Key Points

In multicellular animals, cells form tissues, tissues form organs, organs form organ systems. This hierarchy allows for division of labour and efficient functioning of the organism.

Frogs belong to Class Amphibia of Phylum Chordata. The common Indian frog is Rana tigrina, which lives both on land and in freshwater.

Frogs are cold-blooded (poikilotherms), meaning their body temperature varies with the environment. They undergo summer sleep (aestivation) and winter sleep (hibernation) to survive extreme temperatures.

Frogs can change their color to hide from enemies, a phenomenon called camouflage. This protective coloration is also known as mimicry.

The frog's body is divided into a head and trunk, lacking a neck and tail. The skin is smooth and moist due to mucus, which aids in respiration.

Frogs have four-digited forelimbs and larger, muscular, five-digited hind limbs with webbed feet for swimming. Key sense organs include bulged eyes with a nictitating membrane and a tympanum for hearing.

Male frogs are distinguished from females by the presence of sound-producing vocal sacs and a copulatory pad on the first digit of the forelimbs.

The alimentary canal is short as frogs are carnivores. It includes the oesophagus, stomach, intestine, and rectum, which opens into a common chamber called the cloaca.

Frogs exhibit dual respiration. In water, they use cutaneous respiration through the skin. On land, they use the buccal cavity and lungs (pulmonary respiration).

The vascular system is a well-developed closed type with a three-chambered heart (two atria, one ventricle). They have hepatic and renal portal systems, and their red blood cells are nucleated.

The excretory system consists of a pair of kidneys, ureters, a urinary bladder, and a cloaca. Frogs excrete urea, making them ureotelic animals.

The nervous system includes a central nervous system (brain and spinal cord), peripheral, and autonomic systems. The brain is divided into a fore-brain, mid-brain, and hind-brain.

Males have a pair of testes attached to the kidneys. Sperms pass through vasa efferentia into Bidder's canal, then through the urinogenital duct to the cloaca.

Females have a pair of ovaries not functionally connected to the kidneys. A pair of oviducts opens separately into the cloaca. A mature female can lay 2500 to 3000 ova.

Fertilization is external and occurs in water. The development involves a larval stage called a tadpole, which undergoes metamorphosis to become an adult frog.

Frogs are beneficial as they eat insects, protecting crops. They are an important link in the food web, maintaining ecological balance.