deuterostone

Deuterostome is a term used in biology to describe a group of animals that exhibit a specific pattern of embryonic development. This pattern is characterized by the formation of the anus before the mouth during gastrulation, the process by which a single-layered embryo develops into a three-layered structure. This is in contrast to protostomes, where the mouth develops first. Deuterostomes include a diverse range of animals, such as echinoderms (starfish, sea urchins), hemichordates (acorn worms), and chordates (vertebrates, tunicates).

The term "deuterostome" was coined by Ernst Haeckel in the late 19th century. The classification is based on the pharyngeal slits, which are structures that develop from the pharynx and are involved in respiration, feeding, and sensory functions. In deuterostomes, the pharyngeal slits are formed in a specific sequence and number, which is distinct from that of protostomes.

Deuterostomes are further divided into two main groups: echinoderms and chordates. Echinoderms are characterized by their spiny skin and radial symmetry, while chordates possess a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some point in their life cycle. Chordates include vertebrates, such as 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 550 million years ago, during the Ediacaran period. This divergence is thought to have been influenced by changes in gene expression patterns and regulatory networks. The deuterostome body plan has proven to be highly successful, with many deuterostome species dominating various ecological niches.