Ichthyological terms, not general ones.
Any marine mollusc of the class Cephalopoda which includes octopus, squid, cuttlefish, and nautilus. Characterised by possession of a large head and eyes, prehensile tentacles, and, in most species, a specialised sac containing a dark, inky fluid used in defensive behaviour.
Any member of the crustacean order Decapoda, comprising species possessing 5 pairs of jointed, thorassic limbs such as crabs, crayfish, lobsters and shrimp. Also used historically in reference to molluscs of the Cephalopod superorder Decapodiformes, members of which have 8 short limbs plus 2 longer tentacles and include cuttlefish and squid.
Large, highly diverse invertebrate phylum. Members are difficult to define but generally possess a soft, unsegmented body, and often a shell or shell-like structure secreted by a mantle. Includes all bivalves, cephalopods, and gastropods.