If you’re building a marine aquarium and the thought of putting an octopus in it crosses your mind, consider not doing that. Consider not doing that so hard that you put this ill-conceived notion to permanent rest, no matter how much fun Finding Dory made it sound. An octopus makes for a difficult, finicky, and potentially even dangerous marine-aquarium inhabitant, best left to nature and specialists.
The next big chapter in the course I teach is on the mollusks. The most visible component of any seashore’s biota, mollusks are an incredibly diverse group of animals. One could be forgiven for not knowing in advance that snails, clams, and the colossal squid are about as close together on the tree of life as spiders and lobsters, or humans and pipefish. Mollusks are, in this way, a classic example of adaptive radiation, in which an ancestral body plan somewhat like a very primitive snail was reshaped into widely dissimilar beasts in response to very different selection pressures.