Anatomical Differences
Because they have no internal skeleton, octopi's bodies are very soft. The only hard part on a an octopus body is its beak, which is made of chitin. This beak is used to eat prey.
In a squid, the main body mass is enclosed in the mantle, which has two swimming fins along each side. It should be noted that these fins, unlike in other marine organisms, are not the main source of ambulation in most species. The skin of the squid is covered in chromatophores, which enable the squid to change color to suit its surroundings. The underside of the squid is also found to be lighter than the topside, in order to provide camouflage from both prey and predator.
Both squids and octopi have three hearts each. Two pump blood through the gills, while the third heart is dedicated blood circulation through the body. The blood in both creatures contains a copper-rich protein known as hemocyanin. Both creatures have similar, though not identical, organ systems.