Frog belongs to the Phylum Chordata, Subphylum Vertebrata or Chordata, Subphylum Vertebrata or Craniata, Superclass Gnathostomata, Class Amphibia and Genus Rana. The most common species is known as Rana tigrina. The scientific name of common toad is Bufo melanostictus. The most common frog found in India is the Indian bullfrog. It is the largest frog and is named as bullfrog because of its large size and loud call. Indian bullfrog is found in freshwater marshes, ditches, ponds, and shallow lakes. They are also reported from running waters, and are seen in large numbers during rainy season as they breed during this period. They undergo aestivation (summer sleep) in summer and hibernation (winter sleep) in winter. During scorching summer they protect themselves by burrowing in damp and soft soil. They rest in a squatting posture, i.e. hind-limbs folded inside and fore-limbs upright. They are carnivorous (feeding upon other animals, insects, etc.), poikilotherms i.e. the body temperature changes with environment. They develop protective colouration to camouflage, that is, to hide in surroundings.
Frogs exhibit sexual dimorphism. Male and female are distinguishable externally only during breeding season when the males develop nuptial pad in the thumb. Vocal sac in males produces louder sound as compared to the females, which are devoid of vocal sac.
The mouth is present as a terminal, wide opening. It opens into buccopharyngeal cavity, which contains numerous maxillary teeth arranged along the margin of the upper jaw and vomerine teeth present in the roof of the bucco-pharyngeal cavity. The lower jaw is toothless. Opening of eustachian tube, vocal sac (only in male) gullet and glottis can be seen clearly in the bucco-pharyngeal cavity. The muscular tongue is bilobed at the tip and free from behind. It is used for capturing the prey.
The gullet opens in narrow and short tube-like oesophagus, which continues in large and distended stomach. It contains a thick muscular layer, which helps in converting food into chyme. It secretes gastric juice containing HCl and proteolytic enzymes. Stomach is followed by a coiled small intestine. Intestinal wall has numerous finger-like folds, called villi and microvilli, projecting into its lumen to enhance surface area of absorption for digested food. The first part of the small intestine laying parallel to stomach is called duodenum. Intestine continues into a wider rectum, opening into cloacea. The urinary bladder opens into cloacal chamber through the ureter.
The gastric and intestinal glands occur in the walls of stomach and intestine, respectively. The other important digestive glands associated with the alimentary canal are liver and pancreas. Liver secretes bile, which is temporarily stored in gall bladder before being released into the duodenum. Bile helps in digestion of food by changing its pH from acidic to alkaline and by emulsifying the fat. Liver does not secrete any digestive enzyme. Pancreas is an irregular, elongated gland, situated in a thin mesentery and lies parallel to the stomach. It produces pancreatic juice containing digestive enzymes like trypsin, amylopsin, etc.
Fig: Alimentary Canal of Frog
Frogs respire both on land as well as in water. Skin acts as an aquatic respiratory organ. Exchange of gases takes place by diffusion. Buccal cavity and lungs act as aerial respiratory organs. The respiration by lungs is called pulmonary respiration. The lungs are a pair of elongated, pear-shaped, pink coloured sac-like structures present in thoracic (upper) part of the trunk region. During aestivation and hibernation, they respire by skin (cutaneous respiration).
The circulatory system of frog consists of blood vascular system and lymphatic system. It is of closed type and represents single circulation, which means that both the oxygenated and the deoxygenated blood enters the heart and get mixed in the ventricle. The blood vascular system comprises heart, blood vessels and blood, whereas the lymphatic system consists of lymph, lymph channels and lymph nodes. Both systems are inter-connected because lymph channels open into large venous channels. Heart is a three chambered, muscular structure situated in the upper part of the body cavity. A thin membrane called pericardium covers it. It has two upper chambers (auricles) which open separately into a single lower chamber, ventricle. A short sac-like conus arteriosus is present on the ventral side of the heart over the larger right auricle. On dorsal side, a triangular structure, called sinus venosus is present in which all the three main vena cava (right and left anterior and posterior) are open. A definitely arranged network of arteries and veins form the arterial and venous systems. Frogs also possess two well developed portal systems: renal portal and hepatic portal systems. In these the concerning veins originate from intestine and break up into fine capillaries in the respective organs, kidney (renal portal system). Three types of blood corpuscles are present in plasma, viz. RBC (red blood corpuscles) or erythrocytes, WBC (white blood corpuscles) or leucocytes and thrombocytes. RBC's contain red-coloured respiratory pigment namely haemoglobin. RBCs are nucleated. The lymph is a filtered out fluid from blood capillaries. It is different from blood, lacking a few proteins and RBCs.
The nervous system is organised into a central nervous system (brain and spinal cord), a peripheral nervous system (cranial and spinal nerves) and an autonomic nervous system. There are ten pairs of cranial nerves. Brain is enclosed in a bony structure or brain box, which has two occipital condyles for attachment with the first vertebra (atlas). The brain is divided into fore-brain, mid-brain and hind-brain. Fore-brain includes olfactory lobes, paired cerebral hemispheres and unpaired diencephalon. The mid-brain is characterised by a pair of optic lobes. Hind-brain consists of cerebellum and medulla oblongata. The medulla oblongata passes out through the foramen magnum and continues into spinal cord, which is contained in the vertebral column.
Frog has five types of sense organs, namely organs of touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum and internal ears). Out of these, eyes and internal ears are well-organised structures and the rest are cellular aggregations around nerve endings. Eyes in a frog are a pair of spherical structures situated in the orbit. These are simple eyes (possessing only one unit). The wall of eye is composed of three layers, sclerotic, choroid and retina. The cornea is transparent. External ear is absent in frogs and only tympanum can be seen externally. Middle ear, named as tympanic cavity, is filled with air. It passes vibrations to internal ear or membranous labyrinth. It is present in a fluid-filled otic or auditory capsule. It has two sac-like bodies (utriculus and sacculus) and three semicircular canals. The ear is an organ of hearing as well as balancing (equilibrium).
The main organ of excretion is a pair of kidneys. These are compact, dark red and bean like structures situated little posteriorly in the body cavity on both sides of vertebral column. The frog excretes urea thus, is a ureotelic animal. It is carried by blood into the kidney where it is separated and excreted. Each kidney is composed of several structural and functional units called uriniferous tubules or nephrons. Ureter emerges from the kidney as urinogenital duct in the male. A common ureter opens into the cloaca. A thin-walled urinary bladder is present ventral to rectum, which also open in the cloaca.
The chemical coordination of various organs of the body is manifested by hormones, which are secretions of the endocrine glands. The prominent endocrine glands found in a frog are pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.
Male reproductive organs consist of a pair of yellowish ovoid testes, which are found adhered to the upper part of kidneys by a double fold of peritoneum called mesorchium. Vasa efferentia are 10-12 in number and after arising from testes run through the mesorchium and enter the kidneys of their side. In kidneys, these open into Bidder's canal, which finally communicate with the urinogenital duct, which comes out of the kidneys and finally opens into the cloaca. The cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms to the exterior.
Fig: Male Reproductive System of Frog
A pair of ovaries, situated near kidneys, comprises the female reproductive organs. However, these have no functional connection with kidneys. A pair of oviduct opens into the cloaca, separately. Ovaries produce ova by oogenesis. A mature female can lay 2500 to 3000 ova at a time. Fertilisation is external. Development is indirect involving a larval stage called tadpole.
Fig: Female Reproductive System of Frog
Interaction with Mankind
Frog is a beneficial animal for human-being. Frog eats up insects and thus protects the crop. This saves expenditure on insecticides. Frog has been used as an experimental material for teaching and research. Frogs maintain ecological balance because these serve as an important food link in ecosystem. Due to over use of pesticides, insecticides and other agro-chemcials they are being vanished at a fast rate causing loss to biodiversity.