Final answer:
The first phylum to have two body openings is the Deuterostomes, where the mouth develops opposite the blastopore end of the embryo and the anus develops at the site of the blastopore.
Step-by-step explanation:
Deuterostomes are a group of bilaterally symmetrical, triploblastic eucoelomates that include phyla such as chordates and echinoderms. In deuterostomes, the mouth develops at a site opposite the blastopore end of the embryo, and the anus develops at the site of the blastopore. This is in contrast to protostomes, where the mouth forms at or near the site of the blastopore. The word deuterostome means 'mouth second,' indicating that the mouth is a secondary structure opposite the blastopore.