Life and Diversity of Animals – Chordates (Reptilia to Mammalia) BZO4T07

Syllabus topics (RTMNU)

• 1.1 Reptilia: General characters of Reptilia, Classification of Reptilia on the basis of temporal vacuities.
• 1.2 Calotes: External features, Digestive system, Respiratory system,
• 1.3 Structure and function of CNS (Brain and spinal cord).
• 1.4 Identification of Poisonous snakes and biting mechanism of snakes.

1.1 Reptilia: General characters and classification (temporal vacuities)

Reptilia (L. reptum = to creep) are cold-blooded, scaly, air-breathing vertebrates. They are the first true land vertebrates and the first amniotes (embryo protected by amnion, chorion and allantois).

General characters

• Skin: dry, without glands (few scent glands), covered by horny epidermal scales; bony dermal plates (scutes/osteoderms) in turtles and crocodiles. Skin is shed (ecdysis).
• Poikilothermic (cold-blooded).
• Limbs: two pairs of pentadactyl limbs with clawed digits; limbs absent in snakes and some lizards.
• Skeleton: fully ossified; skull monocondylic (one occipital condyle); ribs with sternum (absent in snakes); two sacral vertebrae.
• Respiration: only by lungs; no gills at any stage.
• Heart: three-chambered (2 auricles + 1 incompletely divided ventricle); four-chambered in crocodiles. Both right and left systemic arches present.
• Excretion: metanephric kidneys; mostly uricotelic (uric acid) to save water.
• 12 pairs of cranial nerves.
• Reproduction: internal fertilization; hemipenes in male lizards and snakes. Mostly oviparous; large yolky cleidoic eggs with leathery or calcareous shell; no larval stage.

Classification on the basis of temporal vacuities

Temporal vacuities (fossae) are openings in the skull behind the orbit that give room to jaw muscles. Their number and position are the basis of classification.

Note: Some books keep Parapsida (ichthyosaurs) and Euryapsida (plesiosaurs) separate.

1.2 Calotes: External features, digestive and respiratory system

Calotes versicolor is the common Indian garden lizard (family Agamidae). It lives on trees, hedges and walls and feeds on insects.

External features

• Body divisible into head, neck, trunk and tail. Tail long and slender (about two-thirds of total length).
• Whole body covered with overlapping, keeled horny scales. A dorsal crest of spiny scales runs from neck to back.
• Head triangular. Mouth wide; a pair of nostrils near snout tip; eyes with movable upper and lower eyelids and a nictitating membrane; tympanum (ear drum) behind the angle of jaws.
• Two pairs of clawed pentadactyl limbs.
• Cloacal aperture is a transverse slit at the base of the tail.
• Male’s head and throat turn red in breeding season (“bloodsucker”).

Digestive system

1. Mouth → buccal cavity: jaws bear small conical acrodont teeth (fused to the jaw edge), used for holding prey. Tongue is thick, fleshy and sticky, free in front.
2. Pharynx → oesophagus: short, muscular tube.
3. Stomach: J-shaped, wide cardiac part and narrow pyloric part with pyloric sphincter.
4. Small intestine: duodenum and coiled ileum held by mesentery.
5. Large intestine: a small caecum at its start, then the wide rectum opening into the cloaca.
6. Glands: large bilobed liver with gall bladder (bile duct to duodenum); pancreas in the duodenal loop.

Respiratory system

• External nares → nasal chambers → internal nares → buccal cavity → glottis → small larynx.
• Trachea supported by cartilaginous rings; divides into two bronchi.
• Two sac-like lungs; anterior part spongy (faveoli), posterior part thin-walled.
• Mechanism: no diaphragm. Intercostal muscles move the ribs; the body cavity expands (inspiration) and contracts (expiration).

1.3 Calotes: Structure and function of CNS (brain and spinal cord)

The central nervous system (CNS) consists of the brain inside the cranium and the spinal cord in the neural canal, both covered by meninges.

