From cells to organ systems
In many-celled animals, similar cells group together to form a tissue. Animals have four basic tissues: epithelial (covering), connective (joining and support, like bone and blood), muscular (movement) and neural (messages). Tissues combine to make organs (like the stomach or heart), and organs work together as organ systems (like the digestive system). This pattern of cell → tissue → organ → organ system is the structural organisation of an animal. We study it using the frog.
The frog: habitat and body form (morphology)
Frogs live both in water and on land, so they are amphibians. The common Indian frog is Rana tigrina (now Hoplobatrachus tigerinus). Frogs are cold-blooded (poikilotherms): their body temperature changes with the surroundings. They can change colour to hide among leaves and mud (camouflage). In hot summers they hide deep in the ground (aestivation, summer sleep) and in cold winters too (hibernation, winter sleep).
- Skin: smooth, slippery, moist because of mucus; olive green on the back with dark spots, pale yellow below. Frogs never drink water; they absorb it through the skin.
- Body: only a head and a trunk. No neck, no tail.
- Head: a mouth, a pair of nostrils above it, bulging eyes covered by a thin nictitating membrane that protects them in water, and a round tympanum (eardrum) on each side behind the eyes that picks up sound.
- Limbs: short forelimbs help to land after a jump; long hind limbs push for jumping and swimming. Hands have 4 digits, feet have 5 larger digits with webs for swimming.
- Sexual dimorphism: males have vocal sacs (to croak) and a copulatory pad on the first digit of the forelimb; females have neither.
Digestive system
The frog is a carnivore (eats insects, worms), so its gut is short. It catches prey with a sticky bilobed (forked) tongue that is attached at the front.
Path of food: mouth → buccal cavity → pharynx → oesophagus (short tube) → stomach → intestine → rectum → out through the cloaca.
- The stomach walls make HCl and gastric juices that partly digest the food; the churned food is called chyme.
- The liver makes bile, stored in the gall bladder. Bile breaks fat into tiny drops (emulsification).
- The pancreas makes pancreatic juice with enzymes for carbohydrates and proteins.
- Digestion finishes in the intestine. Its inner wall has finger-like folds called villi and microvilli that absorb digested food.
- Undigested waste leaves through the cloaca.
Respiratory system
A frog breathes in two ways:
- In water: cutaneous respiration. Oxygen dissolved in water passes straight through the moist skin into the blood (diffusion).
- On land: through the buccal cavity (mouth lining), the skin and the lungs. Breathing with lungs is pulmonary respiration. The lungs are a pair of long, pink, bag-like organs in the upper trunk (thorax). Air enters through the nostrils into the buccal cavity and then into the lungs.
- During aestivation and hibernation, gas exchange happens only through the skin.
Circulatory system
The frog has a well-developed closed circulatory system: blood always stays inside the heart and vessels. It also has a lymphatic system (lymph, lymph channels and lymph nodes).
- Heart: a muscular organ in the upper body cavity, covered by a membrane called the pericardium. It has three chambers: two atria and one ventricle.
- A triangular chamber, the sinus venosus, collects blood from the body through the vena cava and pours it into the right atrium. The left atrium gets blood from the lungs (pulmonary veins).
- The ventricle pumps blood into a sac called the conus arteriosus, from which arteries carry it to all parts (arterial system). Veins bring it back (venous system).
- Special links: the hepatic portal system (between liver and intestine) and the renal portal system (between kidney and the lower body).
- Blood = plasma + cells: red blood cells (RBCs, which have a nucleus and carry red haemoglobin), white blood cells and platelets.
Excretory system
Nitrogen waste is removed by a pair of kidneys: compact, dark red, bean-shaped organs a little behind the middle of the body, near the backbone. Each kidney has many tiny filtering units called uriniferous tubules (nephrons). Two ureters carry urine to the cloaca; a thin-walled urinary bladder lies in front of the rectum and also opens into the cloaca.
In the male, the ureter carries both urine and sperm, so it is called the urinogenital duct. In the female, ureters and oviducts open separately into the cloaca. The frog passes out urea, so it is ureotelic.
Control and coordination: nervous and endocrine systems
Control is done by nerves (fast) and hormones (slow).
