Notochord:
The notochord is a flexible, rod-like structure that runs along the dorsal (back) side of the animal. It provides support and helps the animal to move. In vertebrates, the notochord is replaced by the vertebral column during development.
Dorsal Nerve Cord:
The dorsal nerve cord is a bundle of nerve tissue that runs along the dorsal side of the animal. It is responsible for coordinating the animal's movements and sensory responses. In vertebrates, the dorsal nerve cord becomes the brain and spinal cord.
Pharyngeal Slits:
Pharyngeal slits are openings in the throat that connect the pharynx (the back of the throat) to the outside environment. In some animals, pharyngeal slits are used for respiration, while in others they are used for feeding. In vertebrates, pharyngeal slits develop into structures such as the Eustachian tubes and tonsils.
Post-anal Tail:
A post-anal tail is a tail that extends beyond the anus. In some animals, the tail is used for locomotion, while in others it is used for balance or as a storage organ. In vertebrates, the tail is lost during development in some species, while in others it is retained as an adult.
Chordates are divided into three subphyla:
Urochordata:
Urochordates, also known as tunicates, are marine animals that have a sac-like body. They are the most primitive chordates and include animals such as sea squirts and salps.
Cephalochordata:
Cephalochordates, also known as lancelets, are small, fish-like animals that live in sandy or muddy bottoms. They are the most similar to the ancestral chordates and include animals such as Branchiostoma.
Vertebrata:
Vertebrates are the most diverse and well-known group of chordates. They include animals such as fish, amphibians, reptiles, birds, and mammals. Vertebrates are characterized by the presence of a backbone, which is made up of vertebrae.