• Cavities: lateral ventricles, 3rd ventricle, iter, 4th ventricle.
• 12 pairs of cranial nerves arise from the brain.
• Spinal cord: runs to the tail tip; brachial and lumbar enlargements supply the limbs; inner H-shaped grey matter, outer white matter. Function: conduction of impulses and reflexes.

1.4 Identification of poisonous snakes and biting mechanism

India’s “Big Four” venomous snakes: Indian cobra (Naja naja), common krait (Bungarus caeruleus), Russell’s viper (Daboia russelii) and saw-scaled viper (Echis carinatus).

Key for identification

1. Tail laterally compressed (paddle-like) → sea snake (poisonous).
2. Tail cylindrical and belly scales small, not covering full width → non-poisonous (e.g. python).
3. Belly scales broad (full width): look at the head —
   • Head with small scales: viper. A pit between eye and nostril → pit viper; no pit → Russell’s viper or saw-scaled viper.
   • Head with large shields, 3rd upper labial touching eye and nasal shield, hood present → cobra.
   • Head with large shields, middle row of back scales enlarged and hexagonal, 4th lower labial largest, white cross bands → krait.
   • Otherwise usually non-poisonous (coral snakes are small exceptions).

Poison apparatus and biting mechanism

• Poison glands: modified upper labial (parotid) glands behind the eyes. A duct carries venom to the fang base.
• Fangs: modified maxillary teeth. Types: Proteroglyphous — front, fixed, grooved (cobra, krait); Solenoglyphous — front, long, hollow, movable, fold back (vipers); Opisthoglyphous — rear grooved (mildly poisonous); Aglyphous — no fangs (non-poisonous).

1. The snake opens its mouth very wide (almost 180° in vipers).
2. Quadrate swings forward; pterygoid and ectopterygoid push the maxilla, which rotates and makes the fang erect.
3. The snake strikes and the fangs pierce the skin.
4. Muscles (compressor glandulae) squeeze the gland; venom flows through the duct and fang into the wound.

Venom types: neurotoxic (cobra, krait — paralysis, breathing failure) and haemotoxic (vipers — bleeding, swelling). A poisonous bite shows two fang marks; a non-poisonous bite shows rows of small teeth marks. Treatment: anti-snake venom (ASV).

Likely exam questions

1. (Long, 8 marks) Give the general characters of Reptilia and classify Reptilia on the basis of temporal vacuities with examples and diagrams.
2. (Long, 8 marks) Describe the identification of poisonous snakes and explain the biting mechanism of a poisonous snake.
3. (Short, 4 marks) Describe the respiratory system of Calotes.
4. (Short, 4 marks) Draw a labelled diagram of the brain of Calotes and give functions of its parts.
5. (Short, 4 marks) Write a note on types of fangs in snakes.

Syllabus topics (RTMNU)

• 2.1 Aves: General characters and classification of Aves on the basis of carina (Ratitae and Carinatae).
• 2.2 Columba livia: External features, Digestive system, Respiratory system,
• 2.3 Structure and function of CNS (Brain and spinal cord)
• 2.4 Migration in Birds, Flight adaptation in birds

2.1 Aves: General characters and classification (Ratitae and Carinatae)

Aves (birds) are warm-blooded, feathered, bipedal vertebrates whose forelimbs are modified into wings. Birds are often called “glorified reptiles” (Huxley) because they evolved from reptiles.

General characters

• Exoskeleton: body covered with feathers; legs with horny scales; jaws covered by a horny beak without teeth (in living birds).
• Skin: dry and thin; the only skin gland is the oil (uropygial) gland at the base of the tail.
• Homeothermic (warm-blooded); body temperature about 40–42 °C.
• Skeleton: light, pneumatic bones (air spaces); skull monocondylic; fused bones such as synsacrum, pygostyle and furcula (wish-bone).
• Respiration: compact spongy lungs connected to air sacs; voice box is the syrinx.
• Heart: four-chambered; only the right systemic arch is present; RBCs nucleated.
• Excretion: metanephric kidneys; uricotelic; no urinary bladder (except ostrich).
• Reproduction: internal fertilization; in females only the left ovary and oviduct are functional; oviparous with large calcareous-shelled cleidoic eggs; parental care well developed.

