Which of the following is NOT traditionally classified as an invertebrate?

The classification of animals as invertebrates or vertebrates is based on the presence or absence of a backbone or spinal column. Invertebrates are animals that lack a backbone, while vertebrates possess one.

Sharks are classified as vertebrates because they have a well-defined skeleton made of cartilage rather than bone, which is a characteristic of their group, the Chondrichthyes. This group includes other similar creatures, such as rays and skates, all of which possess a backbone.

In contrast, scallops, octopuses, and crabs belong to various groups of invertebrates. Scallops are mollusks, octopuses are cephalopods, and crabs are crustaceans, all of which lack a backbone. Therefore, the correct answer for this question is indeed the shark, as it does not fit the invertebrate classification due to the presence of a backbone.