annelid vs annelidan

annelid is a noun but annelidan is not a noun.

Word	Noun	Adjective	Verb	Adverb
annelid	Yes	Yes	No	No
annelidan	No	Yes	No	No

As adjectives, annelid and annelidan are synonyms defined as:

annelid and annelidan: relating to or belonging to or characteristic of any worms of the phylum Annelida

annelid (noun)	annelidan (noun)
worms with cylindrical bodies segmented both internally and externally

annelid (adjective)	annelidan (adjective)
relating to or belonging to or characteristic of any worms of the phylum Annelida	relating to or belonging to or characteristic of any worms of the phylum Annelida