🐸 Frog – Morphology and Anatomy

The common frog described in NCERT is the Indian bullfrog — earlier written as Rana tigrina and now commonly accepted as Hoplobatrachus tigerinus. It belongs to:

  • Phylum: Chordata
  • Subphylum: Vertebrata
  • Class: Amphibia

Frogs are amphibians because they show a dual mode of life (land + water). They are poikilotherms/ectotherms (cold-blooded) — their body temperature depends on the environment.

  • Hibernation (winter sleep): during cold season
  • Aestivation (summer sleep): during hot, dry season

1️⃣ External Morphology (Important NCERT Points)

✅ Body Shape and Regions

  • Body is streamlined for swimming.
  • Divided into Head + Trunk (adult frog has no neck and no tail).
  • Skin: smooth, moist, glandular.
  • Mucus glands keep skin moist (very important for respiration).
  • Skin is also highly vascular (rich in blood capillaries), aiding gas exchange.

✅ Colour and Protection

  • Dorsal side: olive green/brown with dark patches → helps in camouflage (protective coloration).
  • Ventral side: pale/yellowish.

✅ Sense Organs and External Features

  • Eyes: prominent; have a nictitating membrane (a transparent third eyelid) that:
  • protects the eye in water,
  • keeps it moist,
  • allows vision underwater.
  • Tympanum: circular membrane behind the eye; acts as an eardrum (frogs lack external pinna).
  • Nostrils: external openings for respiration.

✅ Limbs (Adaptations)

  • Forelimbs: shorter, used mainly for support and balance.
  • Hindlimbs: long, muscular, and webbed toes → adapted for jumping (leaping) and swimming.

✅ Male vs Female (NEET-asked)

  • Male frogs generally have:
  • vocal sacs (for mating call)
  • nuptial/copulatory pads on the first digit of forelimb (to hold female during amplexus)

2️⃣ Internal Anatomy (System-wise)

(A) Digestive System

Frog is carnivorous/insectivorous.

✅ Alimentary Canal (Sequence)

Mouth → Buccal cavity → Pharynx → Oesophagus → Stomach → Small intestine → Large intestine/Rectum → Cloaca → Cloacal aperture

✅ Special Features

  • Tongue: muscular, bilobed at the tip, and attached anteriorly (in front) → can be flipped out quickly to catch insects.
  • Teeth: maxillary teeth help in gripping prey (frog does not chew).

✅ Accessory Glands

  • Liver: secretes bile (stored in gall bladder) → emulsifies fats.
  • Pancreas: secretes digestive enzymes (trypsin, amylase, lipase) into duodenum.

✅ Absorption

  • Intestine has villi + microvilli to increase surface area for absorption.

(B) Respiratory System (Very High-Yield)

Frog can respire by three methods:

  1. Cutaneous respiration (through skin):
  • Most important in water and during resting.
  • Requires moist, vascular skin.
  1. Buccopharyngeal respiration:
  • Through lining of buccal cavity and pharynx.
  • Occurs mainly on land when frog is at rest with mouth closed.
  1. Pulmonary respiration (lungs):
  • Mainly on land during activity.
  • Frog uses positive pressure breathing (buccal pumping) to force air into lungs.

(C) Circulatory System

  • Closed circulatory system.
  • Heart is three-chambered:
  • 2 atria (auricles): Right + Left
  • 1 ventricle

✅ Blood Flow (Key Concept)

  • Right atrium: receives deoxygenated blood from body.
  • Left atrium: receives oxygenated blood from lungs/skin.
  • Both enter a single ventricle → partial mixing occurs.

✅ Incomplete Double Circulation

Frog has:

  • Pulmonary circuit (heart → lungs/skin → heart)
  • Systemic circuit (heart → body → heart)

It is called incomplete double circulation because oxygenated and deoxygenated blood are not completely separated due to the single ventricle.

✅ Frog RBCs

  • Frog RBCs are nucleated and oval (unlike human RBCs which are enucleated).