Classification on the basis of carina (keel)

Modern birds (Neornithes) are commonly divided into Ratitae (Palaeognathae, L. ratis = raft) and Carinatae (Neognathae, L. carina = keel), based on the presence of a keel on the sternum. (Fossil Archaeopteryx is placed in a separate subclass, Archaeornithes.)

Note: Penguins are carinate (keel present) though flightless; they swim with their wings.

2.2 Columba livia: External features, digestive and respiratory system

External features

• Body spindle-shaped (streamlined), about 30 cm; divided into head, neck, trunk and tail.
• Head: beak of upper and lower mandibles; soft swollen cere at base of upper beak with slit-like nostrils; large eyes with eyelids and nictitating membrane; ear opening behind eye, hidden by feathers.
• Neck: long and flexible; head can turn widely.
• Trunk: forelimbs form wings; hind limbs have scaly shank and four clawed toes (3 in front, 1 behind) for perching and walking.
• Tail: short stump bearing a fan of 12 tail feathers (rectrices); oil gland on its dorsal side; cloacal aperture below the tail.
• Feathers: quills (remiges on wing, rectrices on tail), contour feathers, filoplumes and down feathers.

Digestive system

1. Beak → buccal cavity: no teeth; narrow pointed tongue; few salivary glands.
2. Oesophagus enlarges into a thin-walled crop for storing and softening grains. In breeding season the crop of both parents secretes “pigeon milk” (crop milk) to feed the young.
3. Proventriculus: glandular stomach; secretes gastric juice.
4. Gizzard: thick muscular stomach with horny lining; grinds grains with swallowed small stones (grit) — it does the job of teeth.
5. Small intestine: U-shaped duodenum (pancreas lies in its loop), then coiled ileum.
6. Rectum: short, with a pair of small rectal caeca; opens into the cloaca.
7. Glands: bilobed liver (bile ducts open into duodenum; gall bladder absent in pigeon); pancreas with three ducts.

Respiratory system

• Nostrils → nasal chambers → internal nares → glottis → larynx (no voice) → long trachea with complete rings.
• Syrinx: voice box at the junction of trachea and bronchi, with vibrating membranes.
• Lungs: small, compact, non-elastic, fixed to the dorsal wall; air passes through fine tubes (parabronchi) surrounded by air capillaries where gas exchange takes place.
• Air sacs: 9 thin-walled sacs — 1 interclavicular (unpaired), 2 cervical, 2 anterior thoracic, 2 posterior thoracic, 2 abdominal. They extend into hollow bones.
• Mechanism (double respiration): during inspiration the sternum moves down, fresh air enters the posterior air sacs and air from the lungs goes into the anterior sacs; during expiration the posterior sacs push fresh air through the lungs and anterior sacs empty outside. So fresh air passes over the lungs in both inspiration and expiration (one-way flow).

2.3 Columba: Structure and function of CNS (brain and spinal cord)

The pigeon brain is short, broad and fills the cranium. It is large compared to body size.

• 12 pairs of cranial nerves.
• Spinal cord: runs the full length of the neural canal; brachial enlargement (wings) and lumbosacral enlargement (legs). In the lumbosacral region the dorsal halves separate, forming the sinus rhomboidalis filled with a gelatinous (glycogen) body. Inner grey matter, outer white matter; carries impulses and controls reflexes.

2.4 Migration in birds and flight adaptations

Migration is the regular, seasonal, two-way movement of birds from their breeding place to a wintering place and back.

Types

• Latitudinal: north–south (e.g. Siberian crane to India in winter).
• Longitudinal: east–west.
• Altitudinal: up and down mountains (Himalayan birds come down in winter).
• Partial / total: some or all members migrate.

