Chordates

All vertebrates, or animals with backbones, as well as several invertebrates, or living organisms without backbones, are members of the Phylum Chordata, which is a subclass of the Kingdom Animalia. They have a bilaterally symmetrical body and are classified into three distinct sub-phyla.

Classification of Chordates

The three following subphyla make up the Phylum Chordata:

• Urochordata (tunicates)
• Cephalochordata (lancelets)
• Vertebrata (vertebrates).

Urochordata

It is also referred to as a “tunicate” because the adult’s body is covered with a tunic made of a cellulose-like substance called tunicin. Only during the larval stage can the notochord be observed; once an adult, it is gone. In adults, a dorsal ganglion takes the role of the nerve cord that exists in larvae. The larva transforms and has the ability to move.

Example: Ascidia, Salpa, and Doliolum.

Cephalochordata

There is an atrium present in cephalochordata. Both the adult and larval stages are movable. Throughout life, there is a tail. They undergo gradual transformation. All throughout life, the notochord is present. They have numerous, well-developed pharyngeal gill slits.

Example: Lancelets are born with the notochord and nerve cord. But unlike Branchiostoma, they don’t have a brain or a skeletal spinal column.

Vertebrata

These evolved chordates have a skull that surrounds their brains. In adults, a spinal column takes the place of the notochord. This explains the expression that “all chordates are vertebrates, but all chordates are not vertebrates”. High levels of cephalization are seen. The epidermis has several layers. They are made up of three different muscle types: cardiac, striped, and unstriped. They have an advanced coelom. The gastrointestinal tract is developed. There are three or four chambers in the heart. Their respiratory and excretory systems have grown considerably. All mammals have endocrine glands. With the exception of hogfish, all are sexually reproducing and unisexual.

Example: Humans.

Subphylum of Vertebrata

Vertebrata is further sub-divided into:

• Pisces (Fishes): They are aquatic organisms with a sleek body and a pair of fins that are utilized for movement and propulsion. Example: Rohu, Catla.
• Amphibia: They often consist of animals with cold blood that need an aquatic environment to lay their eggs. These organisms can breathe through their two pairs of limbs and their smooth, moist skin. Example: Frog, Toad.
• Reptilia: Ectothermic (cold-blooded) species are included in the class Reptilia. They are distinguished by osteoderms, which produce scales, bony plates, or skin scutes. Example: Tortoise, Snake.
• Aves (Birds): The majority of members have sleek bodies that are especially made to provide little air resistance while flying. Such birds have modified forelimbs that function as wings, with the breast muscles providing the force. Example: Parrot, Pigeon.
• Mammalia: Mammary glands are a defining characteristic of these organisms. They have two sets of limbs for activities such as walking, grabbing, swimming, and flying. The digits have claws, nails, or hooves. Example: Monkeys, Humans.

Characteristics of Chordates

The Phylum Chordata has the following distinguishing characteristics:

Notochord

It is a long, cartilaginous rod that connects the nervous system and the digestive system. In all vertebrates, it serves as a support for the nerve cord and is eventually replaced by the vertebral column.

Dorsal Nerve Cord

The brain and spinal cord are separated from the bundle of nerves that runs along the “back” of the body. It lies dorsal to the notochord and is hollow.

Pharyngeal Slits

They are the holes that connect the mouth and throat and permit water to enter the body through the mouth without going through the digestive system. These holes on the lateral sides of the pharynx are present in all chordates at some point in their development.

Post Anal Tail

It is the body’s extension to the anus. In chordates, the tail is made up of skeletal muscles that aid in locomotion in species that resemble fish. The majority of adult Chordates lack it.

The classification of the animal kingdom is the process of classifying animals in a hierarchy. A predetermined number of levels, such as kingdom, family, or genus, form the basis of the ranking system. Depending on the goal of the categorization, the basis for that classification may change. A biological classification typically uses evolutionary and morphological similarities as its foundation.