Characteristics of Chordates

Chordates are a diverse group of organisms that share several characteristics, including radial symmetry, pharyngeal slits, notochord, dorsal nerve cord, and a post-anal tail. These features are present at some stage during their development, although some may be present only during embryonic stages.

Radial Symmetry

Chordates exhibit radial symmetry, which means that their body is divided into two identical halves by a single plane of symmetry. This symmetry is most evident in certain chordates, such as sea squirts, where the body has a central mouth surrounded by a ring of tentacles.

Pharyngeal Slits

Pharyngeal slits are openings in the pharynx (the region just posterior to the mouth) that extend to the outside environment. In organisms that live in aquatic environments, these slits allow for the exit of water that enters the mouth during feeding. Some invertebrate chordates use the pharyngeal slits to filter food out of the water that enters the mouth. In vertebrate fishes, the pharyngeal slits are modified into gill supports, and in jawed fishes, into jaw supports.

Notochord

The notochord is a flexible, rod-shaped structure found in the embryonic stage of all chordates and in the adult stage of some chordate species. It is located between the digestive tube and the nerve cord and provides skeletal support through the length of the body. In some chordates, the notochord acts as the primary axial support of the body throughout the animal’s lifetime. In vertebrates, the notochord is present during embryonic development and induces the development of the neural tube, serving as a support for the developing embryonic body. However, in vertebrates, the notochord is replaced by the vertebral column (the spine) in the postnatal stage.

Dorsal Nerve Cord

The dorsal nerve cord is part of the chordate central nervous system and is derived from ectoderm that rolls into a hollow tube during development. In chordates, it is located dorsal to the notochord. In contrast, other animal phyla are characterized by solid nerve cords that are located either ventrally or laterally.

Post-Anal Tail

The post-anal tail is a posterior elongation of the body, extending beyond the anus. It contains skeletal elements and muscles, which provide a source of locomotion in aquatic species, such as fishes. In some terrestrial vertebrates, the tail also helps with balance, courting, and signaling when danger is near. In humans, the post-anal tail is vestigial, that is, reduced in size and nonfunctional.

In conclusion, chordates are a diverse group of organisms that share several characteristics, including radial symmetry, pharyngeal slits, notochord, dorsal nerve cord, and a post-anal tail. These features are present at some stage during their development, although some may be present only during embryonic stages.