Which is more complex arthropod or mollusc?

Answer 1

I honestly can't say. Arthropods may have an exoskeleton while molluscs are squishy, they may be more diverse and numerous in species and individuals, but molluscs are pretty intricate on the inside as well (I've dissected squids and snails, they have much the same organs as us), and octopuses and squids are very intelligent. They can count, preform tasks in return for rewards and they require entertainment in aquaria or else they'll turn on the other inhabitants or even short-cirquit the lights by squirting water at them!