Structural Organisation in Animals

2.1 Core concepts

• In multicellular animals, a group of similar cells along with intercellular substances performing a specific function is called a tissue; tissues organise in specific proportion and pattern to form organs, and two or more organs performing a common function form an organ system (NCERT §7 intro, p. 79).
• All complex animals consist of only four basic types of tissues — epithelial, connective, muscular and neural — and the heart is an organ that contains all four (NCERT §7.1, p. 80).
• Morphology is the external appearance of organs/body parts in animals; anatomy is the study of morphology of internal organs (NCERT §7.1, p. 80).
• Frogs belong to class Amphibia of phylum Chordata; the common Indian species is Rana tigrina; they are poikilotherms (cold-blooded), exhibit mimicry (protective camouflage), undergo aestivation in summer and hibernation in winter (NCERT §7.2, p. 80).
• External morphology: skin is smooth, moist and slippery due to mucus; dorsal side olive green with dark irregular spots, ventral side pale yellow; frog never drinks water but absorbs it through the skin; body divisible only into head and trunk — neck and tail are absent (NCERT §7.2.1, pp. 80–81).
• Eyes are bulged and covered by a nictitating membrane; tympanum on either side of eyes receives sound; hind limbs have 5 webbed digits and are larger/muscular, forelimbs have 4 digits; sexual dimorphism — males have vocal sacs and a copulatory pad on the first digit of the forelimb (NCERT §7.2.1, p. 81).
• Digestive system: alimentary canal is short because frogs are carnivores; sequence — mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca; bilobed tongue captures food; stomach secretes HCl and gastric juices; bile from gall bladder and pancreatic juice from pancreas reach the duodenum via a common bile duct; bile emulsifies fat, pancreatic juice digests carbohydrates and proteins; absorption occurs via villi and microvilli (NCERT §7.2.2, pp. 81–82).
• Respiration is by three routes: (a) cutaneous through skin in water (and during aestivation/hibernation), (b) buccopharyngeal through buccal cavity on land, and (c) pulmonary through a pair of pink, elongated, sac-like lungs in the thorax (NCERT §7.2.2, p. 82).
• Vascular system is closed; heart is three-chambered (two atria + one ventricle), enclosed in pericardium; sinus venosus (triangular) joins the right atrium and receives blood via the vena cava; ventricle opens into a sac-like conus arteriosus on the ventral side; hepatic portal (liver–intestine) and renal portal (kidney–lower body) systems are present; RBCs are nucleated and contain haemoglobin; lymph lacks a few proteins and RBCs, and the lymphatic system consists of lymph, lymph channels and lymph nodes (NCERT §7.2.2, p. 82).
• Excretory system: paired kidneys (compact, dark red, bean-like, on both sides of vertebral column), ureters, urinary bladder, cloaca; functional unit is the uriniferous tubule / nephron; in males the ureters function as urinogenital ducts opening into cloaca, in females the ureters and oviducts open separately into the cloaca; frog is ureotelic (NCERT §7.2.2, pp. 82–83).
• Nervous system has three divisions — central (brain + spinal cord), peripheral (cranial + spinal nerves) and autonomic (sympathetic + parasympathetic); 10 pairs of cranial nerves; brain enclosed in cranium and divided into forebrain (olfactory lobes + paired cerebral hemispheres + unpaired diencephalon), midbrain (pair of optic lobes) and hindbrain (cerebellum + medulla oblongata); medulla passes through the foramen magnum into the spinal cord (NCERT §7.2.2, p. 83).
• Endocrine glands listed in frog: pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads (NCERT §7.2.2, p. 83).
• Sense organs of frog: touch (sensory papillae), taste (taste buds), smell (nasal epithelium), vision (eyes — simple, single-unit), hearing (tympanum + internal ear); external ear is absent, only tympanum is visible externally; the ear functions in both hearing and balance/equilibrium (NCERT §7.2.2, p. 83).
• Male reproductive system: pair of yellowish ovoid testes attached to the upper part of kidneys by a double fold of peritoneum called mesorchium; 10–12 vasa efferentia arise from the testes, enter the kidneys, open into Bidder's canal, and ultimately drain into the urinogenital duct → cloaca, which is a small median chamber passing faeces, urine and sperms (NCERT §7.2.2, pp. 83–84).
• Female reproductive system: pair of ovaries situated near kidneys with no functional connection to them; a pair of oviducts opens separately into the cloaca; a mature female lays 2500–3000 ova at a time; fertilisation is external in water; development passes through a tadpole larval stage that undergoes metamorphosis into adult (NCERT §7.2.2, p. 84).
• Ecological/economic value: frogs eat insects and protect crops, serve as an important link in the food chain/web; in some countries their muscular legs are eaten (NCERT §7.2.2, p. 84).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 7.1, p. 80 — External features of frog: labels head, trunk, eye, forelimb, hind limb. Memorise that neck and tail are absent.
• Figure 7.2, p. 81 — Internal organs of frog with complete digestive system: heart, oesophagus, liver, gall bladder, lung, stomach, fat bodies, kidney, ureter, urinary bladder, intestine, rectum, cloaca, cloacal aperture. Note the relative positions — heart and lungs above, kidneys ventral to fat bodies, urinary bladder ventral to rectum.
• Figure 7.3, p. 83 — Male reproductive system: testis, vasa efferentia, kidney, adrenal gland, urinogenital duct, urinary bladder, rectum, cloaca, cloacal aperture. The urinogenital duct (a single tube) is the key male-specific feature.
• Figure 7.4, p. 83 — Female reproductive system: ovary with ova, oviduct, ureter (separate from oviduct), urinary bladder, cloaca, cloacal aperture. The separateness of ureter and oviduct is the key female-specific feature.
• Process — pathway of food: mouth → buccal cavity → pharynx → oesophagus → stomach → duodenum → intestine → rectum → cloaca (p. 81–82).
• Process — brain divisions: forebrain (olfactory lobes + cerebral hemispheres + diencephalon) → midbrain (optic lobes) → hindbrain (cerebellum + medulla oblongata) → spinal cord through foramen magnum (p. 83).

