Structural Organisation in Animals — NEET Notes

From cell to organism — levels of organisation

A unicellular organism does everything — digestion, respiration, reproduction — inside one cell. In a multicellular animal the same work is shared by groups of cells acting together. A tissue is a group of similar cells, with their intercellular substances, that performs a specific function. Tissues of more than one type are organised into organs such as the stomach, lung, heart, and kidney. Two or more organs functioning together by physical or chemical interaction form an organ system — the digestive system, respiratory system, and so on. Cells, tissues, organs, and organ systems are nature's division of labour, and each level adds a new emergent property that the level below cannot manage alone.

NCERT opens the chapter with a quiet but exam-relevant claim: every complex animal is built from only four basic tissues. Even an organ as elaborate as the heart contains all four — epithelial linings, connective tissue scaffolds, cardiac muscle, and neural conducting fibres. The pattern of these four tissues, repeated in different proportions, is what makes a stomach a stomach and a kidney a kidney.

The four animal tissues — at a glance

Before drilling into each, fix the four-way split in your mind. Each tissue has a characteristic location, a signature function, and a NEET-favourite diagnostic feature. The four-card grid below is the single most important slide in this chapter; match-the-column PYQs are recycled from it almost every year.

Epithelial tissue

Epithelia are the body's covering and lining sheets. They cover the skin, line all body cavities and tubes, and form the secretory parenchyma of glands. NCERT lists three diagnostic features that always apply: cells are tightly packed with little intercellular substance, the sheet rests on a non-cellular basement membrane, and one surface is always free, facing either a body fluid or the outside environment. Epithelia are also avascular — they receive nutrients by diffusion from the underlying connective tissue.

Two architectural classes exist. Simple epithelium is a single cell layer thick — built for diffusion, absorption, and secretion across thin surfaces. Compound (stratified) epithelium is many layers thick and lines surfaces exposed to mechanical wear and chemical insult, such as the dry surface of skin, the moist surface of the oral cavity, and the lining of the pharynx and oesophagus. Within the simple class, NCERT recognises five shapes — squamous, cuboidal, columnar, ciliated columnar, and glandular epithelium — and the boundary between epithelium and gland is fuzzier than students remember.

Cell junctions in epithelia

Because epithelial sheets must seal off compartments, the cells are stitched together by three classes of intercellular junctions. NEET tests them as a three-way match, and the wording in NCERT is exactly the wording in the question paper.

Connective tissue — six varieties, one logic

Connective tissues are the most widespread of the four. They share a common architecture: scattered cells embedded in a non-living matrix (ground substance plus fibres). What differs is the matrix. In areolar tissue it is gel; in cartilage it is chondrin; in bone it is mineralised ossein; in blood it is plasma (a fluid matrix). NCERT groups connective tissue into three families — loose (areolar, adipose), dense (regular, irregular), and specialised (cartilage, bone, blood).

Loose connective tissue

Areolar tissue is the most widely distributed connective tissue in the body. It is found beneath the skin and acts as packing between organs. It contains three signature cell types — fibroblasts (make collagen and elastin fibres), macrophages (engulf bacteria and debris), and mast cells (release heparin and histamine). NEET 2023 Q.186 matched "mast cells" to "areolar connective tissue" directly. Adipose tissue is also a loose connective — its cells (adipocytes) store fat as a single large droplet. Found mainly under the skin and around organs, it functions as energy reserve and insulator.

Dense connective tissue

The fibres are tightly packed and predominantly oriented. In dense regular connective tissue, the collagen fibres are aligned in parallel bundles — giving great tensile strength along one axis. This is what tendons (muscle-to-bone) and ligaments (bone-to-bone) are made of. In dense irregular, the fibres run in many directions, providing strength in multiple planes — the dermis of the skin is the classic example.

Specialised connective tissue — cartilage, bone, blood

Cartilage is flexible and resists compression. Its cells (chondrocytes) sit in small fluid-filled spaces called lacunae, embedded in a chondrin matrix. Cartilage is found in the pinna of the ear, tip of the nose, intervertebral discs, between adjacent bones of the vertebral column, and at the joints of long bones. In the adult mammal most cartilage is replaced by bone during development. Bone is rigid and resists both compression and tension. Its cells (osteocytes) sit in lacunae within a matrix of ossein hardened with calcium phosphate. The mammalian shaft of a long bone shows concentric lamellae arranged around a central Haversian canal, the whole unit called a Haversian system. Bone provides skeletal support and houses the marrow that produces blood cells.

