Frog brain has three main regions — forebrain, midbrain, hindbrain. Forebrain (prosencephalon): Olfactory lobes: paired, anterior. Detect smell. Connected to olfactory epithelium. Cerebral hemispheres: paired. Thin-walled. Limited cognitive function in frogs (compared to mammals). Diencephalon: thalamus (relay centre), hypothalamus (temperature regulation, osmoregulation), pineal body (epiphysis — melatonin, regulates photoperiod), optic chiasma (crossing of optic nerves). Midbrain (mesencephalon): Optic lobes: large, paired. Dominant brain region in frogs (vision is primary sense). Cerebral peduncles. Hindbrain (rhombencephalon): Cerebellum: small, unpaired. Limited in frogs (little need for complex movement coordination). Medulla oblongata: connects to spinal cord. Controls autonomic functions (heart rate, respiration). Cranial nerves: 10 pairs (I-X) in frogs. Not 12 — mammals have 12 (XI accessory + XII hypoglossal additional).

Circulatory system: 3-chambered heart (2 auricles + 1 ventricle). Sinus venosus: thin-walled chamber that receives blood from body, empties into right auricle. Conus arteriosus: muscular chamber with spiral valve, distributes blood from ventricle. Truncus arteriosus: main arterial trunk. Branches into: carotid arches (head), systemic arches (body), pulmocutaneous arches (lungs and skin). Portal systems: Hepatic portal system: intestinal veins → hepatic portal vein → liver → hepatic veins → posterior vena cava. Absorbs nutrients. Renal portal system: blood from hind limbs → kidneys → back to circulation. Lymphatic system: frogs have a well-developed lymphatic system. Large lymph spaces under skin (subcutaneous lymph sacs). Lymph hearts: small pulsating structures at several points in the lymphatic system. Actively pump lymph into veins. Present in amphibians. Absent in mammals (which rely on smooth muscle in lymph vessel walls and skeletal muscle contractions).

Kidneys: mesonephric (opisthonephric) type. Paired, reddish-brown, elongated, lying dorsolaterally. Filter blood → urine. Ureters: carry urine from kidneys to cloaca. Male: ureters carry both urine and sperm (ureters also serve as vas deferens). Female: ureters open separately from oviducts into the cloaca. Urinary bladder: bilobed, thin-walled sac ventral to large intestine. Stores urine. Opens into cloaca. Testes (male): paired, yellowish, near kidneys. Vasa efferentia → ureters → cloaca. Ovaries (female): paired, granular, near kidneys. Oviducts: pair of coiled tubes. Carry eggs from ovarian cavity (coelom) to cloaca. Funnel (ostium): wide opening that collects eggs from body cavity. Oviducal gland: jelly coat added to eggs. Cloaca: common chamber at end of digestive, urinary, and reproductive tracts. Single opening (cloacal aperture) to outside. Summary: female frog — ureters open into dorsal cloaca, oviducts open into cloaca separately. Not combined into a single opening.

Frog skeleton provides support and enables movement. Axial skeleton: skull (cranium + splanchnocranium — jaw bones). Vertebral column: 10 vertebrae (9 true + 1 urostyle). No neck or tail. Urostyle (coccyx equivalent): elongated fusion of caudal vertebrae. Extends as a rigid rod → helps in jumping. Appendicular skeleton: Forelimbs: humerus, radio-ulna (fused), carpals, metacarpals, 4 phalanges (4 digits). Hind limbs: femur, tibio-fibula (fused — single elongated bone), tarsals, metatarsals, 5 phalanges (5 digits). Webbed (digits connected by skin for swimming). Pectoral girdle: connects forelimbs. Includes clavicle, coracoid, suprascapula, scapula, epicoracoid, sternum. Pelvic girdle: connects hind limbs. Consists of ilium, ischium, pubis (forming urostyle attachment).

Frog has well-developed muscles for jumping, swimming, and burrowing. Myotomes: segmentally arranged muscles (more evident in fish, less distinct in frogs). Important muscles: Gastrocnemius (calf muscle): large, powerful hind limb muscle. Used in classic physiology experiments (nerve-muscle preparation). Sartorius: thigh muscle. Iliofibularis: flexes knee. Semimembranosus: extends thigh. Gracilis: adducts thigh. Deltoid: shoulder. Biceps brachii: forearm. Dorsal muscles: iliacus externus for jumping power. The powerful hind limbs with elongated tarsals and metatarsals act as levers for jumping. Frogs use energy stored in tendons to jump — the energy is released rapidly for explosive jumping. Record jumps: South African sharp-nosed frog (Ptychadena oxyrhynchus) can jump 17.6 m in one leap.

Hepatic portal system: specialised blood vessel arrangement that carries blood from the gastrointestinal tract, spleen, and pancreas to the liver before it reaches general circulation. Why it is called portal: blood flows through two capillary beds in sequence (intestinal capillaries → portal vein → hepatic capillaries) rather than the usual one capillary bed. Function: First pass metabolism: liver processes nutrients absorbed from intestine (converts glucose to glycogen, metabolises amino acids, detoxifies alcohol and drugs). Liver monitoring: liver is the first organ to encounter absorbed substances → can respond quickly to changes in blood composition. Immune surveillance: Kupffer cells (hepatic macrophages) filter bacteria from portal blood (important since gut bacteria could otherwise reach systemic circulation). If liver damaged (cirrhosis): portal hypertension → blood backs up → varices (dilated veins) in oesophagus, spleen, umbilical area (caput medusae). Frogs also have RENAL PORTAL system: blood from hind limbs → kidneys before returning to heart.

Comparative heart anatomy: Fish: 2-chambered heart (1 auricle + 1 ventricle). Single circulation (blood goes: heart → gills → body → heart). Amphibia (frogs): 3-chambered heart (2 auricles + 1 ventricle). Double but incomplete circulation. Some mixing in single ventricle. Reptilia (except crocodiles): 3-chambered heart (2 auricles + 1 ventricle). Ventricle incompletely divided by interventricular septum. Less mixing than frog ventricle (septum partially separates). Crocodilia: 4-chambered heart. Complete separation. Aves: 4-chambered heart. Complete double circulation. Right aortic arch. Mammalia: 4-chambered heart. Complete double circulation. Left aortic arch. Key evolutionary trend: increasing heart chambers → increasing separation of oxygenated and deoxygenated blood → more efficient oxygen delivery → supports higher metabolic rates → endothermy in birds and mammals.

Coelom: fluid-filled body cavity derived from mesoderm. Types: Acoelomate: no coelom. Body solid mesoderm between gut and body wall. Examples: Platyhelminthes (flatworms — Taenia, Fasciola, Planaria). Pseudocoelomate: false coelom (not lined by mesoderm — lined by mesoderm only on outer side). Examples: Aschelminthes (roundworms — Ascaris, Wuchereria). Coelomate: true coelom (lined by mesoderm on both sides). Two subdivisions: Schizocoelous: coelom formed by splitting of mesodermal mass. Examples: Annelida, Arthropoda, Mollusca. Enterocoelous: coelom formed from outpocketings of archenteron (embryonic gut). Examples: Echinodermata, Chordata. Significance of coelom: hydrostatic skeleton (earthworm), organ suspension and protection, fluid transport, allows complex organ development independent of body wall.