- Notochord: Chordates possess a notochord, which is a flexible rod-like structure that runs along the length of the body. It provides support and acts as a precursor to the vertebral column (spine) in most vertebrates.
- Dorsal Nerve Cord: Chordates have a dorsal nerve cord that is located above the notochord. This nerve cord is hollow and serves as the central nervous system. In vertebrates, it develops into the brain and spinal cord.
- Pharyngeal Gill Slits: Chordates possess pharyngeal gill slits, which are openings in the pharynx (throat) region. In some chordates, these slits are used for filter feeding, while in others, they are modified for respiration.
- Post-anal Tail: Chordates have a tail that extends beyond the anus at some point during their development. This tail may be present in the adult stage in some species, while in others, it may be present only during the embryonic or larval stage.
- Endostyle or Thyroid Gland: Chordates have an endostyle or a thyroid gland located in the throat region. It is responsible for the production of hormones and plays a role in metabolic regulation.
NOTE: Not all chordates exhibit these features in the same way or to the same degree.
For example, invertebrate chordates like lancelets and tunicates exhibit these traits in their larval stages, while some traits may be lost or modified in certain vertebrates. Nonetheless, these characteristics provide a framework for classifying organisms within the phylum Chordata.
Phylum Chordata is divided into three subphyla: Urochordata or Tunicata, Cephalochordata, and Vertebrata. Urochordata and Cephalochordata are often referred to as protochordates and are exclusively marine.
- Subphylum Urochordata: Urochordates, also known as Tunicates, exhibit chordate characteristics only during their larval stage. The notochord is present only in the larval tail, and it is lost or reduced in the adult stage. Examples of Urochordates include Ascidians, Salpa, and Doliolum.
- Subphylum Cephalochordata: Cephalochordates, represented by the genus Branchiostoma (also called Amphioxus or Lancelet), exhibit chordate characteristics throughout their life. They possess a notochord that extends from the head to the tail region and persists throughout their lifespan. Cephalochordates are also marine organisms.
- Subphylum Vertebrata: Vertebrates are the most familiar and diverse group within the phylum Chordata. They possess a notochord during the embryonic stage, which is later replaced by a cartilaginous or bony vertebral column (spine) in the adult stage. Vertebrates have additional characteristics beyond the basic chordate features. These include a ventral muscular heart with two, three, or four chambers, kidneys for excretion and osmoregulation, and paired appendages, which can be in the form of fins or limbs. Vertebrates exhibit a wide range of diversity, including fish, amphibians, reptiles, birds, and mammals.
It's important to note that while all vertebrates are chordates, not all chordates are vertebrates. The subphylums Urochordata and Cephalochordata represent primitive chordate groups that show certain chordate characteristics only during specific stages of their life cycles. Vertebrates, on the other hand, represent a more advanced and diverse group within the phylum Chordata.
The subphylum Vertebrata is further classified as follow: