In which one of the following animals is respiration done by skin ?

1. Classification of the Animal Kingdom: Vertebrates (basic)

To understand the diversity of the animal kingdom, we start with the fundamental division between those with backbones and those without. Vertebrates (subphylum Vertebrata) belong to the phylum Chordata. The defining feature of a vertebrate is the possession of a vertebral column or backbone, which protects the spinal cord and provides a structural framework for the body. This allows vertebrates to grow to much larger sizes and develop complex nervous systems compared to most invertebrates. Environment, Shankar IAS Academy, Chapter 16, p.249

Vertebrates are typically classified into five major classes based on their physical characteristics, reproductive methods, and how they regulate body temperature. These classes represent an evolutionary journey from aquatic life to fully terrestrial existence. For instance, while fish are restricted to water, amphibians like frogs represent a 'middle ground,' capable of living on land but remaining tied to water for reproduction and often for breathing through their skin. Science - Class VII NCERT, Chapter 9, p.133

Here is a quick comparison of the five classes of vertebrates:

Class	Key Features	Examples
Pisces (Fish)	Gills for breathing, scales, cold-blooded, aquatic.	Flying fish, Sea horse
Amphibia	Moist skin, can live on land and water, lay eggs in water.	Frogs, Toads, Salamanders
Reptilia	Dry scaly skin, lay leathery eggs, mostly terrestrial.	Chameleons, Snakes, Turtles
Aves (Birds)	Feathers, hollow bones, warm-blooded, lay hard-shelled eggs.	Eagles, Penguins, Sparrows
Mammalia	Milk-producing glands, hair/fur, warm-blooded, give birth to live young.	Dolphins, Humans, Kangaroos

Understanding these classifications is crucial because their biological structures—such as whether they have gills, lungs, or moist skin—dictate their survival strategies and habitats. For example, mammals like the Dugong (Sea Cow) or the River Dolphin have adapted to aquatic life but still retain mammalian traits like breathing air through lungs. Environment and Ecology, Majid Hussain, BIODIVERSITY, p.16

Sources: Environment, Shankar IAS Academy, Classification or Species, p.249; Science - Class VII NCERT, Life Processes in Animals, p.133; Environment and Ecology, Majid Hussain, BIODIVERSITY, p.16

2. Diverse Mechanisms of Respiration in Animals (basic)

At its core, respiration is the process of exchanging gases—specifically taking in oxygen (O₂) for energy production and releasing carbon dioxide (CO₂) as a waste product Science-Class VII . NCERT(Revised ed 2025), Chapter 9, p.129. While humans rely exclusively on lungs, the animal kingdom displays a fascinating variety of respiratory mechanisms, each finely tuned to the animal's habitat and biological complexity Science-Class VII . NCERT(Revised ed 2025), Chapter 9, p.134. The primary goal is always to maximize the surface area available for gas diffusion.

Animals living in aquatic environments, such as bony fish and sea horses, typically utilize gills (branchial respiration) to extract dissolved oxygen from water. In contrast, terrestrial animals like mammals, birds, and reptiles have evolved lungs. However, the skin also plays a critical role for some. For example, frogs are unique because they are amphibians; while adult frogs have lungs for breathing on land, they also perform cutaneous respiration—breathing through their moist, porous skin—which is essential when they are underwater Science-Class VII . NCERT(Revised ed 2025), Chapter 9, p.133.

Mechanism	Primary Organ	Example Animals
Cutaneous	Skin	Earthworms, Frogs (in water)
Branchial	Gills	Fish, Sea horses, Tadpoles
Pulmonary	Lungs	Mammals, Birds, Chameleons
Tracheal	Air Tubes	Insects (like cockroaches)

Adaptation is the key driver here. A chameleon, being a reptile, has dry and scaly skin designed to prevent desiccation (drying out) in arid environments. Because its skin is thick and waterproof, it cannot be used for gas exchange, making the chameleon entirely dependent on its lungs. On the other hand, birds possess highly efficient lungs adapted for the high-energy demands of flight, even at high altitudes where oxygen levels are significantly lower Science-Class VII . NCERT(Revised ed 2025), Chapter 9, p.136.

