Ryggstrengdyrene

Ryggstrengdyrene, or the Chordata, is a diverse phylum of animals that share key embryological features: a notochord, a dorsal hollow nerve cord, pharyngeal slits, a post-anal tail, and an endostyle. These characteristics are present at some stage of development in all members, though not all persist into adulthood in every lineage. The phylum includes three major groups: Cephalochordata (lancelets), Urochordata (tunicates), and Vertebrata (craniates), the latter encompassing fishes, amphibians, reptiles, birds, and mammals. In Vertebrata, the notochord is typically replaced by a segmented vertebral column, while the dorsal nerve cord develops into the central nervous system.

Chordates are deuterostomes, meaning the anus forms before the mouth during embryogenesis, with radial cleavage in early development. The endostyle of primitive chordates is evolutionarily related to the thyroid gland in higher vertebrates.

Ecology and diversity: Members occupy a wide range of habitats, from marine to freshwater and terrestrial environments in vertebrates. Feeding strategies vary, including filter feeding, predation, and herbivory. Reproduction is diverse, with external and internal fertilization; many species produce larval forms with chordate features that may differ markedly from adults, as seen in tunicates and some amphibians.

Evolutionary history: The chordate lineage appears early in the Cambrian fossil record, with potential early relatives such as Haikouella and Pikaia illustrating chordate traits. Today, Ryggstrengdyrene form a foundational group in the animal kingdom, linking simple deuterostomes to highly derived vertebrates.