A squid is not a crustacean but actually a cephalopod which eats mollusks. One example of a crustacean is a lobster or a crab, though squids do not fit into this category at all. A couple other examples of crustaceans include krill and shrimp. You will find that there are some very specific and distinct differences between crustaceans and cephalopods which is what squids are.
It can be easy for those who are not familiar with all of the different sea organisms to confuse one for another, even though when you closely compare the two they are really nothing alike in terms of their overall physical attributes and behaviors.