Octopuses and squids are both marine invertebrates that are sometimes confused. They both have eight arms, but one also has two tentacles. One has rectangle-shaped pupils but the other's pupils are round. One swims or walks along the ocean floor with strong arms. The other only swims using fins. One lives alone in caves while the other lives in open waters. Learn more about what these animals have in common and how they are different through the latest installment of the Compare and Contrast Book series.