Which one among the following animals does not undergo periodic moulting of their external body covering?

1. Basis of Animal Kingdom Classification (basic)

Welcome to your first step in mastering Animal Diversity! Before we dive into the specific groups of animals, we must understand why and how we categorize them. Classification is not just a naming exercise; it is a tool to organize the staggering complexity of nature. Just as economists divide activities into sectors like primary or secondary to analyze patterns Understanding Economic Development. Class X . NCERT, SECTORS OF THE INDIAN ECONOMY, p.32, biologists use specific criteria to group animals to understand their evolutionary history and functional roles.

The primary basis for animal classification includes levels of organization (whether they are made of simple cells or complex organs), body symmetry, and intrinsic biological habits. For instance, animals are often grouped by their movement mechanisms—such as burrowing, flying, or swimming—which are often dictated by their physical environment Environment and Ecology, Majid Hussain, PLANT AND ANIMAL KINGDOMS, p.9. Another vital criterion is how an animal feeds—whether it is a producer (performing photosynthesis) or a consumer that feeds on plants or other animals Science, Class VIII . NCERT, How Nature Works in Harmony, p.198.

A sophisticated layer of classification involves the nature of the body covering and how the animal grows. Some animals possess a rigid external skeleton (exoskeleton) that does not expand. To grow, these animals must periodically shed this outer layer in a process known as moulting or ecdysis. This is a defining trait for a massive group called Ecdysozoans, which includes insects and spiders. In contrast, animals like the earthworm (Annelida) have flexible, moist skin that grows with them, meaning they do not need to undergo the dramatic shedding seen in arthropods Science-Class VII . NCERT, Life Processes in Animals, p.133.

Criteria	Example/Description
Morphology	Physical structure, such as presence of wings or fins.
Growth Mechanism	Periodic shedding (Ecdysis) vs. continuous growth.
Nutritional Mode	Herbivores, Carnivores, or Omnivores Science, Class VIII . NCERT, p.198.

Sources: Understanding Economic Development. Class X . NCERT, SECTORS OF THE INDIAN ECONOMY, p.32; Environment and Ecology, Majid Hussain, PLANT AND ANIMAL KINGDOMS, p.9; Science, Class VIII . NCERT, How Nature Works in Harmony, p.198; Science-Class VII . NCERT, Life Processes in Animals, p.133

2. Skeletal Systems: Exoskeleton vs. Endoskeleton (basic)

At its core, a skeletal system serves three fundamental purposes: it provides a structural framework for the body, protects delicate internal organs, and works with muscles to facilitate movement. Animals have evolved two primary ways to house this framework: on the outside (Exoskeleton) or on the inside (Endoskeleton).

An Exoskeleton is a hard, external covering found in many invertebrates, most notably the Arthropods (such as insects, spiders, and crustaceans). These skeletons are often made of a tough substance called chitin. While they provide excellent protection against predators and prevent dehydration, they have a major limitation: they do not grow with the animal. To increase in size, these creatures must undergo moulting (or ecdysis), where they shed their old casing and grow a larger one. This group includes common species like cockroaches and dragonflies Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.155.

In contrast, an Endoskeleton is an internal framework made of bone or cartilage. This is the hallmark of Vertebrates, including mammals like cows and buffaloes, birds like peacocks, and reptiles History Class XI (Tamil Nadu State Board), Evolution of Society in South India, p.70. Unlike the exoskeleton, an endoskeleton grows continuously alongside the rest of the body, allowing these animals to reach much larger sizes. Furthermore, it allows for more flexible movement as skeletal muscles are attached to these internal bones, contracting to move the limbs in response to the nervous system NCERT Class X, Control and Coordination, p.109.

