 In animals at least as complex as an earthworm, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the formation of the gut.In deuterostomes, the blastopore becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth.In the protostomes, it used to be thought that the blastopore formed the mouth ("proto–" meaning "first") while the anus formed later as an opening made by the other end of the gut.