Deuterostomit

Deuterostomia is a subphylum of bilaterian animals that includes the chordates and echinoderms. The term "deuterostome" comes from the Greek words "deuteros," meaning second, and "stoma," meaning mouth. This refers to the second opening in the embryo's gastrula stage, which becomes the anus. In contrast, protostomes have the blastopore, the first opening, becoming the mouth. Deuterostomes are characterized by a unique developmental process where the blastopore becomes the anus, and the mouth develops later. This subphylum is further divided into two main groups: the echinoderms, which include starfish, sea urchins, and sea cucumbers, and the chordates, which include vertebrates like mammals, birds, reptiles, amphibians, and fish, as well as invertebrate chordates like tunicates and lancelets. The deuterostome lineage is believed to have diverged from the protostome lineage around 540 million years ago.