What type of animal has a notochord during core developmental stages?

The presence of a notochord is a defining characteristic of chordates, which is a major grouping of animals that includes vertebrates, such as mammals, birds, reptiles, amphibians, and fish, as well as some invertebrate species like tunicates and lancelets. The notochord serves as a flexible rod-like structure that provides support and aids in the development of the nervous system.

During the early stages of development, chordates possess a notochord that runs along the length of the body. In vertebrates, the notochord is often replaced by the vertebral column as they mature, but it plays a crucial role in embryonic development. This shared characteristic among chordates distinguishes them from other animal phyla, where structures like exoskeletons (as seen in arthropods) or hydrostatic skeletons (in some mollusks and echinoderms) are more prevalent.

Arthropods possess an exoskeleton rather than a notochord, mollusks exhibit various body structures that do not include a notochord, and echinoderms have a unique calcareous endoskeleton and lack a notochord as well. Thus, the defining trait of a notochord