Ans. Deuterostomes.
Deuterostomes are organisms, which belong to subtaxon Deuterostomia (means 'second mouth'). In these organisms, the blastopore or first opening present at bottom of developing blastula develops as anus, instead of mouth as in protostomes. Organisms of phyla echinodermata, hemichordata, and chordata are deuterostomes.
Thus, the correct word to complete the sentence is 'deuterostomes.'
