Two "descriptive" chapters — animal tissues and the study of a cockroach and a frog — together contribute a steady 2–4 questions in NEET every year, and they are almost entirely fact-recall. Because the marks are cheap but the details are fussy (numbers of segments, spiracles, ganglia, ducts), the safest strategy is to learn them as organised comparisons rather than as loose paragraphs.
Animal Tissues: The Four Basic Types
A tissue is a group of cells with a common origin that work together on one job. All the organs of a complex animal are built from just four tissue families: epithelial, connective, muscular and neural.
Epithelial tissue is a sheet tissue. Its cells sit tightly packed on a non-cellular basement membrane, with one free (apical) surface facing a lumen, a cavity or the outside. It always covers or lines, and it always forms the boundary through which materials must pass.
- Simple epithelium — a single layer, found on lining and absorptive/diffusion surfaces.
- Squamous: flattened, thin cells; alveolar walls, walls of blood vessels — diffusion and filtration.
- Cuboidal: cube-shaped; kidney tubules, ducts of glands — secretion and absorption. Microvilli may be present.
- Columnar: tall cells with nuclei at the base; stomach and intestinal lining — secretion and absorption.
- Ciliated: cuboidal or columnar cells bearing cilia; bronchioles, fallopian tubes, kidney tubule parts — moves mucus or eggs in one direction.
- Glandular epithelium — columnar/cuboidal cells specialised for secretion. Unicellular (isolated goblet cells of the gut) or multicellular (salivary gland). By mode of release: exocrine glands pour secretions (mucus, saliva, enzymes, milk, oil, earwax) through ducts; endocrine glands are ductless and release hormones straight into blood.
- Compound epithelium — two or more cell layers; lines the dry surface of skin, buccal cavity, pharynx, and the ducts of salivary and pancreatic glands. Its role is protection, not absorption or secretion.
Neighbouring epithelial cells are held and coordinated by three cell junctions: tight junctions seal cells so that substances cannot leak across; adhering junctions cement cytoskeletons of adjacent cells; gap junctions are direct channels allowing rapid passage of ions and small molecules — communication.
Connective tissue is the linking and supporting tissue, and its defining feature is abundant matrix in which few cells lie scattered. Fibres of collagen and elastin lie embedded in this matrix.
- Loose connective tissue — cells and fibres loosely arranged in a semi-fluid matrix, with fibroblasts, macrophages and mast cells scattered through it; found beneath the skin and around organs, where it packs organs together and stores water and salts. Areolar tissue is the commonest example, alongside adipose tissue, whose cells are swollen with stored fat and act as an insulating, shock-absorbing energy reserve.
- Dense connective tissue — fibres are packed much more tightly. In dense regular tissue the collagen fibres run in one orderly direction, giving strength along that axis, as in tendons (muscle to bone) and ligaments (bone to bone, more elastic than tendons due to extra elastin). In dense irregular tissue the fibres are randomly arranged, as in the deeper dermis of skin.
- Specialised connective tissue — cartilage has a solid but pliable matrix (chondrin) secreted by chondrocytes sitting in lacunae; it resists compression at joints, in the nose, ear pinna, trachea and between vertebral discs. Bone, the hardest connective tissue, has a calcified matrix of calcium and phosphorus compounds giving the body its structural framework; osteocytes sit in its lacunae. Blood is technically a connective tissue too — plasma is the fluid matrix carrying RBCs, WBCs and platelets — though its transport role is covered separately under circulation.
Muscular tissue is made of elongated fibres packed with contractile proteins that shorten on stimulation.
- Striated (skeletal) muscle — attached to bones, cylindrical, unbranched, multinucleate, banded appearance; voluntary, fast, fatigues quickly.
- Unstriated (smooth) muscle — spindle-shaped, uninucleate, unbanded; found in gut wall, blood vessels, iris, bladder; involuntary, slow, resists fatigue.
- Cardiac muscle — found only in the heart; cylindrical, branched, uninucleate fibres joined by intercalated discs so the tissue contracts as one unit; involuntary but striated, and myogenic (self-exciting).
Neural tissue is built of cells specialised to receive and transmit stimuli. The functional unit, the neuron, has a cell body (cyton) with the nucleus and Nissl granules, several short dendrites receiving signals, and a single long axon conducting impulses onward. Neuroglia, far more numerous than neurons, provide structural and metabolic support rather than conducting impulses.
The Cockroach: External Features and Internal Systems
Periplaneta americana is the standard NEET example of insect organisation.
