cephalopod vs mollusk

cephalopod and mollusk both are nouns.

Word	Noun	Adjective	Verb	Adverb
cephalopod	Yes	Yes	No	No
mollusk	Yes	No	No	No

As nouns, mollusk is a hypernym of cephalopod; that is, mollusk is a word with a broader meaning than cephalopod:

cephalopod: marine mollusk characterized by well-developed head and eyes and sucker-bearing tentacles
mollusk: invertebrate having a soft unsegmented body usually enclosed in a shell

Other hypernyms of cephalopod include mollusc, shellfish.

cephalopod (noun)	mollusk (noun)
marine mollusk characterized by well-developed head and eyes and sucker-bearing tentacles	invertebrate having a soft unsegmented body usually enclosed in a shell

cephalopod (adjective)	mollusk (adjective)
relating or belonging to the class Cephalopoda