Crabs, which have jointed legs and a hard exoskeleton, belong to which phylum in the animal kingdom?

Introduction / Context:
Crabs are common marine and freshwater animals that many students recognize from daily life or from pictures. Being able to classify them correctly is a core part of zoology. This question focuses on the phylum to which crabs belong and checks whether you can associate key features like jointed legs and an exoskeleton with the correct animal group.

Given Data / Assumptions:

• The organism mentioned is crab.
• Crabs have jointed legs and a hard external skeleton.
• The options are Mollusca, Cnidaria, Arthropoda, and Platyhelminthes.

Concept / Approach:
Crabs are crustaceans, a subgroup within the phylum Arthropoda. Arthropods are characterized by jointed appendages, segmented bodies, and an exoskeleton made of chitin. Mollusca includes snails and clams, Cnidaria includes jellyfish and corals, and Platyhelminthes includes flatworms. Therefore, by matching the defining traits of crabs with the diagnostic features of phyla, we identify Arthropoda as the correct answer.

Step-by-Step Solution:
Step 1: Recall that crabs have a hard external shell and jointed legs. Step 2: Recognize that these features are typical of arthropods, which include insects, spiders, and crustaceans. Step 3: Remember that crustaceans such as crabs, lobsters, and shrimps form a major class within the phylum Arthropoda. Step 4: Note that Mollusca includes soft bodied animals, often with shells, such as snails and octopus, which have different body plans. Step 5: Understand that Cnidaria includes animals with stinging cells and radial symmetry, like jellyfish and corals, not joint legged creatures like crabs. Step 6: Recall that Platyhelminthes consists of flattened worms, such as tapeworms, which bear no resemblance to crabs. Step 7: Conclude that crabs belong to the phylum Arthropoda.
Verification / Alternative check:
Standard classification charts in zoology divide the animal kingdom into phyla and list crab under crustacean arthropods. Textbook figures show crabs as typical representatives of Arthropoda along with insects and spiders. They are never grouped with molluscs, cnidarians, or flatworms. This consistent representation across diagrams and classification tables is a reliable way to confirm the answer for exam purposes.

Why Other Options Are Wrong:
Mollusca: Molluscs are mostly soft bodied animals, sometimes with shells, but their limbs are not segmented and jointed like those of crabs.
Cnidaria: Cnidarians have stinging cells and are usually radially symmetrical; their body organization does not involve jointed appendages or a segmented exoskeleton.
Platyhelminthes: These are flatworms with simple, flattened bodies and no external skeleton or jointed legs, making this option incompatible with crab structure.

Common Pitfalls:
Students may confuse crabs with molluscs because both live in aquatic environments, or may simply pick a familiar sounding phylum. To avoid this, remember that the key diagnostic feature of arthropods is their jointed legs and exoskeleton. Associating any animal with these features to Arthropoda is a safe strategy. Recalling that crabs are crustaceans, and that crustaceans are arthropods, also reinforces the correct classification.

Final Answer:
Crabs belong to the phylum Arthropoda.