Phylum : Chordata
Class : Amphibia
Genus : Rana
Species : tigrina
· Rana tigrina is the most common species in India.
· They are poikilotherms (cold blooded).
· They can change colour to hide them from their enemies (camouflage). This protective coloration is called mimicry.
· During summer and winter, they undergo aestivation (summer sleep) and hibernation (winter sleep) respectively to protect them from extreme heat and cold.
· Body is divisible into head & trunk. Neck and tail absent.
· Skin is moist, smooth and slippery due to the mucus.
· Colour of dorsal side is olive green with dark irregular spots and ventral side is pale yellow.
· The frog never drinks water but absorb it through the skin.
· A mouth, paired nostrils and bulged eyes (covered by nictitating membrane) are present.
· On either side of eyes have a membranous tympanum (ear).
· The forelimbs (4 digits) and hind limbs (5 digits) help in swimming, walking, leaping and burrowing. The hind limbs are larger and muscular than fore limbs.
· Feet have webbed digits that help in swimming.
· Frogs exhibit sexual dimorphism. Male frogs have sound producing vocal sac and also a copulatory (nuptial) pad on the first digit of fore limbs which are absent in female frogs.
- Consists of alimentary canal and digestive glands.
- The alimentary canal is short because frogs are carnivores and hence the length of intestine is reduced.
- Mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca.
- Liver secretes bile that is stored in gall bladder. Pancreas produces pancreatic juice containing digestive enzymes.
- Food is captured by the bilobed tongue.
- Digestion: Gastric juice and HCl secreted from gastric wall digest the food. Partially digested food (chyme) is passed from stomach to the duodenum.
Duodenum receives bile and pancreatic juices through a common bile duct.
Bile emulsifies fat. Pancreatic juice digests carbohydrates and proteins. Digestion completes in the intestine.
- Finger-like villi and microvilli in intestine absorb digested food. The undigested solid waste moves into the rectum and passes out through cloaca.
- Skin acts as aquatic respiratory organ (cutaneous respiration). Dissolved oxygen in the water is exchanged through the skin by diffusion. During aestivation and hibernation respiration takes place through skin.
- On land, the buccal cavity, skin and lungs (pulmonary respiration) act as the respiratory organs.
- The lungs are a pair of elongated, pink coloured sac-like structures present in the thorax. Air enters through the nostrils into the buccal cavity and then to lungs.
- Closed type. Includes Blood vascular system (heart, blood vessels & blood) and lymphatic system (lymph, lymph channels & lymph nodes).
- Heart is 3-chambered, (two atria and one ventricle) and is covered by a membrane called pericardium.
- A triangular structure called sinus venosus joins the right atrium. It receives blood through major veins (vena cava).
- The ventricle opens into a saclike conus arteriosus on the ventral side of the heart.
- The blood pumped from the muscular heart is carried to all parts of the body by the arteries (arterial system).
- The veins collect blood from different parts of body to the heart and form the venous system.
- Hepatic portal system (venous connection between liver and intestine) and renal portal system (between kidney and lower parts of the body) are present in frogs.
- Blood contains plasma and cells (RBC, WBC & platelets). RBCs are nucleated and contain haemoglobin.
- Blood transports nutrients, gases and water to tissues.
- Includes kidneys (2), ureters (2), cloaca & urinary bladder.
- Kidneys are dark red and bean-shaped. Found posteriorly in the body cavity on both sides of vertebral column. Each kidney is formed of uriniferous tubules (nephrons).
- 2 ureters emerge from the kidneys. In male frogs, the ureters act as urinogenital duct which opens into cloaca.
In females, ureters & oviduct open separately in cloaca.
- The thin-walled urinary bladder is present ventral to the rectum which also opens in the cloaca.
- The frog is a ureotelic animal (excretes urea). Nitrogenous wastes are carried by blood into the kidney where it is separated and excreted.
Control and co-ordination
- The endocrine glands secrete hormones.
- Endocrine glands: pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals & gonads.
§ Central nervous system (brain & spinal cord),
§ Peripheral nervous system (cranial & spinal nerves)
§ Autonomic nervous system (sympathetic & parasympathetic).
- There are 10 pairs of cranial nerves arising from brain.
- Brain is enclosed in a bony brain box (cranium).
- The brain is divided into
§ Fore-brain: Includes olfactory lobes, paired cerebral hemispheres and unpaired diencephalon.
§ Mid-brain: Includes a pair of optic lobes.
§ Hind-brain: Includes cerebellum & medulla oblongata.
- Medulla oblongata passes out through the foramen magnum and continues into spinal cord, which is enclosed in the vertebral column.
- Sense organs include organs of
§ Sensory papillae: For touch
§ Taste buds: For taste
§ Nasal epithelium: For smell
§ Simple eyes: For vision. Paired and situated in orbit
§ Tympanum with internal ears: For hearing and balancing (equilibrium).
- 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 (mesorchium).
- Vasa efferentia (10-12 in number) arise from testes. They enter the kidneys on their side and open into Bidder’s canal. It communicates with urinogenital duct that comes out of the kidneys and opens into cloaca.
- The cloaca is a small, median chamber that is used to pass faecal matter, urine and sperms to the exterior.
- The female reproductive organs include a pair of ovaries. The ovaries are situated near kidneys and there is no functional connection with kidneys.
- A pair of oviduct arising from the ovaries opens into the cloaca separately.
- A mature female can lay 2500 to 3000 ova at a time.
- Fertilisation is external and takes place in water.
- Development involves a larval stage called tadpole.
- Tadpole undergoes metamorphosis to form the adult.
- Frogs are beneficial for mankind because they eat insects and protect the crop.
- Maintain ecological balance by serving as an important link of food chain and food web in the ecosystem.
- In some countries the muscular legs of frog are used as food by man.