Blood is the fluid connective tissue. Its cells — red blood cells (erythrocytes), white blood cells (leucocytes), and platelets — float in a liquid matrix called plasma. Plasma carries fibrinogen, albumin, and globulin. Blood is classified as a specialised connective tissue because matched-the-column PYQs (NEET 2023 Q.186) test this label specifically.

Muscular tissue — three contractile fabrics

Muscle cells (also called fibres because of their elongated shape) contain parallel arrays of contractile microfilaments — actin and myosin — and shorten when stimulated. Three histologically distinct muscle types share this contractile machinery but differ in striation, branching, nucleation, and voluntary control. NEET asks at least one question on these distinctions almost every year, usually as a match-the-column or a "which is incorrect" stem.

NEET 2016 Q.82 matched smooth muscle to the wall of the intestine; that combination is the textbook anchor. The four functional properties of every muscle — excitability, extensibility, contractility, elasticity — are also recyclable PYQ material.

Neural tissue — neurons and neuroglia

Neural tissue exerts the body's fastest control system. It contains two cell types. Neurons are excitable cells that generate and conduct electrical impulses. Neuroglia (or glial cells) are the supportive scaffolding that surrounds them — they do not conduct impulses, but they make up more than half the volume of all neural tissue. NEET 2022 Q.171 tested precisely this: neuroglia is part of nervous tissue, not connective tissue.

A typical neuron has a central cyton (cell body) containing a nucleus and cytoplasm rich in Nissl bodies — granular structures made of rough endoplasmic reticulum and free ribosomes. From the cyton arise multiple short branched dendrites (which receive impulses) and a single long process — the axon — which transmits impulses to the next neuron, muscle, or gland. The axon may be sheathed in myelin (myelinated/medullated) or not (non-myelinated). The myelin sheath is interrupted at intervals called nodes of Ranvier, which speed up impulse conduction by allowing saltatory transmission.

Organ and organ system — the bigger picture

Tissues do not work in isolation. Two or more tissue types come together to form an organ; two or more organs interacting to perform a common task form an organ system. NCERT highlights that the same organ — for example, the heart — contains all four tissue types: an epithelial endocardium, connective tissue framework, cardiac muscle, and conducting neural fibres. As the animal kingdom is climbed, the number of organ systems and the sharpness of division of labour increase. NCERT calls this an evolutionary trend — Class XII covers it in detail.

NCERT illustrates the organ-system level with three model organisms. Earthworm (Pheretima) — a tube-within-a-tube body plan with true coelom and segmentation. Cockroach (Periplaneta americana) — segmented body with jointed appendages, exoskeleton of chitin, and an open circulatory system. Frog (Rana tigrina) — the simplest tetrapod with closed circulation, three-chambered heart, and a urinogenital duct in the male. Each animal is examined under six organ systems: digestive, circulatory, respiratory, excretory, nervous, and reproductive.

Earthworm — Pheretima posthuma

The Indian earthworm is a soil-dwelling annelid with a long, cylindrical, metamerically segmented body. The body is divided into roughly 100 to 120 segments. Segment 1 (the peristomium) bears the mouth; an overhanging fleshy lobe called the prostomium sits above the mouth and acts as a sensory wedge to push apart soil cracks — NEET 2021 Q.197 tested exactly this point. The clitellum, a prominent glandular band that secretes the egg cocoon, occupies segments 14, 15, and 16 in Pheretima. The male genital pores open on the ventral side of segment 18, and the female genital pore is on segment 14.

Digestion. The alimentary canal is a straight tube running the length of the body. Earthworms are detritivores. The mouth opens into a buccal cavity (segments 1–3), then a muscular pharynx (segments 4), an oesophagus (5–7), a small gizzard (in segment 8–9) which grinds soil and decaying leaves, and a long stomach (9–14) where calciferous glands neutralise the humic acid of soil. The intestine begins in segment 15 and continues to the last segment; the typhlosole — a dorsal infolding of the intestinal wall — increases absorptive surface. Faeces (worm castings) leave through the terminal anus.

Circulation. The system is closed. Blood flows entirely within vessels. There is a dorsal vessel (the main collecting trunk), a ventral vessel (the main distributing trunk), and lateral hearts in segments 7–11 — four pairs of pulsatile vessels — that pump blood from dorsal to ventral. Blood contains haemoglobin dissolved in plasma; corpuscles are nucleated. Respiration is cutaneous: gases diffuse across the moist, vascularised skin. There are no specialised respiratory organs.

