Which of the following is not classified as a chordate?

The classification of organisms into groups is based on shared characteristics, and chordates are defined by specific features, including having a notochord at some stage of their life cycle, a dorsal nerve cord, pharyngeal slits, and a post-anal tail.

Among the options provided, squid do not possess these defining characteristics of chordates. Squid belong to a different group of animals called cephalopods, which are invertebrates. Unlike sharks, fish, and turtles, which all possess a backbone—making them vertebrate chordates—squid do not have a spinal column or the other features associated with the phylum Chordata.

Therefore, identifying squid as the organism that is not classified as a chordate is accurate because they represent a distinct evolutionary lineage within the animal kingdom that lacks the defining characteristics of chordates.