Feature	Exoskeleton	Endoskeleton
Location	Outside the body	Inside the body
Growth	Periodic shedding (moulting) required	Grows continuously with the body
Examples	Insects, Crabs, Scorpions	Humans, Cows, Fish, Birds
Primary Function	Protection and moisture retention	Structural support and internal protection

Sources: Environment, Shankar IAS Academy (10th ed.), Indian Biodiversity Diverse Landscape, p.155; History, Class XI (Tamilnadu State Board 2024 ed.), Evolution of Society in South India, p.70; Science, Class X (NCERT 2025 ed.), Control and Coordination, p.109

3. Phylum Annelida: Characteristics and Cuticle (intermediate)

Welcome back! Today, we are diving into one of nature’s most efficient engineers: Phylum Annelida. The name comes from the Latin annellus, meaning “little ring,” which perfectly describes their most striking feature—segmentation. Unlike simpler worms, annelids have bodies divided into repeated linear segments, both externally and internally, which allows for sophisticated movement and specialized organ development Environment (Shankar IAS Academy), Indian Biodiversity Diverse Landscape, p.155.

One of the most critical aspects of an annelid’s biology is its cuticle. Unlike the hard, crunchy exoskeleton of an insect or the scaly skin of a reptile, the annelid cuticle is a thin, transparent, and flexible layer secreted by the epidermis. It is intentionally kept moist by mucus. This moisture isn’t just for texture; it is essential for cutaneous respiration. Earthworms, for instance, do not have lungs; they rely on the exchange of oxygen and carbon dioxide directly through this moist skin Science-Class VII (NCERT), Life Processes in Animals, p.133.

A common point of confusion in biology is whether all “skin-shedding” animals are related. It is vital to distinguish annelids from the group known as Ecdysozoans (like arthropods and nematodes). While animals like cockroaches or snakes must periodically shed their rigid outer covering (a process called moulting or ecdysis) to grow, annelids do not moult. Because their cuticle is flexible and elastic, it expands as the animal grows. They possess well-developed internal organs and a closed circulatory system, but they lack the jointed limbs seen in their arthropod cousins Environment (Shankar IAS Academy), Indian Biodiversity Diverse Landscape, p.155.

Feature	Phylum Annelida (e.g., Earthworm)	Phylum Arthropoda (e.g., Cockroach)
External Covering	Thin, moist, flexible cuticle.	Hard, chitinous exoskeleton.
Growth Process	Continuous growth (flexible skin).	Discontinuous growth (requires moulting).
Respiration	Often via skin (Cutaneous).	Tracheal system or Gills.

Sources: Environment (Shankar IAS Academy), Indian Biodiversity Diverse Landscape, p.155; Science-Class VII (NCERT), Life Processes in Animals, p.133

4. Phylum Arthropoda: Diversity and Life Cycles (intermediate)

Phylum Arthropoda is the largest and most successful group in the animal kingdom, comprising over 80% of all known species. The name comes from the Greek words arthros (jointed) and podos (foot), referring to their most distinctive feature: jointed appendages. These limbs allow for sophisticated movement, from the rapid scuttling of a crab to the precise flight of a dragonfly Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.155.

The secret to their success is a specialized "suit of armor" known as the exoskeleton. This hard, external covering is made of chitin and provides both protection and structural support. However, because this shell is rigid, it does not grow with the animal. To increase in size, arthropods must undergo moulting (or ecdysis) — a process where they periodically shed their old exoskeleton to reveal a soft, new one underneath that expands before hardening Science, Class VII NCERT, Life Processes in Animals, p. 133. This distinguishes them from animals like earthworms (Annelids), which have flexible, moist skin that grows without the need for shedding an entire external skeleton Science, Class VII NCERT, Life Processes in Animals, p. 133.

While all arthropods share these traits, they are highly diverse. We primarily categorize them based on their body segments and the number of legs they possess:

Feature	Insects (e.g., Bees, Butterflies)	Arachnids (e.g., Spiders, Scorpions)
Body Parts	3 parts: Head, Thorax, Abdomen	2 parts: Cephalothorax, Abdomen
Legs	6 legs (3 pairs)	8 legs (4 pairs)
Antennae	Present	Absent

Arthropods also play critical roles in ecosystems. For instance, the life cycle of a dragonfly begins as an aquatic larva. In a balanced pond ecosystem, fish often prey on these larvae. If fish populations decline, the dragonfly population may surge, which in turn reduces the number of bees and butterflies, as adult dragonflies are formidable predators Science, Class VIII NCERT, How Nature Works in Harmony, p.195.

