigment cells or chromatophores present in dermis. They are:
i) Melanophores containing black pigment called melanin.
ii) Xenthophores containing yellow pigment called xanthophyll.
iii) Guanophores containing a white refracting pigment called guanine.
These pigment provide specific colouration to the skin. The dark green colour of the dorsal surface of skin of frog is a combined effect of all these pigments, while the black patches are due to the concentration of melanin only. The animals like frog, toads, salamanders, some fishes and lizards respond to changes in their surrounding environment by reversible change of skin colour. This ability of animals is called metachrosis. The stratum compactum is made up of compact fibrous connective tissue, smooth muscle fibres and blood vessels and capillaries. It does not contain cutaneous glands but may have few chromatophores.
Functions of the Skin:
1) The skin provides a protective covering over the body. it also protects the frog against the invasion of many germs.
2) It exhibits protective colouration to the body.
3) The mucous glands secrete a colourless watery fluid that keeps the skin moist and sticky.
4) The moist surface of skin helps in exchange of respiratory gases between the environment and the body. Thus, it takes part in respiration.
5) The keratinized cells shed off periodically and helps in removing certain wastes of the body.
6) The poison glands secrete poisonous substance which serves to protect it from enemies.
7) Frogs never drink water through mouth but they absorb water through their skin.
8) It acts as an important sensory organ.
9) The membrane bones usually found in the skull are formed of the connective tissue sheaths of dermis.
10) The skin of the embryos of frog produces hatching enzymes, which help in hatching by dissolving the egg membranes.