Amphibia (amphibious = double life) live in two worlds — water (larval stage) and land (adult stage). Key features: ectothermic (cold-blooded, poikilothermic). Moist, glandular skin (no scales) — skin breathing (cutaneous respiration). Two pairs of limbs (except caecilians — limbless). Three-chambered heart (2 auricles + 1 ventricle). Double circulation (incomplete — oxygenated and deoxygenated blood mix in single ventricle). External fertilisation (in water), oviparous. Larval stage (tadpole) — aquatic, gill breathing. Metamorphosis to adult — lung + skin breathing. Examples: Rana tigrina (Indian bullfrog), Bufo (toad), Hyla (tree frog), Salamandra (salamander), Ichthyophis (limbless caecilian). Amphibia were the first vertebrates to colonise land (~365 mya). They never fully left water — must return to breed.

External features of frog (Rana tigrina): Head: triangular, continuous with trunk (no neck). Eyes: large, prominent, on top of head for 360 degree vision. Protected by nictitating membrane (transparent third eyelid). Tympanic membrane (eardrum): circular membrane behind each eye — detects sound vibrations. Nostrils (nares): on top of snout — can breathe at water surface while submerged. Mouth: wide gape for capturing prey. Skin: smooth, moist, glandular. Contains mucous glands (keeps skin moist) and granular/poison glands (especially in toads). Dorsal surface: olive green with dark irregular spots (camouflage). Ventral surface: paler. Limbs: forelimbs — short, 4 digits (tetradactyl). Hind limbs — longer (for jumping and swimming), 5 webbed digits. Cloaca: common opening for urinary, reproductive, and digestive systems. Sexual dimorphism: males have vocal sacs and copulatory pads, females are generally larger.

Reproduction in frogs is aquatic. Breeding season: during monsoon/wet season. Male calls using vocal sacs to attract females. Amplexus: male grips female behind forelimbs (axillary amplexus in Rana). Female lays eggs in water as male simultaneously releases sperm → external fertilisation. Eggs: covered in jelly coat (protects against desiccation, pathogens). Laid in clumps (Rana) or strings (Bufo). Tadpole larva: herbivorous. Gills for breathing. Tail for swimming. Gradually metamorphoses into adult (metamorphosis regulated by thyroid hormone, T3). Adult: carnivorous (insects, worms, small animals). Lungs replace gills. Limbs develop (hind limbs first). Tail absorbed. Unique: Rana bombina (fire-bellied toad) shows parental care. Darwin frog (Rhinoderma darwinii): male keeps eggs in vocal sac until metamorphosis — unusual parental care. Most frogs: no parental care after egg laying.

Frog has a 3-chambered heart: 2 auricles (right and left) + 1 ventricle. Right auricle: receives deoxygenated blood from body (via right precaval and postcaval veins). Left auricle: receives oxygenated blood from lungs and skin (via pulmonary veins). Ventricle: single, receives mixed blood from both auricles. Spongy internal structure (columnae carneae) partially separates blood streams. Conus arteriosus: thick-walled, contains spiral valve that partially directs blood. Pulmo-cutaneous artery: carries blood to lungs and skin. Systemic aorta: carries blood to body. Portal systems: hepatic portal (gut → liver). Renal portal (body → kidneys → back to heart). RBCs: nucleated in frogs (unlike non-nucleated RBCs in mammals). Largest RBCs: Protopterus (lungfish). Smallest RBCs: musk deer. Comparison: fish = 2-chambered, amphibia = 3-chambered, reptiles = 3-chambered (with partial septum in ventricle), birds + mammals = 4-chambered.

Central nervous system: Brain: enclosed in cranium (skull). 10 pairs of cranial nerves (not 12 — that is mammals). Parts of brain: olfactory lobes (forebrain, smell), cerebral hemispheres (forebrain), diencephalon (thalamus, hypothalamus), optic lobes (midbrain, vision), cerebellum (very small in frogs — limited coordination needs), medulla oblongata (hindbrain). Spinal cord: protected by vertebral column. 10 pairs of spinal nerves. Peripheral nervous system: somatic (voluntary) and autonomic (involuntary). Autonomic: sympathetic and parasympathetic. Sense organs: eyes (no eyelids in fish, nictitating membrane in frogs), tympanic membrane (hearing), nasal chambers (olfaction), tongue (taste), lateral line system — absent in adult frogs (present in tadpoles and aquatic amphibians).

Frogs are ureotelic (excrete urea as main nitrogenous waste — like humans). Not ammoniotelic (fish, aquatic organisms) or uricotelic (birds, reptiles, insects). Kidneys: mesonephric (opisthonephric) kidneys — functional kidney type. Located dorsolaterally. Nephridia not present (those are in annelids and other invertebrates). Urine production: glomerular filtration → tubular reabsorption → urine. Ureters: carry urine from kidneys to cloaca (in male) or cloaca (in female — ureters and oviducts open separately into cloaca in females). Urinary bladder: thin, bilobed, stores urine. Cloaca: common chamber for digestive, urinary, and reproductive systems. Skin: also plays role in osmoregulation (absorbs water from moist substrate). Amphibians are more osmoconforming in water, use active uptake mechanisms on land.

Frog is carnivorous (insectivorous). Tongue: bifid (forked), protrusible, attached at front of mouth, used to catch prey. Teeth: maxillary teeth on upper jaw only. Vomerine teeth on roof of mouth. No teeth on lower jaw. Digestive tract: mouth → buccal cavity → pharynx → oesophagus → stomach → small intestine (duodenum + ileum) → large intestine → cloaca. Liver: bilobed with gall bladder. Bile: emulsifies fats. Pancreas: produces digestive enzymes (pancreatic juice) and insulin (endocrine). Ileum: main absorption site (has finger-like villi). Complete digestive system (mouth to anus) — unlike Platyhelminthes (incomplete — single opening). Hepatic portal system: special venous connection between liver and intestine — carries absorbed nutrients from gut to liver for processing.

Amphibians are the most threatened vertebrate class — ~32-41% of species threatened. Causes of decline: Chytrid fungus (Batrachochytrium dendrobatidis — Bd): causes chytridiomycosis, disrupts skin function → electrolyte imbalance → cardiac arrest. Has caused mass die-offs globally (possibly >90 species extinct). Second chytrid: Batrachochytrium salamandrivorans (Bsal) — threatens European and Asian salamanders. Habitat loss: wetland draining, deforestation, agricultural runoff. Pesticides: absorbed through permeable skin. Climate change: drying of breeding ponds, phenological mismatches. UV radiation: ozone depletion → more UV → damages eggs and larvae (transparent = no protection). Invasive species: predation (American bullfrog, trout). Amphibians as bioindicators: their sensitivity to environmental changes makes them excellent indicators of ecosystem health. Conservation: captive breeding (Amphibian Ark programme), habitat protection, chytrid treatment (antifungal baths), disease-free insurance populations.