Internal Anatomy of Amphibians

The internal anatomy of amphibians reflects their evolutionary transition from water to land. Their systems are designed to accommodate a dual lifestyle, featuring organs that handle both aquatic and terrestrial physiological demands.

1. Digestive System

Amphibians possess a complete digestive tract that begins at the mouth and terminates at the cloaca.

• Mouth and Tongue: Most amphibians have a “protrusible” tongue attached at the front of the lower jaw, allowing it to flip forward to capture prey.
• Teeth: Many possess vomerine teeth (on the roof of the mouth) and maxillary teeth (along the upper jaw), used primarily to grip prey rather than chew.
• Alimentary Canal: Consists of the esophagus, stomach (where chemical digestion begins), small intestine (nutrient absorption), and large intestine (water reabsorption).
• Accessory Organs: The liver (produces bile) and pancreas (produces digestive enzymes) are well-developed. The liver also serves as a primary site for glycogen storage.

2. Circulatory System

Amphibians exhibit a closed circulatory system with a “double circulation” pattern (pulmonary and systemic).

• • Three-Chambered Heart: The heart consists of two atria and one ventricle.
    • The Right Atrium receives deoxygenated blood from the body.
    • The Left Atrium receives oxygenated blood from the lungs/skin.
    • The Ventricle pumps the mixed blood to both the lungs and the rest of the body.
  • Spiral Valve: Located in the conus arteriosus, it helps direct oxygenated blood toward the brain and deoxygenated blood toward the lungs, minimizing the inefficiency of blood mixing in the single ventricle.

3. Respiratory System

Amphibians utilize a unique “tri-modal” respiratory strategy depending on their life stage and environment.

• Cutaneous Respiration: Gas exchange through the moist skin. This is the primary method during hibernation or while submerged.
• Buccopharyngeal Respiration: Exchange of gases across the moist lining of the mouth and pharynx.
• Pulmonary Respiration: Utilization of lungs. Amphibians use positive pressure breathing (buccal pumping), where they actively “swallow” air into the lungs because they lack a diaphragm.
• Gills: Present in all larval stages and retained in some “obligate aquatic” adult salamanders.

4. Excretory and Osmoregulatory System

Maintaining water balance is a major challenge for amphibians due to their permeable skin.

• Kidneys: A pair of mesonephric kidneys filter nitrogenous waste from the blood.
• Waste Product: * Larval (aquatic) stages typically excrete ammonia (ammonotelic).
  • Adult (terrestrial) stages primarily excrete urea (ureotelic) to conserve water.
• Urinary Bladder: A large, thin-walled sac that stores urine. It can also act as a water reservoir during dry periods.

5. Nervous and Sensory Systems

• Brain Structure: The brain is divided into the forebrain (olfaction), midbrain (vision), and hindbrain (hearing and balance). The optic lobes (midbrain) are particularly prominent.
• Tympanum and Columella: The tympanum (external ear) vibrates and transmits sound through a single bone called the columella (homologous to the stapes in humans) to the inner ear.
• Jacobson’s Organ: A specialized chemoreceptive organ located in the nasal cavity used to detect pheromones and chemical signals.

6. Reproductive System

Amphibians are typically dioecious (separate sexes) with distinct internal reproductive organs.

• Males: Possess a pair of testes located near the kidneys. Sperm is transported through the vasa efferentia into the ureters (uromas).
• Females: Possess paired ovaries. Eggs pass through long, coiled oviducts where they are coated in a jelly-like substance before exiting through the cloaca.
• Fat Bodies: Yellow, finger-like projections attached to the gonads that store energy required for gamete production and hibernation.

Comparison of Larval vs. Adult Internal Anatomy

UPSC Prelims Fact File

• Positive Pressure Ventilation: Unlike humans who create a vacuum (negative pressure) to breathe, amphibians must force air into their lungs.
• Erythrocytes: Amphibian Red Blood Cells (RBCs) are nucleated and oval-shaped, unlike the non-nucleated RBCs of mammals.
• Hibernation and Aestivation: During these periods of dormancy, the metabolic rate drops significantly, and the animal relies almost entirely on cutaneous respiration and stored fat bodies.