cephalopodan vs cephalopod

cephalopodan is not a noun while cephalopod is a noun.

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

As adjectives, cephalopodan and cephalopod are synonyms defined as:

cephalopodan and cephalopod: relating or belonging to the class Cephalopoda

cephalopodan (noun)	cephalopod (noun)
	marine mollusk characterized by well-developed head and eyes and sucker-bearing tentacles

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