The external anatomy of amphibians is a result of evolutionary adaptation to a “dual life.” Their structure must facilitate locomotion and protection on land while maintaining physiological functions (like respiration) that are traditionally aquatic.
1. Integumentary System (The Skin)
The skin is the most distinctive external feature of an amphibian, serving as a respiratory organ and a primary defense mechanism.
- Texture and Moisture: Unlike reptiles, amphibians lack scales. The skin is smooth and kept moist by mucus glands. This moisture is vital for cutaneous respiration, where oxygen is dissolved in the liquid film and absorbed into the bloodstream.
- Granular (Poison) Glands: These glands secrete toxic or distasteful substances to deter predators. In toads, these are concentrated in the parotoid glands located behind the eyes.
- Chromatophores: Specialized pigment cells in the skin that allow for color changes. These assist in thermoregulation, camouflage (cryptic coloration), and signaling (aposematism).
2. Head and Sensory Structures
The head is usually broad and flat, housing several specialized organs adapted for terrestrial sensing.
- Eyes and Eyelids: Amphibians possess protruding eyes for a wide field of vision. Most have movable eyelids and a nictitating membrane (a transparent third eyelid) that protects and moistens the eye while underwater.
- Tympanic Membrane: Also known as the tympanum, this is a circular patch of skin behind the eye that functions as an external eardrum to transmit sound vibrations to the inner ear.
- Nares (Nostrils): External openings that lead to the nasal cavity. They are equipped with valves that close when the animal is submerged.
- Sensory Tentacles: Found exclusively in Caecilians (Order Gymnophiona), these are retractable structures between the eye and nostril used for chemosensory perception in subterranean environments.
3. Appendages and Locomotion
Amphibian limbs are adapted based on their specific order and habitat.
- Forelimbs and Hindlimbs: In Anurans (frogs/toads), the hindlimbs are significantly longer and more muscular, adapted for jumping and swimming. Salamanders possess four limbs of roughly equal size for walking.
- Webbing: The digits of the hindlimbs are often connected by thin membranes (webbing) to increase surface area for efficient swimming.
- Adhesive Pads: Arboreal species, such as tree frogs, have specialized toe pads that create surface tension or friction, allowing them to cling to vertical surfaces.
- Nuptial Pads: During the breeding season, males of many species develop thickened, rough patches on their thumbs to help them grip females during amplexus (mating).
4. Cloaca: The Common Exit
Amphibians do not have separate openings for digestive, urinary, and reproductive systems. Instead, they possess a cloaca, a single posterior chamber and opening (the cloacal vent) through which waste and gametes are expelled.
5. Comparison of External Morphology by Order
| Feature | Anura (Frogs/Toads) | Caudata (Salamanders) | Gymnophiona (Caecilians) |
| Body Shape | Short, tailless, “squat” | Elongated with a distinct tail | Cylindrical and worm-like |
| Limb Presence | Four (unequal size) | Four (equal size) | Completely absent |
| Visible Ears | Distinct tympanum | Internal/Hidden | Reduced or absent |
| Skin Folds | Dorsolateral folds (some) | Costal grooves on sides | Annuli (ring-like folds) |
UPSC Prelims Trivia: External Adaptations
- Fossorial Adaptations: Burrowing amphibians often have hardened “spurs” or tubercles on their hind feet to act as spades for digging.
- Absence of Claws: Most amphibians lack true claws (except for the Xenopus or African Clawed Frog), having soft, rounded tips on their digits.
- External Gills: While most adults lose them, some aquatic salamanders (like the Axolotl) retain bushy, red external gills behind the head throughout their lives.