Endocrine glands
Pituitary, thyroid, parathyroid, thymus, pineal body, pancreatic islets, adrenals and gonads.
Nervous system
- Central nervous system: brain + spinal cord. The brain is kept inside a bony box, the brain box (cranium). It has a forebrain (olfactory lobes, paired cerebral hemispheres, diencephalon), a midbrain (a pair of optic lobes) and a hindbrain (cerebellum and medulla oblongata). The medulla continues as the spinal cord through the foramen magnum, inside the vertebral column.
- Peripheral nervous system: 10 pairs of cranial nerves from the brain, plus spinal nerves.
- Autonomic nervous system: sympathetic and parasympathetic, controlling organs without our will.
Sense organs
Touch (sensory papillae on skin), taste (taste buds), smell (nasal epithelium), vision (eyes) and hearing (tympanum with inner ears). Eyes and inner ears are well organised; the rest are simple nerve endings. The eyes are a pair of simple, round eyes in bony sockets.
Reproductive system and development
Male and female frogs are separate, and their reproductive organs are well organised.
Male
A pair of yellowish, oval testes stuck to the upper part of the kidneys by a double fold of peritoneum called the mesorchium. Vasa efferentia (10–12) come out of the testes, enter the kidney on its side and open into Bidder's canal, which joins the urinogenital duct. That duct opens into the cloaca, a small middle chamber that passes out faeces, urine and sperm.
Female
A pair of ovaries near the kidneys but not connected to them. A pair of oviducts opens separately into the cloaca. A mature female lays 2500 to 3000 eggs at a time.
Fertilisation and development
Fertilisation is external: it happens in water. Development has a larval stage called the tadpole, which changes into an adult frog by metamorphosis.
Why frogs matter
They eat insects and protect crops, keep the ecosystem in balance as a link in food chains, and their legs are eaten as food in some countries.
Key formulas and definitions
- Cell → tissue → organ → organ system → organism
- Frog heart: 2 atria + 1 ventricle (3 chambers) + sinus venosus + conus arteriosus
- Food path: mouth → buccal cavity → pharynx → oesophagus → stomach → intestine → rectum → cloaca
- Breathing: water = skin (cutaneous); land = buccal cavity + skin + lungs (pulmonary)
- Male sperm path: testis → vasa efferentia → Bidder's canal → urinogenital duct → cloaca
- Nitrogen waste = urea → ureotelic; 10 pairs of cranial nerves
Worked examples
1. A frog sits buried in mud for the whole summer. How does it breathe, and what is this sleep called?
This summer sleep is aestivation. The lungs are not used; the frog exchanges gases only through its moist skin (cutaneous respiration).
2. Trace a sperm from the testis to the outside in a male frog.
Testis → vasa efferentia → kidney → Bidder's canal → urinogenital duct (ureter) → cloaca → outside.
3. You have two frogs. One has vocal sacs and a pad on the first finger of the forelimb. Which one is male and why?
The one with vocal sacs and a copulatory pad is male. These are male-only features (sexual dimorphism); the female has neither.
4. Name the chamber that receives blood from (a) the body and (b) the lungs, and the one that sends blood out.
(a) Body blood → sinus venosus → right atrium. (b) Lung blood → pulmonary veins → left atrium. The single ventricle pumps it out through the conus arteriosus.
5. Why does a frog not need to drink water?
Its skin is thin and moist, so water enters the body directly through the skin by absorption. That is enough for the frog.
6. List three organs whose products enter the frog's cloaca.
Rectum (undigested waste), kidneys via ureters and bladder (urine), and gonads (sperm through the urinogenital duct in males; eggs through oviducts in females).
Common mistakes
- Saying the frog heart has 4 chambers. It has 3: two atria and one ventricle (plus sinus venosus and conus arteriosus, which are not counted as chambers).
- Thinking the ovaries are joined to the kidneys. The testes are attached (by mesorchium); the ovaries only lie near the kidneys.
- Writing 12 pairs of cranial nerves (that is humans). The frog has 10 pairs.
- Saying frogs breathe only with lungs. In water and during aestivation/hibernation they use only the skin.