chaetognath vs sagitta

chaetognath and sagitta both are nouns.

Word	Noun	Adjective	Verb	Adverb
chaetognath	Yes	No	No	No
sagitta	Yes	No	No	No

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

chaetognath: any worm of the Chaetognatha; transparent marine worm with horizontal lateral and caudal fins and a row of movable curved spines at each side of the mouth
sagitta: any arrowworm of the genus Sagitta

chaetognath (noun)	sagitta (noun)
any worm of the Chaetognatha; transparent marine worm with horizontal lateral and caudal fins and a row of movable curved spines at each side of the mouth	any arrowworm of the genus Sagitta