Structural Organisation in Animals

All complex animals are built from just four basic tissue types — epithelial, connective, muscular and neural — and this chapter begins with the first two. Epithelial tissue (epithelium) covers the body's surfaces and lines its cavities and organs. Its cells are compactly packed with little intercellular material, and they always rest on a non-cellular basement membrane. Epithelia are classified by the number of cell layers and cell shape.

Simple epithelium is a single cell layer used for lining and for diffusion, secretion and absorption. Squamous epithelium is a thin flat layer for diffusion and filtration (walls of blood vessels, air sacs of lungs); cuboidal epithelium lines ducts of glands and tubular parts of the nephron for secretion and absorption; and columnar epithelium of tall cells lines the stomach and intestine for secretion and absorption. When columnar or cuboidal cells bear cilia they form ciliated epithelium (bronchioles, fallopian tubes) that moves materials along. Some epithelial cells are glandular, either unicellular (goblet cells of the gut) or multicellular (salivary glands), and glands are exocrine (secrete through ducts) or endocrine (secrete hormones directly into blood). Compound epithelium of many layers is mainly protective and covers the dry skin and lines the buccal cavity and pharynx. Neighbouring epithelial cells are held by three cell junctions: tight junctions (seal cells to stop leakage), adhering junctions (cement cells together) and gap junctions (cytoplasmic channels for communication).

Connective tissue is the most abundant and widely distributed tissue; its cells are loosely spaced in an abundant matrix (with fibres) that they secrete. Loose connective tissue has cells and fibres loosely arranged in a semi-fluid matrix: areolar tissue (beneath the skin, between organs, with fibroblasts, macrophages and mast cells) acts as a support and packing tissue, while adipose tissue stores fat. Dense connective tissue has densely packed fibres: when the collagen fibres are parallel it is dense regular — the tendons that join muscle to bone and the ligaments that join bone to bone — and when irregular it forms much of the skin.

Specialised connective tissues are cartilage, bone and blood. Cartilage has a solid yet pliable matrix (chondroitin) resisting compression, with cells (chondrocytes) in spaces called lacunae; it occurs in the tip of the nose, external ear, and between adjacent bones. Bone has a hard matrix rich in calcium salts and collagen, with cells (osteocytes) in lacunae; it provides the skeletal framework and houses marrow. Blood is a fluid connective tissue with a liquid matrix, plasma, carrying red cells, white cells and platelets, and serves as the main transport medium. Distinguishing tendon from ligament and remembering the matrix of each specialised tissue are reliable NEET marks.

Muscular tissue is made of long, contractile cells (muscle fibres) containing the proteins actin and myosin; their contraction produces movement. NCERT recognises three types, and distinguishing them by structure, control and location is a classic NEET table. Skeletal (striated) muscle is attached to bones, shows alternating light and dark bands (striations), has many nuclei per fibre, and is voluntary (under conscious control); it powers body movements. Smooth (unstriated) muscle has spindle-shaped cells with a single central nucleus, no striations, and is involuntary; it lines the walls of internal hollow organs such as the stomach, intestine and blood vessels.

Cardiac muscle is found only in the wall of the heart. It combines features of the other two: it is striated like skeletal muscle but involuntary like smooth muscle, and its branched fibres are joined end-to-end by special junctions called intercalated discs that allow the heart to contract as a coordinated unit. A quick way to remember the trio is: skeletal = striated + voluntary; smooth = unstriated + involuntary; cardiac = striated + involuntary (with intercalated discs).

Neural tissue exerts the greatest control over the body's responsiveness to changing conditions. It is made of two kinds of cells. Neurons are the excitable, impulse-conducting units; a typical neuron has a cell body (with the nucleus), branched dendrites that receive signals, and a long axon that carries the impulse away. When a neuron is suitably stimulated, an electrical disturbance is generated and swiftly propagated along its membrane.