Sources: Science-Class VII . NCERT(Revised ed 2025), Chapter 9: Life Processes in Animals, p.129, 133, 134, 136

3. Thermoregulation: Ectotherms vs. Endotherms (intermediate)

At the heart of animal survival is the ability to manage body heat, a process known as thermoregulation. Animals are broadly categorized into two groups based on how they generate and maintain this heat. Endotherms (commonly called 'warm-blooded') are animals like mammals and birds that maintain a relatively constant internal body temperature primarily through internal metabolic processes Environment, Shankar IAS Academy, Environmental Issues and Health Effects, p.419. This allows them to remain active in diverse climates, but it comes at a high 'fuel' cost; they must consume significant amounts of food to keep their internal furnace burning.

In contrast, ectotherms (or 'cold-blooded' animals), such as reptiles, amphibians, and fish, do not use their metabolism to maintain a steady body temperature. Instead, their internal temperature fluctuates with the environment Environment, Shankar IAS Academy, Environmental Issues and Health Effects, p.419. Because they rely on external heat sources, you will often see a lizard basking in the sun to warm up or a frog retreating into the mud to cool down. While this makes them vulnerable to extreme weather, it is incredibly energy-efficient — an ectotherm can survive on a fraction of the food required by an endotherm of the same size.

Temperature changes have profound effects on the biology of these organisms. For ectotherms, especially aquatic ones, a rise in environmental temperature can lead to a dangerous cycle: it increases their metabolic rate, forcing them to consume more food just to stay alive, which can lead to population crashes if food sources are limited Environment, Shankar IAS Academy, Environmental Pollution, p.78. Even a shift of 1-2°C can cause significant adverse effects at the cellular level, highlighting why climate stability is so critical for biodiversity Environment, Shankar IAS Academy, Environmental Pollution, p.78.

Feature	Endotherms (Warm-blooded)	Ectotherms (Cold-blooded)
Heat Source	Internal (Metabolism)	External (Environment)
Energy Needs	High (need to eat often)	Low (can survive longer without food)
Examples	Birds, Humans, Dogs Science, Class VIII NCERT, p.223	Snakes, Fish, Frogs Science, Class VII NCERT, p.133

Sources: Environment, Shankar IAS Academy, Environmental Issues and Health Effects, p.419; Environment, Shankar IAS Academy, Environmental Pollution, p.78; Science, Class VII NCERT (Revised ed 2025), Life Processes in Animals, p.133; Science, Class VIII NCERT (Revised ed 2025), Our Home: Earth, a Unique Life Sustaining Planet, p.223

4. Reptilian Adaptations and Water Conservation (intermediate)

To understand reptilian adaptations, we must first look at the greatest challenge faced by animals transitioning from water to land: desiccation (drying out). Unlike their amphibian cousins, who must stay near water to keep their skin moist for breathing, reptiles have evolved to be the true masters of dry land. The defining feature of a reptile is its dry, scaly skin. This skin is rich in keratin, the same protein found in your fingernails, which creates a waterproof barrier. While this protects the animal from losing internal moisture to the environment, it creates a physiological trade-off: because the skin is impermeable, reptiles cannot breathe through it. Consequently, they rely almost entirely on lungs for gas exchange, a significant shift from the cutaneous (skin) respiration seen in amphibians Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.133.

Beyond their skin, reptiles exhibit several structural features that aid their survival in diverse landscapes. Most reptiles are ectothermic (cold-blooded), meaning they regulate their body temperature through external sources like sunlight. Morphologically, they are characterized by having scales, ear holes instead of external ear flaps, and typically possess four legs (though some, like snakes, have lost them through evolution) Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.154. These physical traits allow them to inhabit niches ranging from scorching deserts to tropical rivers.

