planarian vs planaria

planarian and planaria both are nouns.

Word	Noun	Adjective	Verb	Adverb
planarian	Yes	No	No	No
planaria	Yes	No	No	No

As nouns, planarian and planaria are synonyms defined as:

planarian and planaria: free-swimming mostly freshwater flatworms; popular in laboratory studies for the ability to regenerate lost parts

planarian (noun)	planaria (noun)
free-swimming mostly freshwater flatworms; popular in laboratory studies for the ability to regenerate lost parts	free-swimming mostly freshwater flatworms; popular in laboratory studies for the ability to regenerate lost parts