"Invertebrate is a term that describes any animal without a spinal column. The group includes 97% of all animal species — all animals except those in the Chordate subphylum Vertebrata (fish, reptiles, amphibians, birds and mammals)." Squid are cephalopods, which is a group of molluscs, and with the exception of the nautilus, none have an external shell but instead have an internal mantle.

http://en.wikipedia.org/wiki/Invertebrate

This is a picture of a Carribean Reef Squid http://en.wikipedia.org/wiki/Image:Caribbean_reef_squid.jpg