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How do chordate embryos form their mouths differently from many other animals?

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A translucent X-ray fish (Pristella maxillaris) swims in an aquarium, showing its visible backbone and internal structures through its body. — Chordate X-ray fish (*Pristella maxillaris*), is an example of a chordate with a visible backbone.

Chordates belong to a major animal division known as Deuterostomia (Greek for "second mouth"). Unlike many other animals, such as those in the group Protostomia (where the mouth develops from the first embryonic opening, or blastopore), chordates form their mouths differently.123

In chordate embryos, the blastopore typically develops into the anus. The mouth, by contrast, forms later as a completely new opening that arises from a different location on the embryonic gut. This developmental pattern is a defining characteristic used to classify chordates alongside other deuterostomes, such as echinoderms.1452

During later stages of development, the mouth is specifically derived from the stomodaeum—an external pit where ectoderm and endoderm fuse to form a membrane that eventually ruptures to connect the exterior to the pharynx.6