Sources: Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.155-156; Science, Class VII NCERT, Life Processes in Animals, p.133; Science, Class VIII NCERT, How Nature Works in Harmony, p.195

5. Class Reptilia: Skin and Scales (intermediate)

To understand the success of Class Reptilia on land, we must first look at their 'armor' — the skin. Unlike amphibians, which possess moist, permeable skin used for breathing, reptiles have evolved a dry, cornified skin. This skin is covered with epidermal scales made of keratin, the same tough protein found in human fingernails. This adaptation was a revolutionary step in evolution, as it provided a waterproof barrier that prevented desiccation (drying out), allowing reptiles to inhabit diverse environments ranging from lush rainforests to arid deserts Environment and Ecology, Majid Hussain, MAJOR BIOMES, p.21.

Because this keratinized layer is tough and relatively inelastic, it creates a unique challenge for a growing animal. To increase in size, many reptiles (particularly squamates like snakes and lizards) must periodically shed their outer skin in a process known as ecdysis or moulting. For example, the Indian Cobra sheds its skin to allow for growth and to remove external parasites. This process is highly distinct from animals like the earthworm (Annelida), which possesses a moist, flexible cuticle that is used for gas exchange and does not require periodic shedding for growth Science-Class VII, NCERT, Life Processes in Animals, p.133.

The skin and scales also serve specialized sensory and protective roles. For instance, snakes have a specialized notch in their lips that allows them to flick their forked tongue to collect scent particles from the air without opening their mouths. These particles are then analyzed by the Jacobson’s organ located on the roof of the mouth Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.157. In larger species, such as the Indian Water Monitor, the skin is exceptionally thick and heavy, providing the necessary protection for one of the world's largest lizard species Environment, Shankar IAS Academy, Animal Diversity of India, p.189.

Feature	Reptilian Skin (e.g., Snake)	Annelid Skin (e.g., Earthworm)
Texture	Dry, keratinized scales	Moist, thin cuticle
Growth Mechanism	Periodic shedding (Ecdysis)	Flexible expansion (No moulting)
Function	Protection and water retention	Gas exchange (Respiration)

Sources: Environment and Ecology, Majid Hussain, MAJOR BIOMES, p.21; Science-Class VII, NCERT, Life Processes in Animals, p.133; Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.157; Environment, Shankar IAS Academy, Animal Diversity of India, p.189

6. The Science of Moulting (Ecdysis) (exam-level)

Moulting, scientifically known as ecdysis, is the biological process of shedding an old, outgrown external covering to make room for new growth. In the animal kingdom, this process is the defining characteristic of the superphylum Ecdysozoa. The most prominent members are Arthropods—including insects like dragonflies, crustaceans, and arachnids like spiders and scorpions Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.156. These animals possess a hard exoskeleton made of chitin that provides protection and support for their jointed limbs Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.155. Because this "suit of armor" is rigid and cannot expand, the animal must periodically cast it off, expand its soft body, and wait for a new, larger exoskeleton to harden. While we often associate moulting with insects, it is also a vital process for squamates (snakes and lizards). For a cobra, ecdysis involves shedding the entire outer layer of keratinous skin, which not only allows for growth but also helps remove external parasites Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.157. However, it is a common misconception that all invertebrates moult. For example, Annelids like the earthworm have a thin, moist cuticle used primarily for gas exchange Science-Class VII, NCERT, Life Processes in Animals, p.133. Since their outer layer is flexible and segmented, they can grow incrementally without the need for the periodic, wholesale shedding seen in arthropods.

