urochord vs ascidian

urochord and ascidian both are nouns.

Word	Noun	Adjective	Verb	Adverb
urochord	Yes	No	No	No
ascidian	Yes	No	No	No

As nouns, ascidian is a hyponym of urochord; that is, ascidian is a word with a more specific, narrower meaning than urochord:

urochord: primitive marine animal having a saclike unsegmented body and a urochord that is conspicuous in the larva
ascidian: minute sedentary marine invertebrate having a saclike body with siphons through which water enters and leaves

Other hyponyms of urochord include salp, salpa, doliolum, larvacean, appendicularia.

urochord (noun)	ascidian (noun)
a notochord of a larval tunicate typically confined to the caudal region	minute sedentary marine invertebrate having a saclike body with siphons through which water enters and leaves
primitive marine animal having a saclike unsegmented body and a urochord that is conspicuous in the larva