Correct Option
The octopus is classified under Phylum Mollusca, specifically within the Class Cephalopoda. Molluscs are soft-bodied invertebrates, many of which possess an external or internal shell, though octopuses typically lack a shell. Key characteristics of octopuses include a distinct head, a mantle, and eight arms equipped with suckers. They are marine animals known for their advanced nervous system and complex behaviors.
Incorrect Options
(a) An arthropod: Arthropods are characterized by a segmented body, jointed appendages, and a chitinous exoskeleton. Examples include insects, spiders, and crustaceans. Octopuses do not possess these features.
(b) An echinoderm: Echinoderms, such as starfish and sea urchins, typically exhibit radial symmetry in their adult stage and possess a water vascular system for locomotion and feeding. Octopuses display bilateral symmetry and lack a water vascular system.
(c) A hemichordate: Hemichordates are marine deuterostome animals, often worm-like, that possess a stomochord and pharyngeal gill slits. They represent an evolutionary link between invertebrates and chordates. Octopuses have a distinct body plan and anatomical features that differentiate them from hemichordates.