What is the definition of a cephalopod?

The correct response identifies a cephalopod as an organism characterized by its unique anatomical features, which include a well-developed head, bilateral symmetry, and tentacles. Cephalopods belong to the class Cephalopoda and include creatures such as octopuses, squids, and cuttlefish. They are distinguished from other marine animals by their soft bodies, which typically have an internal shell (or external in some cases, like squids) and the ability to expel water for rapid movement, a behavior known as jet propulsion.

While one of the incorrect options describes a fish, it's important to note that cephalopods are not classified as fish; they are mollusks, which is a different group of marine animals. Additionally, the description of a thin, wide fish with both eyes on one side is associated with flatfish, not cephalopods. Another response that refers to crustaceans does not apply, as cephalopods lack the exoskeleton characteristic of crustaceans. Finally, the term shellfish is commonly used to refer to various marine animals with shells, but it generally includes both mollusks and crustaceans, making it too broad to specifically refer to cephalopods alone.