Excretion. Carried out by segmentally arranged nephridia — coiled tubular structures of three types: septal nephridia (on intersegmental septa from segment 15 onwards, open into the gut), integumentary nephridia (on the inner body wall, open onto the skin), and pharyngeal nephridia (in clusters in segments 4–6). Earthworms can be ammonotelic or ureotelic depending on water availability.

Nervous system. A pair of cerebral ganglia (the "brain") in segment 3, connected by circumpharyngeal connectives to a ventral nerve cord that runs the body's length and bears segmental ganglia. Reproduction. Earthworms are hermaphrodites — both sexes in one body. Two pairs of testes in segments 10 and 11; one pair of ovaries on the intersegmental septum of segments 12–13. Despite being hermaphroditic, self-fertilisation is avoided — cross-fertilisation occurs during copulation, after which the clitellum secretes a cocoon that receives the eggs and stored sperm. Development is direct.

Periplaneta americana — the cockroach

The cockroach is an arthropod with a metamerically segmented body, a chitinous exoskeleton (composed of N-acetylglucosamine), and a haemocoel as the body cavity. The body is divided into head, thorax, and abdomen. The head, formed by fusion of six segments, bears compound eyes, a pair of long thread-like antennae, and biting-and-chewing mouthparts: labrum, a pair of mandibles, a pair of maxillae, labium, and a median tongue-like hypopharynx lying within the cavity enclosed by the mouthparts (NEET 2021 Q.152). The thorax has three segments — prothorax, mesothorax, metathorax — each bearing a pair of walking legs. The forewings (tegmina) are dark, leathery, and arise from the mesothorax; the membranous hindwings used in flight arise from the metathorax (NEET 2022 Q.177). The abdomen has 10 segments. The 10th segment in both sexes bears a pair of jointed sensory anal cerci; only the male shows a pair of short anal styles on the ninth sternum — the standard NEET diagnostic for sexual dimorphism (asked in 2018 and again in 2023).

Digestion. The alimentary canal of the cockroach has three regions. The foregut begins with the mouth, leads into the pharynx and oesophagus, then a sac-like crop for food storage, and a muscular gizzard (proventriculus) armed with six chitinous teeth for grinding. At the junction of foregut and midgut is a ring of finger-like hepatic (gastric) caecae that secrete digestive enzymes — NEET 2021 Q.152 tested this location (the caecae are not at the midgut-hindgut junction). The midgut (mesenteron) is where final digestion and absorption occur. The hindgut consists of ileum, colon, and rectum and ends at the anus. Six rectal papillae help reabsorb water. NEET 2019 asked this exact sequence: mouth → pharynx → oesophagus → crop → gizzard → midgut → ileum → colon → rectum → anus.

Circulation is open. There are no proper veins or arteries; haemolymph fills the haemocoel and bathes the viscera directly. The 13-chambered tubular heart lies mid-dorsally beneath the terga of the thorax and the first seven abdominal segments. Each chamber receives haemolymph through a pair of lateral ostia. Haemolymph is colourless because cockroach respiratory pigment is not haemoglobin — oxygen is delivered directly by tracheae.

Respiration. A network of branching tracheal tubes opens to the exterior at 10 pairs of spiracles (2 thoracic + 8 abdominal). Each spiracle is guarded by sphincters. The tracheae conduct air directly to tissues — no involvement of haemolymph in gas transport.

Excretion is carried out by Malpighian tubules — 100 to 150 thread-like, blind-ending yellow tubules attached at the junction of the midgut and hindgut. They extract uric acid from the haemolymph. Cockroach is therefore uricotelic. The fat body (stores uric acid), nephrocytes (large binucleate cells), and urecose glands (in male cockroaches, synthesise uric acid) all contribute to nitrogen excretion. NEET 2023 Q.193 tested precisely this combination: Malpighian tubules, fat body, nephrocytes, urecose glands.

Nervous system. A nerve ring of three pairs of fused ganglia in the head (supra-oesophageal "brain" + sub-oesophageal ganglion), a double ventral nerve cord with three thoracic ganglia and six abdominal ganglia. Sense organs include compound eyes (each with about 2,000 ommatidia), antennae, and sensory anal cerci. Reproduction is by sexual dimorphism. Male: a pair of testes lie on the lateral sides of segments 4–6; vas deferens open into the ejaculatory duct; a phallic (mushroom) gland in segments 6–7 secretes the spermatophore. Female: two large ovaries lie on the lateral sides of segments 2–6; oviducts unite to form a common oviduct that opens into the genital chamber. The 7th sternum is boat-shaped and, with the 8th and 9th sterna, forms a genital pouch (NEET 2021 Q.152). The collaterial glands secrete the ootheca — a hard egg-case which the female deposits in a sheltered crack and which contains 14–16 fertilised eggs. Cleavage in the embryo is spiral and determinate (NEET 2016 Q.57 tested this — the question stated "indeterminate and radial" and was therefore the incorrect option).

