What it is
A living sea animal with a soft body and no backbone. Squid are mollusks, in the same big group as snails, clams and octopuses.
Inside a squid
A squid has three hearts. Two pump blood through its gills, and one pumps it around the body. Its blood is blue, because it carries oxygen with copper instead of the iron in yours. In the colossal squid, the brain is shaped like a doughnut and wraps around the throat, so food has to be cut into small pieces before it can be swallowed.
Why it matters
Inside the Tully monster's eyes, a team found grains of color stacked in layers, the way a backboned animal's eye stacks them. Then somebody checked squid. Squid stack those same grains in layers too. The strongest single piece of evidence had stopped being evidence.