(D) Excretory System

  • Kidneys: paired mesonephric kidneys.
  • Frog is ureotelic → excretes urea.
  • Ureters:
  • In male, ureters also carry sperms → urinogenital duct.
  • In female, ureters carry only urine; reproductive ducts are separate.
  • Urinary bladder present (stores urine).

(E) Nervous System

  • Central nervous system (CNS): brain + spinal cord.
  • Peripheral nervous system (PNS): cranial nerves + spinal nerves.
  • Sense organs: eyes, tympanum, olfactory sacs.

(F) Reproductive System and Development

  • Frog is dioecious (male and female separate).
  • Fertilization is external and occurs in water.
  • Female lays eggs in gelatinous masses.
  • Development is indirect:
  • Egg → Tadpole larva → Metamorphosis → Adult frog

3️⃣ Economic Importance

  • Frogs help control insects (biological pest control).
  • Important link in food chains (food for snakes, birds).
  • Used historically for biology studies (specimens).

🧠 NEET Memory Hooks

  • Moist skin → cutaneous respiration
  • 3-chambered heart → incomplete double circulation
  • Cloaca → common chamber
  • External fertilization + tadpole → metamorphosis

💡 Questions and Answers

Q1. Why are frogs called amphibians?

A1.

Frogs live both on land and in water. They breed in water, their larva (tadpole) is aquatic, and the adult can survive on land as well. Hence they show a dual mode of life.

Q2. What is the significance of the frog’s moist and vascular skin?

A2.

The skin has mucus glands that keep it moist and it has many blood capillaries (it is vascular). Oxygen dissolves in this moisture and diffuses into blood, while CO₂ diffuses out. This makes skin a major respiratory surface for cutaneous respiration, especially in water.

Q3. Explain how respiration differs in water vs on land in frogs.

A3.

  • In water: Respiration is mainly cutaneous (through skin).
  • On land: Frog uses lungs (pulmonary) and also buccopharyngeal respiration along with skin.

Q4. Why is frog circulation called ‘incomplete double circulation’?

A4.

Frogs have two circuits (pulmonary and systemic), so blood passes twice through the heart in one cycle. But the heart has a single ventricle, so oxygenated and deoxygenated blood can partially mix, making separation incomplete.

Q5. What is a cloaca? Mention systems opening into it.

A5.

The cloaca is a common chamber at the posterior end of the body. The openings of:

  1. Digestive system (rectum),
  2. Excretory system (ureters), and
  3. Reproductive system (urinogenital ducts) open into it, and materials exit through the cloacal aperture.

Q6. Why does a frog require metamorphosis?

A6.

The tadpole is aquatic and has larval adaptations, while the adult frog is amphibious and has lungs and limbs. Metamorphosis converts the larval body plan into the adult form suitable for life on land and water.

💡 Short Questions and Answers

Q1. Name two features that help frog in swimming.
A1. (i) Streamlined body reduces water resistance, and (ii) webbed hindlimbs act like paddles for efficient swimming.

Q2. What is the function of the tympanum in frog?
A2. Tympanum is the eardrum. It receives sound vibrations and helps in hearing.

Q3. Why is the frog’s tongue attached in front?
A3. The tongue is attached anteriorly, so it can be flipped outward quickly to capture insects. It is sticky and bilobed at the tip to hold prey.

Q4. Write one difference between frog RBC and human RBC.
A4. Frog RBCs are nucleated and oval, while human RBCs are enucleated and biconcave.

Q5. What is meant by ureotelic nature of frog?
A5. Frog excretes nitrogenous waste mainly as urea, so it is ureotelic.

Q6. Why is external fertilization linked with water in frogs?
A6. In frogs, eggs and sperms are released outside the body; water provides a medium for sperm movement and prevents drying of gametes and eggs. Hence fertilization occurs in water.

Q7. Name two male-specific features in frogs.
A7. (i) Vocal sacs (for mating call), and (ii) Nuptial pads on the first digit of forelimb (helps in holding female during amplexus).