External features. The dorsoventrally flattened, dark brown body is divided into head, thorax and abdomen, encased in a chitinous exoskeleton. The head bears a pair of long antennae, compound eyes and biting-chewing mouthparts. The thorax has three segments (prothorax, mesothorax, metathorax), each with one pair of jointed walking legs, and the meso- and metathorax each bear one pair of wings — front pair thickened into tegmina, hind pair membranous for flight. The ten-segmented abdomen ends in a pair of anal cerci and, in males, a pair of anal styles.
Digestive system. A complete canal grouped into foregut (buccal cavity, oesophagus, crop, and a gizzard with cuticular teeth for grinding — cuticle-lined, so no absorption here), midgut (true stomach, where digestion and absorption occur, aided by hepatic caecae secreting digestive juices), and hindgut (ileum, colon, rectum), which absorbs water and forms faecal pellets.
Circulatory system is open: a dorsal, tubular, 13-chambered heart pumps colourless haemolymph forward through an aorta into the haemocoel, where it bathes tissues directly before draining back through paired ostia. Haemolymph carries no respiratory pigment, so it plays no role in oxygen transport.
Respiratory system. Air enters through ten pairs of spiracles (two thoracic, eight abdominal) into a branching tracheal tube network that delivers air directly to tissues — gas exchange bypasses the blood entirely.
Nervous system is a ventral, ladder-like chain: paired cerebral ganglia ("brain") connected by a ring to a sub-oesophageal ganglion, then three thoracic and six abdominal ganglia joined by paired longitudinal connectives — a double ventral cord, the reverse of the chordate plan.
Reproductive system. Cockroaches are dioecious with sexual dimorphism (only males have anal styles). Males have paired testes opening via a common ejaculatory duct; females have paired ovaries and oviducts uniting into a single median oviduct, plus a sperm-storing spermatheca. Fertilisation is internal; fertilised eggs are enclosed in a hardened case, the ootheca, which the female carries before depositing it.
The Frog: External Features and Internal Systems
Rana tigrina is the standard vertebrate/chordate comparison to the cockroach.
External features. The body has only a head and trunk (no neck or tail in the adult), a wide mouth, external nostrils, bulging eyes with a protective nictitating membrane, and a tympanic membrane behind each eye for hearing. Shorter forelimbs and longer, muscular, webbed hindlimbs support jumping and swimming. The moist, glandular skin also respires.
Digestive system. Buccal cavity (with a bilobed, protrusible tongue for catching prey), oesophagus, stomach, small intestine and large intestine ending in a cloaca — a single chamber shared by the digestive, excretory and reproductive tracts, unlike the cockroach or humans. Liver and pancreas supply digestive secretions.
Circulatory system is closed, with a three-chambered heart (two atria, one ventricle), so some mixing of oxygenated and deoxygenated blood occurs — unlike the fully separate four-chambered mammalian heart.
Respiratory system uses three surfaces at different times: cutaneous (moist skin, dominant during hibernation), buccopharyngeal (mouth lining), and pulmonary (a pair of simple sac-like lungs, dominant when active) — this trio is one of the most frequently tested frog facts.
Nervous system has a brain (olfactory lobes, cerebral hemispheres, optic lobes, cerebellum, medulla continuing as spinal cord) in a bony cranium, plus cranial and spinal nerves — a dorsal, single, hollow nerve cord, the diagnostic chordate feature.
Reproductive system. Frogs are dioecious. Males have paired testes connected by vasa efferentia to the kidney's own excretory ducts (sperm and urine share a duct, unlike in the cockroach or mammals); there is no penis, and fertilisation is external, in water. Females have paired ovaries releasing ova into the coelom, picked up by ciliated oviducts and passed to the cloaca for external fertilisation during amplexus (the male clasping the female to synchronise gamete release). Development is indirect, via an aquatic, gilled tadpole larva.
Common Mistakes and Exam Traps
- Mixing up the cockroach's and frog's circulation. The cockroach has an open system with pigment-free haemolymph; the frog has a closed system with a three-chambered heart and haemoglobin-bearing blood.
- Forgetting the cloaca is a frog-specific feature in this comparison. The frog's digestive, excretory and reproductive tracts share one cloacal opening; the cockroach (and humans) keep these separate.
- Attributing lungs to the cockroach. The cockroach breathes through a tracheal system opening via spiracles, with no lungs and no role for haemolymph in oxygen transport.
- Reversing tendon and ligament. A tendon joins muscle to bone; a ligament joins bone to bone and is more elastic — a classic one-line question.