endostome

Endostome is a major group of animals that includes the majority of bilaterian animals, which are those with a bilateral symmetry. This group is characterized by the presence of a blastopore, which is an opening in the blastula stage of development that becomes the anus in the adult. The term "endostome" comes from the Greek words "endon," meaning within, and "stoma," meaning mouth, referring to the blastopore's position within the blastula.

The endostomes are divided into two main subgroups: the deuterostomes and the protostomes. The deuterostomes are characterized by the development of the anus from the blastopore, while the protostomes have the mouth developing from the blastopore. The deuterostomes include animals such as echinoderms, hemichordates, and chordates, while the protostomes include animals like arthropods, mollusks, and annelids.

The endostomes are believed to have evolved from a common ancestor that had a bilateral symmetry and a blastopore. This ancestor is thought to have given rise to the protostomes and deuterostomes through a process of evolutionary divergence. The endostomes are a diverse group of animals that includes many of the most familiar and important organisms in the animal kingdom.