Water conservation also extends to how reptiles manage waste and reproduce. To save every drop of water, many reptiles excrete nitrogenous waste as uric acid, a semi-solid paste that requires very little water to expel. Furthermore, their amniotic eggs—which have a leathery or hard shell—allow the embryo to develop in a self-contained "aquatic" environment even on dry land Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.154. This independence from water bodies for reproduction is what allowed reptiles to colonize the interior of continents. Consider the Gharial (Gavialis gangeticus), a specialized crocodile with a long, thin snout; even though it lives in the water, it retains these reptilian hallmarks of scaly skin and lung-based breathing, showing that these adaptations remain vital even in aquatic reptiles Environment and Ecology, Majid Hussain, BIODIVERSITY, p.47.

Sources: Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.133; Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.154; Environment and Ecology, Majid Hussain, BIODIVERSITY, p.47

5. Amphibian Physiology: The Dual Life (exam-level)

The term Amphibian literally means "double life" (from the Greek amphi meaning both and bios meaning life). This dual existence is not just about where they live, but how they function biologically. To survive in both aquatic and terrestrial environments, amphibians like frogs have evolved a highly flexible respiratory strategy. While the core chemical process of respiration remains the same—breaking down glucose in the presence of oxygen to release energy, CO₂, and water—the physical organs used to capture that oxygen change throughout their life cycle Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.149.

During the larval stage, such as a tadpole, amphibians are strictly aquatic and rely on gills (branchial respiration) to extract dissolved oxygen from water. However, as they undergo metamorphosis into adults, their physiology transforms. Adult frogs develop lungs for breathing on land, but these lungs are relatively simple and often insufficient on their own. To supplement this, adult amphibians utilize their skin for gas exchange, a process known as cutaneous respiration Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.133.

For cutaneous respiration to work, the skin must remain moist and porous. This allows oxygen to dissolve and pass directly into the blood vessels near the skin's surface. This is why you will often find frogs in humid environments or near water bodies; if their skin dries out, they lose a vital source of oxygen. Unlike reptiles like chameleons, which have dry, scaly skin to prevent water loss (desiccation), the frog's permeable skin acts as a secondary respiratory organ, often handling more than half of the animal's oxygen intake when submerged or in very damp conditions.

Life Stage/Condition	Primary Respiratory Organ	Mechanism Name
Larval (Tadpole)	Gills	Branchial Respiration
Adult (on Land)	Lungs	Pulmonary Respiration
Adult (in Water/Damp Soil)	Skin	Cutaneous Respiration

Sources: Science-Class VII . NCERT(Revised ed 2025), Life Processes in Plants, p.149; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.133; Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.134

6. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental principles of gas exchange, this question asks you to apply the concept of Cutaneous Respiration to specific biological classifications. You’ve learned that for skin to act as a respiratory surface, it must be thin, highly vascularized, and kept moist to allow gases to dissolve and diffuse. This question essentially tests your ability to identify which animal possesses these specific physiological adaptations among the various classes of vertebrates.

To arrive at the correct answer, look for the animal that bridges the gap between aquatic and terrestrial environments. The Frog, belonging to the class Amphibia, is the quintessential example of an organism with dual respiratory capabilities. While adult frogs use lungs on land, they rely heavily on their moist, porous skin for oxygen intake, particularly when submerged or in damp habitats. As detailed in Science-Class VII . NCERT (Revised ed 2025), this cutaneous exchange is vital for their survival. Therefore, the correct answer is (C) Frog.

UPSC often includes taxonomic distractors to test if you can look past "exotic" names to the underlying biology. Options (A) Flying fish and (B) Sea horse are both bony fish; regardless of their unique appearances, they use gills for branchial respiration. Option (D) Chameleon represents the Reptilia class. Reptiles have evolved dry, keratinized scales to prevent water loss, a feature that physically blocks gas exchange through the skin, forcing them to rely exclusively on lungs. Distinguishing between the permeable skin of an amphibian and the waterproof barrier of a reptile is the key to avoiding this common trap.