Final answer:
Cephalopods are unique among mollusks in that they possess a closed circulatory system, in which their blood is fully contained within vessels.
Step-by-step explanation:
Cephalopods are the only mollusks with a closed circulatory system. This means that their blood is entirely contained within vessels, unlike other mollusks where the blood is in a hemocoel, an open body cavity. Cephalopods include organisms like octopuses, squids, cuttlefish, and nautiluses. Members of the Class Cephalopoda are known for their complex eyes and well-developed nervous system, both of which support their predatory lifestyle. While most are soft-bodied, some retain internal shells, and others, like the nautilus, have external spiral shells filled with gas or water for buoyancy.
Cephalopods are the only mollusks with a closed circulatory system (circulatory system in which the blood is contained in vessels) and have a mantle that produces a shell. They are also the only mollusks that reproduce sexually, as they have separate sexes and mate to reproduce. However, they are not hermaphrodites (have both male and female reproductive organs) like some other mollusks.