Here's a breakdown of why:
* Bilateral Symmetry: Like all mollusks, squid have a body that can be divided into two symmetrical halves.
* Mantle: A squid's body is covered by a fleshy, protective layer called a mantle. This is a defining feature of all mollusks.
* Foot: The squid's foot has evolved into a specialized structure called a siphon, used for jet propulsion. While different from the foot of other mollusks, it is still considered a modified foot.
* Visceral Mass: Like other mollusks, squid have a concentrated internal organ mass called a visceral mass.
* Radula: Squid, like many other mollusks, have a rasping tongue-like structure called a radula, used for feeding.
While squid have adaptations specific to their aquatic lifestyle, the fundamental anatomy and evolutionary history clearly place them within the Mollusca phylum, making them close relatives to snails, clams, and octopuses.