One Animal, Two Worlds, and the Organs That Straddle Both

In unicellular organisms, a single cell performs every function alone, digestion, respiration, reproduction, all at once. In the complex body of a multicellular animal, those same basic functions instead get carried out by different groups of cells working in organised coordination. A group of similar cells, together with the intercellular substances around them, performing one or more shared functions, is called a tissue; astonishingly, all complex animals are built from just four basic tissue types, organised in specific proportions and patterns to form organs like the stomach, lung, heart or kidney. When two or more organs interact, physically or chemically, to perform a common function, they together form an organ system, the digestive system and respiratory system among the clearest examples. This hierarchy, cells into tissues, tissues into organs, organs into organ systems, exists because it lets an animal's body exhibit genuine division of labour, and division of labour, in turn, is what lets millions or billions of cells act as one coordinated, surviving whole rather than a disorganised mass.

Frogs, belonging to class Amphibia within phylum Chordata, can live both on land and in freshwater; Rana tigrina is the most common species found in India. Unlike warm-blooded animals, frogs are poikilotherms, or cold-blooded, meaning their body temperature simply varies with the surrounding environment rather than being internally regulated. Frogs can also change their skin colour while moving between grass and dry land, a protective colouration called mimicry that helps camouflage them from predators. Frogs are notably absent during peak summer and peak winter, because during these extremes they retreat into deep burrows, entering a summer sleep called aestivation to escape excessive heat, and a winter sleep called hibernation to escape excessive cold. Externally, the frog's smooth, slippery, mucus-covered skin stays permanently moist, coloured olive green with dark irregular spots dorsally and uniformly pale yellow ventrally; remarkably, a frog never drinks water at all, instead absorbing it directly through the skin. The body is divisible into just head and trunk, with no neck or tail; a pair of nostrils sits above the mouth, bulging eyes are protected underwater by a nictitating membrane, and a membranous tympanum on either side of the head receives sound. Forelimbs and hind limbs enable swimming, walking, leaping and burrowing, the larger, more muscular hind limbs ending in five webbed digits for swimming and the forelimbs in four; male frogs are distinguished by sound-producing vocal sacs and a copulatory pad on the first forelimb digit, both entirely absent in females.

The frog's body cavity houses well-developed digestive, circulatory, respiratory, nervous, excretory and reproductive systems. Because frogs are carnivores, the alimentary canal is comparatively short, its intestine reduced in length. Food captured by the bilobed tongue enters the mouth, passes through the buccal cavity, pharynx and a short oesophagus, and reaches the stomach, where hydrochloric acid and gastric juices break it down into a partially digested mixture called chyme. Chyme then passes to the duodenum, the first part of the small intestine, which receives bile from the gall bladder, secreted by the liver, and pancreatic juice from the pancreas via a common bile duct; bile emulsifies fat, while pancreatic juices digest carbohydrates and proteins, with final digestion completed in the intestine. Digested food is absorbed through numerous finger-like folds lining the intestine's inner wall, the villi and microvilli, while undigested solid waste moves into the rectum and exits through the cloaca, a single common opening shared with the excretory and reproductive systems.

Frogs respire using two genuinely different methods depending on where they are. In water, the skin itself acts as an aquatic respiratory organ, called cutaneous respiration, with dissolved oxygen exchanged through it by simple diffusion; this only works because the skin stays permanently moist and highly vascularised. On land, the buccal cavity, skin and lungs together act as respiratory organs, with lung-based gas exchange called pulmonary respiration. The lungs themselves are a pair of elongated, pink, sac-like structures in the upper trunk, air entering through the nostrils into the buccal cavity before reaching them. During aestivation and hibernation specifically, when the frog is sealed inside a burrow and not actively breathing air, gaseous exchange happens exclusively through the skin, meaning cutaneous respiration is not merely a backup method but the frog's sole lifeline during both of its dormant seasons.

The frog's vascular system is a well-developed closed type, involving heart, blood vessels and blood, alongside a separate lymphatic system of lymph, lymph channels and lymph nodes. The heart, a muscular structure covered by the pericardium membrane, has three chambers, two atria and one ventricle; a triangular sinus venosus joins the right atrium, receiving blood through the major veins, the vena cava, while the ventricle opens into a sac-like conus arteriosus on its ventral side. Arteries carry blood from the heart to the whole body as the arterial system, while veins collect blood back to the heart as the venous system, with two special venous detours worth noting: the hepatic portal system, a direct venous connection between liver and intestine, and the renal portal system, a similar connection between kidney and the lower body. Blood itself is composed of plasma and cells, nucleated red blood cells, or erythrocytes, carrying the pigment haemoglobin, white blood cells, or leucocytes, and platelets; lymph differs from blood mainly in lacking some proteins and red blood cells. Blood carries nutrients, gases and water throughout the body, its circulation driven entirely by the pumping action of the muscular heart.

The frog's excretory system consists of a pair of compact, dark red, bean-like kidneys, positioned posteriorly on either side of the vertebral column, each composed of numerous structural and functional units called uriniferous tubules, or nephrons. In males, two ureters emerge from the kidneys and double as a urinogenital duct opening into the cloaca; in females, ureters and oviduct open separately into the cloaca instead. A thin-walled urinary bladder, ventral to the rectum, also opens into the cloaca. Because frogs excrete nitrogenous waste mainly as urea, they are classified as ureotelic. Coordination in the frog relies on both a neural system and a set of endocrine glands, pituitary, thyroid, parathyroid, thymus, pineal, pancreatic islets, adrenals and gonads, secreting hormones. The nervous system organises into central (brain and spinal cord), peripheral (ten pairs of cranial nerves plus spinal nerves) and autonomic (sympathetic and parasympathetic) divisions; the brain, enclosed in a bony cranium, divides into forebrain (olfactory lobes, cerebral hemispheres, diencephalon), midbrain (a pair of optic lobes) and hindbrain (cerebellum and medulla oblongata, which continues into the spinal cord). Sense organs cover touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with internal ears); of these, only the eyes and internal ears are well-organised structures, the rest being simple cellular aggregations around nerve endings, and the ear itself does double duty as both a hearing organ and a balance organ.

Frogs have well-organised, separate male and female reproductive systems. Male reproductive organs consist of a pair of yellowish, ovoid testes, adhered to the upper kidneys by a double fold of peritoneum called the mesorchium; ten to twelve vasa efferentia arise from the testes, enter the kidneys, and open into Bidder's canal, which communicates with the urinogenital duct emerging from the kidneys and opening into the cloaca, the single small chamber through which faecal matter, urine and sperm all exit. Female reproductive organs consist of a pair of ovaries, situated near the kidneys but with no functional connection to them, and a pair of oviducts arising from the ovaries and opening separately into the cloaca. A mature female frog can lay 2,500 to 3,000 ova at a time; fertilisation is entirely external, taking place in water, and development passes through a larval stage called the tadpole, which undergoes metamorphosis to become the adult frog. Beyond their own biology, frogs matter ecologically and economically too: they eat large numbers of insects, protecting crops, and serve as an important link in food chains and food webs, while in some countries their muscular hind legs are eaten as food.