Examples: Arctic tern (longest migration, pole to pole), bar-headed goose (crosses the Himalayas), Amur falcon, flamingo, ducks and waders visiting Keoladeo (Bharatpur), Chilika lake and Nal Sarovar.

Causes: cold and short days, shortage of food, need of a safe breeding place. Stimulus: change in day length (photoperiod) acts through the pituitary and gonads; birds store fat as fuel before migrating.

Navigation: sun compass (day), star compass (night), Earth’s magnetic field, landmarks (rivers, coasts), and in some birds smell.

Significance: escape from cold, more food, safe breeding.

Flight adaptations

1. Streamlined body reduces air resistance.
2. Feathers give lift and insulation; forelimbs form wings.
3. Light skeleton: pneumatic bones; toothless beak.
4. Rigid skeleton: fused bones — synsacrum, pygostyle, carpometacarpus, furcula.
5. Keel on sternum for strong flight muscles: pectoralis major (down-stroke) and pectoralis minor (up-stroke).
6. Air sacs and double respiration give plenty of oxygen.
7. Four-chambered heart, high metabolism, warm blood.
8. Large eyes and large cerebellum for vision and balance.
9. Weight saving: no urinary bladder, single ovary.

Likely exam questions

1. (Long, 8 marks) Describe the respiratory system of Columba livia and explain the mechanism of double respiration with the role of air sacs.
2. (Long, 8 marks) What is migration? Describe its types, causes and navigation in birds, adding flight adaptations.
3. (Short, 4 marks) Distinguish between Ratitae and Carinatae with examples.
4. (Short, 4 marks) Describe the digestive system of pigeon with a labelled diagram.
5. (Short, 4 marks) Draw a labelled diagram of the brain of pigeon.

Syllabus topics (RTMNU)

• 3.1 General characters of Mammalia. Comparision of Prototherians, Metatherians and Eutherians.
• 3.2 Aquatic, arboreal and Volant adaptation in mammals.
• 3.3 Rabbit: Structure and function of digestive and respiratory system.
• 3.4 Rabbit: Structure and function of CNS (Brain and spinal cord).

3.1 General characters of Mammalia; comparison of Prototheria, Metatheria and Eutheria

Mammalia (L. mamma = breast) are warm-blooded vertebrates with hair and mammary glands. They are the most highly evolved animals.

General characters

• Skin: body covered with hair; skin has sweat, sebaceous (oil) and scent glands; mammary glands produce milk.
• External ear (pinna) present; middle ear with three ossicles (malleus, incus, stapes).
• Teeth: heterodont (incisors, canines, premolars, molars), thecodont (in sockets) and diphyodont (milk and permanent sets).
• Skull: dicondylic; secondary palate; lower jaw of a single bone (dentary) on each side.
• Seven cervical vertebrae (except sloth, manatee).
• Diaphragm separates thoracic and abdominal cavities.
• Heart: four-chambered; only the left systemic arch; RBCs non-nucleated and biconcave (oval in camel).
• Homeothermic; ureotelic; metanephric kidneys; urinary bladder present.
• Brain: large cerebrum with corpus callosum (in Eutheria); 12 pairs of cranial nerves.
• Testes usually in a scrotum; penis present; internal fertilization; mostly viviparous with a placenta; young fed on milk.

Comparison of the three groups

Significance: Prototheria link reptiles and mammals (eggs, cloaca).

3.2 Aquatic, arboreal and volant adaptations in mammals

A. Aquatic adaptations

Whales, dolphins, porpoises, dugong, seals. They are secondarily aquatic (ancestors lived on land).

• Streamlined body; no visible neck, no pinna.
• Hair reduced; thick blubber (fat) for warmth, buoyancy and food reserve.
• Forelimbs become flippers; hind limbs lost in whales; horizontal tail flukes for swimming; dorsal fin for balance.
• Nostrils on top of head as a blow-hole.
• Large lungs, much myoglobin, slow heart rate while diving.

