Octopus, a soft bodied marine animal with tentacles, belongs to which phylum in the animal kingdom?

Introduction / Context:
Classification of animals into phyla is a fundamental part of zoology. Octopus is a very familiar marine animal, and knowing its correct phylum helps students relate body structure and lifestyle with classification. This question checks whether you can correctly place octopus among the major animal phyla and distinguish it from groups like Cnidaria, Echinodermata, and Chordata.

Given Data / Assumptions:

• The animal mentioned is octopus, a soft bodied marine invertebrate with tentacles.
• The options given are Mollusca, Cnidaria, Echinodermata, and Chordata.
• We assume basic familiarity with the characteristics of major animal phyla.

Concept / Approach:
Octopus is a cephalopod, a subgroup within the phylum Mollusca. Molluscs generally have soft bodies and many have a shell, but cephalopods include animals like octopus, squid, and cuttlefish that have highly developed heads and tentacles. Cnidaria includes jellyfish and corals, Echinodermata includes starfish and sea urchins, and Chordata includes animals with a notochord, such as vertebrates. Therefore, the correct approach is to associate octopus with cephalopods and then with its parent phylum, Mollusca.

Step-by-Step Solution:
Step 1: Recall that cephalopods are a class of molluscs that include octopus and squid. Step 2: Note that octopus has a soft body, a prominent head, and tentacles with suckers, which fits typical cephalopod features. Step 3: Identify the phylum of cephalopods as Mollusca, which also includes snails, clams, and other related animals. Step 4: Recognize that Cnidaria includes animals with stinging cells, such as jellyfish and sea anemones, which do not resemble octopus in body plan. Step 5: Remember that Echinodermata includes radially symmetrical animals like starfish and sea urchins, which are structurally different from octopus. Step 6: Recall that Chordata includes animals with a notochord or backbone; octopus does not have a backbone, so it does not fit this phylum. Step 7: Conclude that octopus belongs to the phylum Mollusca.
Verification / Alternative check:
Another way to verify is to remember that molluscs are often divided into classes such as Gastropoda, Bivalvia, and Cephalopoda. Octopus is a textbook example of a cephalopod, and exam questions frequently mention it in association with Mollusca. Cnidarians and echinoderms have very different symmetry and body organization compared to octopus, and chordates are defined by a notochord or vertebral column, which octopus lacks. These contrasts support the conclusion that Mollusca is the correct phylum.

Why Other Options Are Wrong:
Cnidaria: Members of this phylum have specialized stinging cells called cnidocytes and often show radial symmetry; octopus does not share these features.
Echinodermata: Echinoderms have spiny skin and mostly radial symmetry in adults; this body plan is unlike the bilateral, highly mobile octopus.
Chordata: Chordates possess a notochord or backbone at some stage; octopus is an invertebrate without such a structure, so it is not a chordate.

Common Pitfalls:
Students sometimes confuse octopus with jellyfish or other marine animals and mistakenly associate it with Cnidaria simply because they all live in the sea. Another pitfall is to assume that any complex animal might be a chordate. Remembering that octopus is a cephalopod mollusc is the key to correct classification. Visualizing textbook pictures that place octopus under Mollusca is a helpful memory aid.

Final Answer:
Octopus belongs to the phylum Mollusca.