Animal Group	External Covering	Growth Strategy
Arthropods (e.g., Cockroach)	Rigid Chitinous Exoskeleton	Periodic Ecdysis (Moulting)
Reptiles (e.g., Cobra)	Keratinous Scales/Skin	Periodic Shedding
Annelids (e.g., Earthworm)	Moist, Flexible Cuticle	Continuous growth (No Ecdysis)

Sources: Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.155; Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.156; Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.157; Science-Class VII, NCERT, Life Processes in Animals, p.133

7. Comparing Growth Mechanisms Across Taxa (exam-level)

In the animal kingdom, growth is not always a simple matter of getting larger; for many species, it involves a complex process of Ecdysis, or moulting. This is the periodic shedding of an external body covering to allow the organism to increase in size. This mechanism is most famously seen in the superphylum Ecdysozoa, which includes arthropods like cockroaches and dragonflies. These animals possess a hard exoskeleton made of chitin that provides protection but does not grow with the body. To expand, they must secrete a new, softer layer underneath, shed the old casing, and quickly expand before the new one hardens.

While we often associate moulting with insects, it is also a vital process for Squamates (snakes and lizards). For instance, a cobra periodically sheds its skin to accommodate growth and remove parasites. In contrast, animals like the earthworm (Phylum Annelida) follow a different growth strategy. Earthworms possess a hydrostatic skeleton — a fluid-filled cavity that provides structural support through internal pressure Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p. 155. Because their outer layer is a thin, moist, and flexible cuticle primarily used for gas exchange, they can grow continuously and segmentally without the need to shed their entire external covering Science-Class VII . NCERT(Revised ed 2025), Chapter 9, p. 133.

Understanding these mechanisms helps us distinguish between the rigid structural constraints of arthropods and the flexible, fluid-based growth of soft-bodied invertebrates. Below is a comparison of how these different taxa manage their outer "packaging":

Group	Skeleton Type	Growth Mechanism	Examples
Arthropods	Rigid Exoskeleton (Chitin)	Moulting (Ecdysis)	Cockroach, Dragonfly
Squamates	Internal Bone + Rigid Scales	Periodic Skin Shedding	Cobra, Lizard
Annelids	Hydrostatic Skeleton	Flexible Expansion (No Moulting)	Earthworm, Leech

Sources: Science-Class VII . NCERT(Revised ed 2025), Life Processes in Animals, p.133; Environment, Shankar IAS Academy, Indian Biodiversity Diverse Landscape, p.155

8. Solving the Original PYQ (exam-level)

To solve this question, you must synthesize your knowledge of animal classification with the physiological process of ecdysis (moulting). As you have learned, growth in the animal kingdom often requires overcoming a rigid external barrier. While Arthropods belong to the superphylum Ecdysozoa—a group defined by the periodic shedding of a hard chitinous exoskeleton—certain reptiles like the Cobra also undergo a similar process to replace keratinized skin that cannot expand. The core logic here is identifying which animal possesses a body covering that is elastic and permeable enough to grow without being shed in its entirety.

Walking through the options, we see that the Cockroach and Dragonfly are both insects; they cannot increase in size without discarding their old shells. Similarly, the Cobra must shed its skin to accommodate growth and remove parasites. In contrast, the Earthworm belongs to the phylum Annelida. According to Science-Class VII . NCERT (Revised ed 2025), earthworms possess a thin, moist cuticle that is primarily used for respiration (gas exchange). Because this cuticle is flexible and the earthworm's body is segmented and hydrostatic, it can stretch and grow without the need for the periodic, synchronized shedding seen in ecdysozoans. Therefore, the Earthworm is the correct answer.

UPSC often sets taxonomic traps by mixing different phyla that share similar-sounding biological terms. A common pitfall is confusing the 'cuticle' of an annelid with the 'cuticle/exoskeleton' of an arthropod. While both are external coverings, the Annelid cuticle is non-chitinous and does not restrict growth, whereas the Arthropod exoskeleton is a rigid suit of armor. By recognizing that moulting is a specific adaptation for rigid coverings, as described in Britannica, you can logically eliminate the insects and the reptile to isolate the flexible earthworm.