If the anus develops before the mouth, the organism is a deuterostome, which means "second mouth." It is a sister clade of Protostomia, with which it forms the Nephrozoa clade. The proterostomic develop the mouth first. Deuterostomes are also known as enterocoelomates because their coelom develops through enterocoely.
Answer : The correct for the blank is-
Deuterostomes are the organisms in which anus develops first from the first opening of the embryonic development, which is blastopore.
In other words, blastopore develops into anus first (unlike protostomes where mouth is develped from the blastopore) in case of deuterostome.
Example of Deuterostomes - Chordates and Echinoderms.
Explanation: In embryonic development, protostomes’s first opening develops as a mouth, while the deuterostome develops as an anus, prior to a mouth.