Short answers, pulled from the story.
Bilateria refers to the large animal grouping defined by bilateral symmetry during embryonic development, in which the body has a distinct front, rear, back, and belly. Over 98% of all known animal species belong to this lineage, including all vertebrates, arthropods, annelids, and molluscs.
The Austrian embryologist Berthold Hatschek named the Bilateria in 1888. His original classification placed within the group the Zygoneura, the Ambulacraria, and the Chordonii. In 1910, Austrian zoologist Karl Grobben renamed the Zygoneura to Protostomia and created the Deuterostomia.
The distinction concerns which end of the embryo becomes the mouth. In protostomes, the first embryonic opening becomes the mouth; in deuterostomes, it becomes the anus. Protostomes include arthropods, annelids, and molluscs, while deuterostomes include echinoderms, hemichordates, and chordates.
The first recognized bilaterian body fossil is Kimberella, found in Ediacaran sediments. An earlier candidate, Vernanimalcula, may be an even older bilaterian but its identification is contested; it may represent an infilled bubble rather than an animal fossil.
The nature of the urbilaterian, the hypothetical ancestor of all bilaterians, is actively debated. Johanna Taylor Cannon and colleagues proposed it was a bottom-dwelling worm with a single body opening similar to Xenoturbella. Lewis I. Held argued it was segmented, based on the shared genetic mechanism for segmentation between vertebrates and arthropods.
Echinoderms such as sea stars display pentaradial symmetry as adults but are bilaterally symmetric as embryos. Because Bilateria is defined by bilateral symmetry during embryonic development, echinoderms qualify for inclusion despite their adult body plan, making them the most notable exception within the group.