In India, the most common species of frog is Rana tigrina. The frogs are cold-blooded or poikilotherms. They have the ability to camouflage. The frogs also show mimicry as a tool for protection. During summers, the frogs live in summer sleep (aestivation) and during winters, they live in winter sleep (hibernation).
The skin of frog is moist and slippery due to the presence of mucus. The dorsal side of body is usually olive green with dark irregular spots. The skin on the ventral side is uniformly pale yellow.
The body of a frog is divisible into head and trunk. A pair of nostrils is present above the mouth. Eyes are bulged and covered by a nictitating membrane. These membranes protect the eyes while the frog is under water. Ears are represented by membranous tympanum on either side of the eyes.
The forelimbs and hind limbs help in swimming, walking, leaping and burrowing. The hind limbs have five digits. The hind limbs are large and more muscular than forelimbs. The forelimbs have four digits. Webbed digits help in swimming. Sexual dimorphism is present in frogs. Sound producing vocal sacs and a copulatory pad (on the first digit of the fore limb) are present in male frogs.
The alimentary canal is short because frogs are carnivorous. The alimentary canal is composed of buccal cavity, pharynx, oesophagus, stomach, intestine and rectum. The rectum opens out by cloaca. Liver produces bile and the pancreas produces pancreatic juice. Digestive enzymes are present in the pancreatic juice. The bilobed tongue helps in capturing prey.
Gastric juices and HCl are secreted in the stomach; where partial digestion of food takes place. Bile and pancreatic juice are received in the duodenum. Bile emulsifies fat. Pancreatic juices digest carbohydrates and protein. Final digestion takes place in the intestine.
Numerous finger-like folds are present in the inner wall of intestine. These are called villi and microvilli and facilitate absorption of food. The undigested food goes to the rectum from where it is expelled out through cloaca.
Frogs respire through lungs when they are on land. The exchange of gases takes place through skin when the frog is in water. The lungs are a pair of elongated, pink-coloured, sac-like structures. Lungs are present in the upper part of the thorax. During aestivation and hibernation, exchange of gases takes place through skin.
Blood Vascular System
The vascular system is closed type and is well developed. Lymphatic system is also present in frogs. The blood vascular system of frog is composed of a heart, blood vessels and blood. The lymphatic system consists of lymph, lymph channels and lymph nodes.
The heart is situated in the upper part of the body cavity. There are three chambers in the heart of frog. There are two atria and one ventricle. The heart is covered by a membrane; called pericardium. A triangular structure; called sinus venosus; joins the right atrium. Blood from the vena cava reaches the sinus venosus. The ventricle opens into a sac-like conus arteriosus on the ventral side of the heart.
Arteries and Veins
Arteries carry blood from the heart to all parts of the body. The veins carry blood from all parts of the body to the heart. Hepatic portal system and renal portal system are present in frogs. The hepatic portal system is a system of special venous connection between liver and intestine. The renal portal system is a system of special venous connection between the kidneys and the lower parts of the body.
The blood is composed of plasma and cells. RBCs and WBCs and platelets are present in the blood of frogs. RBCs are nucleated and contain haemoglobin. Lymph lacks few proteins and RBCs and hence is different from blood.