B. Arboreal adaptations

Monkeys, apes, squirrels, sloth, loris (tree-living).

• Grasping hands and feet with opposable thumb; nails, or claws in squirrels.
• Long forelimbs for swinging (brachiation in gibbon); sloth hangs upside down with hook-like claws.
• Prehensile tail in New World monkeys.
• Eyes face forward giving stereoscopic (binocular) vision to judge distance.
• Well-developed cerebellum for balance.

C. Volant (flying) adaptations

True flight: bats (order Chiroptera), the only flying mammals. Gliding: flying squirrel (Petaurista), flying lemur (Cynocephalus), sugar glider (Petaurus, a marsupial).

• Patagium: thin skin membrane stretched between very long fingers (2nd–5th), body sides, hind limbs and tail (uropatagium). Thumb is short, free and clawed.
• Light bones; keeled sternum.
• Small rotated hind limbs; bats hang upside down.
• Small bats use echolocation (ultrasound); large pinna with tragus.
• Gliders have a furred skin fold between limbs; they glide but cannot flap.

3.3 Rabbit: Structure and function of digestive and respiratory system

Oryctolagus cuniculus is a herbivorous mammal of order Lagomorpha.

Digestive system

1. Mouth: upper lip cleft (hare lip); vestibule between lips and teeth.
2. Teeth: dental formula I 2/1, C 0/0, PM 3/2, M 3/3 = 28. Incisors chisel-like, rootless, ever-growing; a small second pair of upper incisors (peg teeth) lies behind the first; gap called diastema (no canines).
3. Buccal cavity: muscular tongue; four pairs of salivary glands — parotid, submaxillary (submandibular), sublingual and infraorbital; saliva contains amylase.
4. Pharynx → oesophagus → stomach (cardiac and pyloric parts); gastric juice digests proteins.
5. Small intestine: duodenum (receives bile and pancreatic juice), jejunum and ileum; ileum ends in a rounded sacculus rotundus.
6. Caecum: very large, ending in a finger-like vermiform appendix; bacteria here digest cellulose.
7. Colon (sacculated) and rectum forming faecal pellets; opens by anus.
8. Glands: liver (five lobes) with gall bladder; diffuse pancreas in duodenal loop with separate pancreatic duct.

Coprophagy: rabbits re-eat soft night pellets to absorb vitamins made by caecal bacteria.

Respiratory system

• External nares → nasal chambers with folded turbinal bones (warm, moisten and filter air) → internal nares → pharynx.
• Glottis guarded by epiglottis → larynx (voice box) with thyroid, cricoid and arytenoid cartilages and vocal cords.
• Trachea: supported by C-shaped cartilaginous rings (incomplete dorsally); divides into two bronchi that branch into bronchioles.
• Lungs: pink, spongy, in the thoracic cavity, covered by double pleura; right lung 4 lobes, left lung 2 lobes; millions of alveoli for gas exchange.
• Breathing: Inspiration — diaphragm contracts and flattens, intercostals lift ribs; chest volume rises, air enters. Expiration — mostly passive; air goes out.

3.4 Rabbit: Structure and function of CNS (brain and spinal cord)

The brain lies in the cranium and the spinal cord in the neural canal; both are covered by three meninges — dura mater, arachnoid and pia mater — with cerebrospinal fluid (CSF) between them.

Brain

• Forebrain: large olfactory lobes (good smell); large cerebral hemispheres, almost smooth (few grooves), joined by the corpus callosum. Diencephalon has pineal body on roof and optic chiasma, infundibulum and pituitary on floor.
• Midbrain: four optic lobes called corpora quadrigemina (anterior pair — vision; posterior pair — hearing); crura cerebri on the floor.
• Hindbrain: large cerebellum (median vermis, lateral lobes and floccular lobes); pons Varolii on the ventral side; medulla oblongata continues as spinal cord.
• Ventricles: lateral ventricles (1st and 2nd), 3rd ventricle, iter (aqueduct of Sylvius) and 4th ventricle, filled with CSF.
• 12 pairs of cranial nerves.