2.4 Common confusions / NTA trap points

• Four digits vs five digits — Forelimbs end in 4 digits, hind limbs in 5 webbed digits. NTA loves to flip these.
• Heart chambers — The frog heart is three-chambered (2 atria + 1 ventricle), not four. The sinus venosus and conus arteriosus are accessory chambers but the chamber count is three.
• RBCs of frog are nucleated — A common distractor states "non-nucleated like mammals." NCERT explicitly states frog RBCs are nucleated (p. 82).
• Male vs female urogenital plumbing — In males the ureter doubles as the urinogenital duct (sperm + urine share the same exit); in females ureters and oviducts open separately into the cloaca. Ovaries have no functional connection with kidneys (pp. 83–84).
• Vasa efferentia count — 10–12 (not 10 pairs). Cranial nerves are 10 pairs. NTA mixes these two "tens."
• Aestivation vs hibernation — Aestivation = summer sleep, hibernation = winter sleep. During both, gas exchange takes place through skin (p. 82).
• Bile route — Bile is secreted by liver, stored in gall bladder, and reaches the duodenum via the common bile duct along with pancreatic juice (p. 82).
• Skin respiration vs lung respiration — Skin is the only respiratory surface in water and during hibernation/aestivation; lungs operate on land along with skin and buccal cavity (p. 82). A common trap is "only lungs on land."
• Tongue attachment — The frog's tongue is bilobed at the tip and attached to the front of the floor of the buccal cavity (free posteriorly), allowing it to be flicked out (p. 81).
• Hepatic vs renal portal system — The frog has both a hepatic portal system (intestine to liver) and a renal portal system (hind limbs to kidneys) (p. 82); the renal portal system is absent in mammals — an NTA contrast trap.

2.5 Quick comparison table — frog systems at a glance