Frog — Rana tigrina

The Indian bullfrog is an amphibian in the class Amphibia, phylum Chordata. Frogs live both on land and in fresh water. They are cold-blooded (poikilothermic), change colour for camouflage (mimicry), and survive extremes by aestivation in summer and hibernation in winter inside deep burrows. The body is divided into head and trunk only — no neck, no tail. The smooth, slippery skin is kept moist by mucus and is the frog's third respiratory organ. The frog does not drink water; it absorbs it through the skin. Sexual dimorphism — males have a pair of sound-producing vocal sacs and a copulatory pad on the first digit of the forelimb; females have neither. Hind limbs are larger, end in five webbed digits for swimming; forelimbs are smaller, end in four digits.

Digestion. Frogs are carnivores, so the alimentary canal is short. The bilobed muscular tongue captures prey. The mouth opens into a buccal cavity → pharynx → short oesophagus → stomach (where HCl and gastric juice initiate digestion) → small intestine. The first part of the intestine — the duodenum — receives bile from the gall bladder and pancreatic juice from the pancreas through a common bile duct. Bile emulsifies fat; pancreatic juices digest carbohydrates and proteins. Final digestion and absorption (through villi and microvilli) take place in the small intestine. The undigested mass passes to the rectum and exits via the cloaca — the common opening for the digestive, urinary, and reproductive systems.

Respiration is by three methods. In water — cutaneous respiration through the moist skin (and the only mode used during aestivation and hibernation). On land — buccal cavity, skin, and lungs all act as respiratory organs; pulmonary respiration uses the pair of pink, elongated, sac-like lungs in the thorax. Air enters through the nostrils → buccal cavity → lungs.

Circulation is well-developed and closed. The three-chambered heart (two atria, one ventricle) is enclosed in a pericardium. Veins enter the heart via the sinus venosus on the right atrium; the ventricle pumps into the conus arteriosus. The frog has two distinctive portal systems — the hepatic portal system (intestine to liver) and the renal portal system (lower body to kidney). RBCs are nucleated, contain haemoglobin, are biconvex. A separate lymphatic system with lymph, lymph channels, and lymph nodes runs in parallel — lymph lacks some plasma proteins and lacks RBCs.

Excretion. A pair of compact, dark red, bean-shaped kidneys lies posteriorly on either side of the vertebral column. Each kidney is composed of nephrons. From each kidney emerges a ureter. In the male, the ureter doubles as the genital duct — called the urinogenital duct — that opens into the cloaca. In the female, the ureter and oviduct open separately into the cloaca. A thin-walled urinary bladder sits ventral to the rectum. The frog excretes urea — it is therefore ureotelic.

Nervous system. Highly evolved. Includes the central nervous system (brain + spinal cord), peripheral nervous system (10 pairs of cranial nerves + spinal nerves), and autonomic nervous system (sympathetic + parasympathetic). The brain is enclosed in the cranium and divided into forebrain (olfactory lobes, paired cerebral hemispheres, unpaired diencephalon), midbrain (paired optic lobes), and hindbrain (cerebellum + medulla oblongata). The medulla oblongata passes out through the foramen magnum and continues as the spinal cord enclosed in the vertebral column. Sense organs include sensory papillae (touch), taste buds, nasal epithelium (smell), eyes (vision), and tympanum-internal ear (hearing + balance). External ear is absent — only the tympanum is visible.

Endocrine glands in frog include the pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals, and gonads.

Reproduction. Sexes are separate. Male: a pair of yellowish ovoid testes adhered to the upper kidneys by the mesorchium. 10–12 vasa efferentia arise from each testis, enter the kidney, open into Bidder's canal, and pass via the urinogenital duct to the cloaca — NEET 2017 Q.75 tested this exact sequence. Female: a pair of ovaries; oviducts open separately into the cloaca; a mature female lays 2,500–3,000 ova at a time. Fertilisation is external in water; development passes through a free-living tadpole larval stage which undergoes metamorphosis to form the adult.