gastropod vs mollusc

gastropod and mollusc both are nouns.

Word	Noun	Adjective	Verb	Adverb
gastropod	Yes	No	No	No
mollusc	Yes	No	No	No

As nouns, mollusc is a hypernym of gastropod; that is, mollusc is a word with a broader meaning than gastropod:

gastropod: a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes
mollusc: invertebrate having a soft unsegmented body usually enclosed in a shell

Other hypernyms of gastropod include mollusk, shellfish.

gastropod (noun)	mollusc (noun)
a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes	invertebrate having a soft unsegmented body usually enclosed in a shell