Spinal cord

A cylindrical cord from the foramen magnum along the neural canal, with cervical and lumbar enlargements; it ends as a thin filum terminale surrounded by nerve roots (cauda equina). In cross-section: central canal, H-shaped grey matter inside and white matter outside, with dorsal and ventral fissures. About 37 pairs of spinal nerves arise from it. Function: carries impulses to and from the brain and is the centre of reflex actions.

Likely exam questions

1. (Long, 8 marks) Give the general characters of Mammalia and compare Prototheria, Metatheria and Eutheria.
2. (Long, 8 marks) Describe the digestive system of rabbit with a labelled diagram.
3. (Short, 4 marks) Write on aquatic adaptations in mammals.
4. (Short, 4 marks) Describe volant adaptations in bats.
5. (Short, 4 marks) Draw a labelled diagram of the brain of rabbit and give functions of any four parts.

Syllabus topics (RTMNU)

• 4.1 Comparative accounts of integuments and its derivatives in Reptilia, Aves and Mammalia.
• 4.2 Comparative accounts of urinogenital systems in Reptilia, Aves and Mammalia.
• 4.3 Comparative accounts of heart and aortic arches in Reptilia, Aves and Mammalia.
• 4.4 Autonomous nervous system in mammals (Sympathetic and Parasympathetic).

4.1 Comparative account of integument and its derivatives in Reptilia, Aves and Mammalia

Integument is the skin with its derivatives. It has an outer epidermis (ectoderm) and inner dermis (mesoderm). It protects, prevents water loss and, in birds and mammals, helps keep body temperature.

Reptilia

• Skin thick, dry and nearly glandless (only a few scent/musk glands, e.g. femoral pores in lizards).
• Epidermis forms horny keratin scales; shed periodically (ecdysis) — whole slough in snakes, in pieces in lizards.
• Dermis forms bony plates — osteoderms in crocodiles; turtle shell of horny scutes over dermal bone (carapace and plastron).
• Other derivatives: claws, rattle of rattlesnake, crests and spines. Chromatophores allow colour change (Calotes, chameleon).

Aves

• Skin thin, dry and loose; no sweat glands; only gland is the uropygial (oil) gland (for preening).
• Main derivative: feathers (keratin) — quill, contour, down, filoplume; give flight, insulation and colour.
• Horny scales on legs and toes (reptilian heritage), beak, claws, spurs, comb and wattles.

Mammalia

• Skin thick and glandular; stratified epidermis with outer dead stratum corneum and inner Malpighian layer; dermis with blood vessels, nerves and fat.
• Hair: keratin from hair follicles; insulation; modified as spines (porcupine), whiskers (vibrissae).
• Glands: sweat (cooling, excretion), sebaceous (oil), scent, mammary (milk), wax (ear).
• Horny structures: nails, claws, hoofs, horns (bony core with keratin sheath in cattle; rhino horn of compact keratin fibres; antlers of deer are bony and shed yearly), scales of pangolin, baleen plates of whales.

4.2 Comparative account of urinogenital systems in Reptilia, Aves and Mammalia

Excretory part

Male reproductive system

• Reptilia: paired testes → vasa efferentia → epididymis → vas deferens → cloaca. Copulatory organs: paired hemipenes in lizards and snakes; single penis in turtles and crocodiles.
• Aves: paired bean-shaped testes in abdomen (enlarge in breeding season); vas deferens dilates into a seminal vesicle and opens into the cloaca. Penis absent in most (present in ratites and ducks); sperm passed by “cloacal kiss”.
• Mammalia: testes in scrotum (cooler for sperm); epididymis → vas deferens → ejaculatory duct → urethra through the penis. Accessory glands: seminal vesicles, prostate, Cowper’s glands.

