Phylum Chordata is an important phylum. Its name is derived from the notochord. Endoskeleton is the chief basic character of the Chordata phylum. This endoskeleton is an important factor in the development and specialization of the higher animals. Man is the most important chordate. The chordates show a great variety. They live in all kinds of habitats.
Characteristics of Phylum Chordata
All the chordates possess three basic characters:
1. Presence of notochord
It is an important character of chordates. The notochord is present in all the chordates. It may be present in larval or embryonic stages or throughout life. The notochord is a semi-rigid rod-like structure. It is composed of vacuolated cells (cells containing large vacuoles like plant cells). These cells are filled with proteinaceous material.
Notochord mostly extends throughout the length of the body. It is present between the enteric canal (digestive canal) and the dorsal hollow central nervous system. Its primary function is to support the body. It acts as a skeletal axis and stiffens the body.
2. Hollow Dorsal Central Nervous System
All the chordates have a central nervous system. It is hollow and dorsal in position.
3. Presence of Gill Slits
All chordates develop paired gill slits or gill openings in their embryonic stages. These gill slits are non-functional in some chordates. In some chordates, these are functional during a small period of their life history e. g. frog. In other chordates, these are functional throughout their life, e.g. Amphioxus and fishes.
Classification of Chordates
Chordates are divided into two major groups:
In these animals, the braincase or skull is absent. Notochord remains intact throughout life. These are also called lower chordates e.g. Amphioxus. Acrania has two sub-phyla, Urochordata and Cephalochordate.
In these animals, a braincase or skull is present. These are called higher chordates. The notochord is replaced by the vertebral column. So they are also called vertebrates. Craniata has a single subphylum, Vertebrata.
- Notochord and nerve chord is present only in the free-swimming larva. These are absent in adults.
- Adult animals are sessile (cannot move).
- Their body is enclosed in a covering called a tunic. So they are also called tunicates.
Notochord and nerve cord extends along the entire length of the body. Notochord persists throughout life e.g. Amphioxus.
The brain of these animals is enclosed in a braincase or cranium. They have a vertebral column. Chordates are commonly divided into two superclasses:
- Pisces (fish): They are all strictly aquatic animals. It includes all types of fishes.
- Tetrapoda: They are four-footed animals. It includes class amphibian, reptilia, saves, and Mammalia.
Vertebrates may be divided into two groups on the basis or absence of embryonic or fetal membrane called the amnion.
- Anamniota: Foetal membrane or amnion is absent in these animals. It includes Cyclostomata, Chondrichthyes, Osteichthyes, and Amphibia.
- Amniota: Foetal membrane or amnion is present in these animals. It includes Reptilia, Aves, and Mammalia.
This phylum has a combination of both invertebrate (Echinoderm) and chordate characteristics. Hemichordata, Echinodermata, and Chordata form a branch of the animal kingdom called Deutrostome. As they have a close relationship with the chordates, so they are called chordates. The common examples of this phylum are Blanoglossus and Saccoglossus.
General Characteristics of Phylum Hemichordata
- Their bodies are soft and worm-like.
- The body is divided into the anterior probosci’s collar and trunk (main body).
- The body wall is made up of the unicellular epidermis. This epidermis contains mucous secreting cells.
- The digestive tube is straight. It may show variations.
- Coelomic cavities are also divided into three regions of the body in the proboscis, collar, and coelomic pouches.
- The circulatory system consists of a median dorsal and a median vessel.
- The respiratory system is composed of gill-slits. These gill slits form a dorsal row behind the collar.
- The excretory system is composed of a single glomerulus. This glomerulus is connected to blood vessels.
- The nervous system has a sub-epidermal plexus of cells and fibers.
Other Related Phyla: