What are the chordates and their derived Characteristics?

Chordates are the organisms that are members of the phylum Chordata. This phylum has vertebrates, which are highly organized organisms. It also comprises two other sub-phyla that are:

• Sub-phylum Tunicata

• Sub-phylum Cephalochordata

Sometimes, hemichordates of phylum Hemichordata are also related to Chordates.

Derived Characteristics of Chordata(Chordates)

The chordates have some essential characteristics which are also sometimes called derived characteristics of chordates. They are the following:

• Dorsal (Hollow) Nerve Cord

• Notochord

• Pharyngeal Slits

• Post-anal (Muscular) Tail

Dorsal Nerve Cord

The “dorsal nerve cord” is a distinctive feature of chordates, and it is especially found in the vertebrates,  phylum Vertebrata, and subphylum Chordata. The dorsal nerve cord is the embryonic feature found in all chordates but sometimes it is also found in the adult chordates, among the other features of chordates namely a notochord, a muscular post-anal tail, and pharyngeal slits. The dorsal hollow nerve cord is a hollow cord that is present on the dorsal side of the notochord. It is formed from ectoderm cells which roll to form this hollow tube. It is an important feature because it differentiates chordates from all other phyla of animals, like  Arthropods and Annelids, which have tubes that are solid in nature. 

It is performed by a process which is called invagination. As discussed earlier, the cells basically tangled into the body cavity /coelom, organizing themselves on the dorsal surface above the notochord. The evolutionary justification for this adaptation from a solid cord to a hollow nerve cord is still unrecognized. In vertebrates, the dorsal nerve cord is altered into the central nervous system, which consists of the spinal cord and brain.

Notochord

The primitive beginning of the backbone is Notochord. It emerges in embryos as a small stretchy rod made from the layer of mesodermal cells, it is one of the three layers of cells called, embryos. Notochords are special characters and are only found in  Chordates, the phylum Chordata, a group of animals that also includes humans. We can say that it is a particular feature of this particular phylum. Notochords, and backbones, are features that distinguish us from the more primitive living organisms.

The Notochord and the Nerve cord are the properties of chordates. Both these appendages,( notochord and nerve cord )are present from the neck to the tail in the dorsal part of the body of organisms. The notochord is a skeletal rod, and a nerve cord is a hard structure made up of nerve tissue. The major difference between the notochord and nerve cord is that the notochord involves the skeleton whereas the nerve cord involves the central nervous system of chordates. Notochord occurs between the nerve cord and therefore the central nervous system. The nerve cord occurs below the notochord. Notochord gives sites for the joining of the skeletal muscles. The nerve cord corresponds to the functions of the body.

The notochord is present from head to tail and is found between the digestive tube and the nerve cord. Since it is composed of stiffer tissue, it allows for skeletal support in organisms. In certain chordates, like the lamprey and the sturgeon, the notochord remains till their whole life. In vertebrates, like humans, a more complex and complicated backbone is formed and only small portions of the notochord remained as remnants. When our backbone is fully formed, the vertebral discs in our back are where the portions of notochord are found which are made up of sticky, gelatinous material.

Pharyngeal Slits

 The Pharyngeal slits are filter-feeding organs found in deuterostomes. Pharyngeal slits are reprised voids that appear along the pharynx caudal to the mouth. With this position, they allow the movement of water in the mouth and out from the pharyngeal slits. It is proposed that this is how pharyngeal slits first help the filter-feeding organisms, and later on, by the addition of gills along their walls, aiding in the respiration of water-living chordates. These repeated appendages are also controlled by identical developmental mechanisms.

In all chordates, the walls of the pharynx are perforated or nearly perforated by the longitudinal sequence of openings. In human beings, the pharynx consists only of vacant space between the oral cavity and nasal cavity, and larynx. In rudimentary chordates and fish, the pharyngeal slits perform the function of respiration and feeding,i.e, water that comes in the mouth moves out of the body through the pharyngeal slits. Organisms filter this water and extract their food and in fish, gills are associated with these slits.

In humans, these slits with their concurrent tissues are important in the formation of blood vessels, cartilages, glands, and bones in the mouth, throat, and upper chest areas. Periodically, a pharyngeal slit remains open in a person, that is cured with the help of surgery.

Post-anal- tail

All chordates have a post-anal tail that is an extension of the notochord and nerve cord past the anus. This segment is also lost in the grown-up stages of many chordates, like frogs and humans.

The post-anal tail is a posterior extension of the body, elongated beyond the anus. The tail is made up of skeletal components and muscles, which provide a way of locomotion and motion in aquatic organisms such as fishes. In other terrestrial vertebrates, this tail also helps to maintain balance, and wooing, and to produce signals when danger is close. In humans and other organisms like great apes, the post-anal tail is diminished to a vestigial coccyx which is also called “tail bone”. that helps during sitting.