Female reproductive system

• Reptilia: paired ovaries and oviducts; oviduct secretes albumen and shell; opens into cloaca.
• Aves: only left ovary and left oviduct functional. Oviduct parts: infundibulum (fertilization) → magnum (albumen) → isthmus (shell membranes) → uterus/shell gland (calcareous shell) → vagina → cloaca.
• Mammalia: paired ovaries; fallopian tubes; uterus (duplex in rabbit, bicornuate in cow, simplex in man); vagina opening by vulva. Cloaca absent (present only in monotremes); separate anus and urogenital opening.

4.3 Comparative account of heart and aortic arches in Reptilia, Aves and Mammalia

The vertebrate embryo has six pairs of aortic arches. In amniotes arches 1, 2 and 5 disappear. Arch 3 forms the carotids, arch 4 the systemic arch, arch 6 the pulmonary arch.

Reptilia

• Heart three-chambered: two auricles and one ventricle incompletely divided by a partial septum, so some mixing of blood occurs. Crocodiles have a four-chambered heart.
• Sinus venosus present; no conus — three trunks arise directly from the ventricle.
• Three arches: pulmonary arch (to lungs), right systemic arch (from left side of ventricle, oxygenated blood; gives carotids) and left systemic arch (from right side, mixed blood). Both systemic arches join to form the dorsal aorta.
• In crocodiles the two systemic arches connect by the foramen of Panizza.

Aves

• Heart four-chambered; complete double circulation, no mixing.
• Sinus venosus merged into right auricle; right auriculo-ventricular valve is a muscular flap.
• Only the right systemic arch (right 4th) remains; carotids from 3rd; pulmonary from 6th.

Mammalia

• Heart four-chambered; sinus venosus reduced to the SA node (pacemaker).
• Valves: tricuspid (right), bicuspid/mitral (left), semilunar valves at the base of aorta and pulmonary trunk.
• Only the left systemic arch remains (aorta); right 4th arch forms part of the right subclavian artery. Carotids from 3rd, pulmonary from 6th. Embryonic ductus arteriosus becomes the ligamentum arteriosum.

4.4 Autonomic nervous system in mammals (sympathetic and parasympathetic)

The autonomic nervous system (ANS) is the part of the peripheral nervous system that controls involuntary organs — heart, smooth muscles, glands. It works through a chain of two neurons: preganglionic and postganglionic, with a ganglion between them.

Sympathetic system (thoraco-lumbar)

• Preganglionic fibres arise from the lateral horn of the spinal cord in thoracic and upper lumbar segments.
• Ganglia form two sympathetic chains on either side of the vertebral column, plus collateral ganglia (coeliac, mesenteric).
• Preganglionic fibres short; postganglionic long.
• Neurotransmitter: acetylcholine at the ganglion, mostly noradrenaline at the organ (adrenergic).
• Prepares the body for emergency — “fight or flight”.

Parasympathetic system (cranio-sacral)

• Fibres arise from the brain in cranial nerves III, VII, IX and X (vagus) and from sacral spinal segments.
• Ganglia lie near or in the wall of the organ; preganglionic fibres long, postganglionic short.
• Neurotransmitter: acetylcholine at both ends (cholinergic).
• Conserves energy — “rest and digest”.

Key point: most organs get dual innervation; the two systems act opposite to each other (antagonistic) and keep a balance (homeostasis).

Likely exam questions

1. (Long, 8 marks) Give a comparative account of heart and aortic arches in Reptilia, Aves and Mammalia.
2. (Long, 8 marks) Describe the autonomic nervous system of mammals and differentiate between sympathetic and parasympathetic systems.
3. (Short, 4 marks) Compare the integument and its derivatives in reptiles, birds and mammals.
4. (Short, 4 marks) Describe the female reproductive system of a bird.
5. (Short, 4 marks) Write a note on the excretory system of Reptilia, Aves and Mammalia.