The octopus has a round head and eight arms that radiate from it like the spokes of a wheel. Squids and octopuses are built similarly, but you can tell they’re different creatures just from their appearance. And, unlike other mollusks, neither one has a shell. They’re also both invertebrates and mollusks, and they both have eight arms. Biological Classificationīoth squids and octopuses are cephalopods, which means “head-footed” as they have no real feet and their head is their most prominent feature. To help you understand the differences between squid vs octopus better, we’ll describe their differences and similarities when it comes to biological classification, anatomy, size, lifespan, habitat, and hunting behavior.