Why are octopuses considered mollusks?

The easiest separation point between mollusc/mollusk and fish is whether it is a vertebrate. Vertebrate means if it's an animal that possesses a skeletal structure, vertebral column or backbones, regardless if it's made of bones or cartilage. Another separate term is craniate which means an animal with a skull. read more

In squid, it is called a “pen.” Cuttlefish bones, which are sold in pet stores, are this same internal shell. Octopuses have reduced their pens to the point that they no longer exist. The other main reason why they are not considered fish is because all fish are vertebrates. All mollusks, including octopuses, are invertebrates. read more

Octopuses are cephalopod mollusks characterized by having eight arms, no tentacles, and the internal shell lost or considerably reduced. There are over 100 species of octopuses (Genus Octopus) in the world as well as numerous species of deep-water and pelagic octopuses (Order Octopoda). read more

Well that's pretty straight forward they are considered mollusks because they have soft unsemented bodies and live in damp to fully aquatic environments much like their cousins the sea slugs and snails : ) read more