Octopus belongs to the phylum Mollusca.

Introduction
The phylum Mollusca is one of the largest and most diverse animal phyla, comprising over 100,000 known species. It includes a wide variety of organisms such as snails, clams, squids, and octopuses. Octopuses, including the common octopus (Octopus vulgaris), are fascinating creatures known for their intelligence, unique body structure, and intriguing behavior.

Characteristics of Mollusca
Mollusks are characterized by several key features:

1. Soft Body: Mollusks have soft bodies that are typically covered by a hard protective shell. However, in the case of octopuses, the shell is either absent or greatly reduced, allowing for a flexible and agile body.

2. Bilateral Symmetry: Mollusks have bilateral symmetry, which means their bodies can be divided into two similar halves along a vertical plane.

3. Mantle and Foot: Mollusks have a specialized structure called the mantle, which secretes the shell in many species. They also possess a muscular foot used for locomotion.

4. Radula: Mollusks have a unique feeding structure called a radula, which is a specialized tongue-like organ with tiny teeth used to scrape or shred food.

5. Coelom: Mollusks have a coelom, which is a fluid-filled body cavity that surrounds their internal organs.

The Octopus and its Molluscan Features
Octopuses are members of the class Cephalopoda within the phylum Mollusca. While they possess some distinct characteristics, they also share several features with other mollusks:

1. Soft Body: Like other mollusks, octopuses have a soft body, which is highly flexible and muscular. Their body is divided into a head and a mantle.

2. Mantle: The mantle in octopuses is highly developed and encompasses most of their body. It plays a crucial role in locomotion by contracting and expelling water, allowing the octopus to move rapidly.

3. Radula: Although octopuses have a radula, it is not as well-developed as in other mollusks. Instead, they have evolved specialized beak-like jaws to capture and manipulate their prey.

4. Cephalopod Traits: Octopuses possess several unique cephalopod traits, such as well-developed eyes, a highly advanced nervous system, and the ability to change color and texture for camouflage.

Conclusion
Based on their shared characteristics with other mollusks, especially the presence of a soft body, mantle, and some features of a radula, octopuses are classified within the phylum Mollusca. However, they have also evolved distinct adaptations that make them highly specialized and fascinating creatures in their own right.