Why are squid unique among invertebrates?

Squid are unique among invertebrates because it consists of three heart and a well developed nervous system.

Explanation:

There are three hearts of squid i. e. two branchial hearts and one systemic heart. Branchial hearts carries the blood to gills for oxygen and this blood reaches to the systematic heart where the blood is distributed to all parts of the body. The colour of squid blood is blue because it contains copper. They are the most intelligent organisms in invertebrates.