Rana tigerina, the common Indian bull frog is including in:
Phylum – Chordata,
Sub-phylum – Vertebrata,
Class – Amphibia.
The skin of Rana tigerina is rough and slimy. The dorsal surface is olive coulor the ventral surface is whitish. There are no scales in the skin. Body is bilaterally symmetrical comprising of head and trunk. The neck is absent. There is no tail behind the trunk in adult frog. The head is semi-circular in shape. Anteriorly the head is produced into a snout. Mouth is located at the terminal end of head and is bounded by upper and lower jaws.On the dorsal side of the snout there is a pair of opening called external nostrils.
Two prominent eyes are situated behind the nostril. Each eye has a well-developed, thick and immovable upper eyelid and poorly developed lower eyelid. A nictitating membrane or third eyelid is attached to the lower eyelid. It is transparent membrane that stretched upward across the eye to clean the cornea. Behind the eyes, there is a circular membrane called the drum of the ear or tympanic membrane.
The tympanic membrane covers the cavity of the middle ear on the outside. In case of male, there are two vocal sacs at the base of jaws. These are pouches of skin opening on the floor of the mouth. They function as resonator amplifying the croaking. The trunk is cylindrical in shape. At the posterior end of trunk, there is a small aperture called cloacal aperture through which the faeces, urine and gamates (ova sperm) are expelled to the outside.
The trunk bears a pair of fore limbs and a pair of hind limbs. The hind limbs are much longer and more powerful. The forelimb consists of 3 parts, namely a proximal upper arm or brachium, middle fore arm or antebrachium, and a distal hand or manus.
The hand in turn is divided into 3 parts, namely a wrist (carpus), a palm (metacarpus) and the fingers. In the case of male, a cushion like pad develops on the index finger during breeding season. It is called nuptial pad, which helps to grip the male during ampelxus. The hind limb consists of three parts namely a proximal thigh (femur), a middle shank (crus) and a distal foot (pes). The foot in turn is divided into 3 parts namely an ankle, and instep and the toes.
Sexual dimorphism is a phenomenon in which two sexes are morphologically different. The male is smaller in size. The ventral surface of trunk is smooth in female but rough in male. The male has two vocal sacs located near the ankles of jaws. The males have a nuptial pad in the index finger.