The second component of neural tissue is the neuroglia (glial cells), which make up more than half the volume of neural tissue and support, protect and nourish the neurons without conducting impulses themselves. Together, neurons and neuroglia let the nervous system detect stimuli and coordinate rapid responses. For NEET, remember that neurons conduct impulses while neuroglia support them, and that neural tissue is what makes an animal's response to its environment fast and precise.

NCERT studies three animals to show increasing organ-system complexity. The earthworm (Pheretima) has a long, cylindrical, metamerically segmented body (about 100–120 segments) and is a reddish-brown burrowing annelid. A prominent glandular band, the clitellum, rings segments 14–16. Each segment (except the first, last and clitellum) bears S-shaped bristles, setae, that grip the soil for locomotion. The earthworm is a hermaphrodite (both sexes in one animal) but cross-fertilises.

Internally the earthworm's digestive tract is a straight tube — pharynx, oesophagus, muscular gizzard (grinds soil and organic matter), stomach and intestine; a median internal fold of the intestinal wall, the typhlosole, increases the absorptive surface area. The circulatory system is closed, with blood confined to vessels and contractile 'hearts'. Excretion is carried out by segmentally arranged nephridia, and the nervous system has a nerve ring and a ventral nerve cord. Earthworms are 'friends of farmers' because their burrowing and castings aerate and enrich the soil.

The cockroach (Periplaneta americana) is a brown-black insect with a body clearly divided into head, thorax and abdomen and covered by a hard chitinous exoskeleton of plates (sclerites). The head bears compound eyes and antennae; the thorax carries three pairs of legs and two pairs of wings (the leathery forewings are tegmina, the hindwings membranous); and the abdomen has ten segments. Cockroaches have separate sexes with clear sexual dimorphism.

The cockroach's organ systems are high-yield. Its digestive system has a crop (food storage) and a gizzard/proventriculus with chitinous teeth for grinding, plus gastric caeca; at the junction of the midgut and hindgut lie the Malpighian tubules, the organs of excretion (the cockroach is uricotelic, excreting uric acid). The circulatory system is open, with blood (haemolymph) bathing the tissues, and the heart is 13-chambered. Respiration is by a network of air tubes, the tracheae, opening through ten pairs of spiracles. Remembering 'crop + gizzard, Malpighian tubules (uricotelic), open circulation, 13-chambered heart, tracheal breathing' covers most cockroach questions.

The frog (Rana tigrina) is an amphibian whose body is divided into a head and trunk with no neck or tail. Its moist, slippery skin helps in respiration and the mottled colour provides camouflage; it has two pairs of limbs, the hind ones long and webbed for swimming. Being a cold-blooded (poikilothermic) animal whose body temperature follows the surroundings, the frog survives temperature extremes by aestivation in summer and hibernation in winter.

The digestive system is a complete alimentary canal — mouth, buccal cavity, oesophagus, stomach, intestine and rectum — opening into a common chamber, the cloaca, through which faeces, urine and gametes are released. The liver secretes bile (stored in the gall bladder) and the pancreas adds digestive enzymes. A frog feeds by capturing prey with its sticky bilobed tongue.

Respiration is a defining amphibian feature: in water the frog breathes only through its moist skin (cutaneous respiration), and on land it uses both the skin and a pair of lungs (pulmonary respiration), along with the buccal cavity lining. Its circulatory system is closed, with a three-chambered heart of two auricles and one ventricle; the blood has nucleated red cells, and special hepatic-portal and renal-portal systems route blood through the liver and kidneys.

For excretion the frog has a pair of kidneys that drain via ureters into the cloaca; it is ureotelic, excreting urea. The nervous system has a brain (in the cranium), a spinal cord and nerves, and the sense organs include eyes, internal ears and a sense of smell. The sexes are separate; males possess sound-producing vocal sacs and copulatory pads. Fertilisation is external in water and development passes through a larval tadpole stage that undergoes metamorphosis into the adult. The exam-favourite facts here are the three-chambered heart, the dual (cutaneous + pulmonary) respiration, ureotelism and external fertilisation with metamorphosis.