(Greek-: chorda, a string, or Latin -: chordatus, a chord, i.e., the notochord) is the largest of the deuterostome phyla. It is the highest and the most important phylum comprising a vast majority of animals living and extinct such as tunicates, lancelets, lampreys, fishes, amphibians, reptiles, birds and mammals including man.
All chordates are bilaterally symmetrical, with 3 germ layers, a segmented body, complete alimentation and a well-developed coelom. While groups of chordates differ widely from each other, they, however, possess three outstanding common characters which separate them from non-chordates. These three fundamental common chordate traits are as follows :
- Notochord (Gr., noton, back; chorde, cord) :- It is a slender longitudinal stiff skeletal rod of connective tissue cells present just dorsal to the digestive tract. It is this structure which gives the phylum its name. It furnishes support to the body and is not to be confused with the nerve cord. It persists throughout life in the lower chordates such as urochordates, cephalochordates and cyclostomes. But in the higher chordates (fishes to mammals) it is present only in the embryos and later replaced by the vertebral column which is made up of different Vertebrae.
- Hollow dorsal nerve chord :- A hollow or tubular nerve cord is present extending lengthwise of the body dorsal to the notochord. It arises as an infolding of the dorsal surface ectoderm of the embryo.
- Pharyngeal gill-clefts :- Paired lateral openings, commonly referred to as gill-clefts or gillslits, develop on the sides of the embryonic pharynx, leading to outside. They persist in all lower chordates and serve for aquatic pharyngeal respiration. In higher Chordates they soon close before hatching or birth as the adults of these animals develop lungs for breathing air.
Classification of Chordata
Phylum Chordata is classified as follows :
GROUP A :- LOWER CHORDATA OR PROTOCHORDATA OR ACRANIATA
Exclusively marine. Mostly small-sized. Cephalization, cranium, brain and vertebral column absent Notochord present.
Subphylum I. Hemichordata
Small soft bodied, marine enterocoelus coelomate creatures. Notochord confined to head region. Hemichordates are divided into class.
Class :- Enteropneusta. Burrowing animal. Ex. Balanoglossus, Saccoglossus and Ptychodera.
Subphylum II. Urochordata (Tunicata)
Typically short, thick-bodied, with a leathery covering called test or tunic. Notochord and nerve cord present only in larval tail.
- Class 1. Ascidiacea :- Tunic with scattered muscles. Many pharyngeal gill-slits.
- Class 2. Thaliacea :- Tunic with circular muscle bands.
- Example :-. Pyrosoma, Doliolum, Salpa.
- Class 3. Larvacea :- Minute. Tunic temporary. 2 gill-slits.
- Example :-. Oikopleura.
Subphylum III. Cephalochordata
Small, elongated, fish-like. Without paired fins. Notochord and nerve cord along entire body and persistent. Pharynx with numerous gill-slits.
Class 1. Leptocardii :- Example :- Branchiostoma (= Amphioxus).
Class 2. Plerobranchie :- Encased body Example :- Rhabdopleura, Cephalodiscus Atubaria.
GROUP B. HIGHER CHORDATA OR CRANIATA
Small to large-sized, head distinct with cranium, visceral arches, vertebrae and brain. The higher chordates include a single subphylum, the Vertebrata. Notochord broken into different vertebra.
Subphylum IV. Vertebrata
Small to very large. With marked cephalization. Brain enclosed in cranium. Notochord replaced partially or wholly by vertebral column.
Division I. Agnatha :- No true jaws or paired appendages.
Class 1. Ostracodermi :- Extinct fishes. Skin covered by large bony scales or plates. Example:- Cephalaspis.
- Class 2. Cyclostomata :- Skin without scales. Mouthsuctorial. Gills 5 to 16 pairs. Skeletoncartilaginous. Example :- Petromyzon, Myxine, Bdellostoma.
Division II. Gnathostomata :- With true jaws and paired appendages.
Superclass 1. Pisces :- Paired fins, gills and skin with scales. Example :- Fishes.
- Class 1. Placodermi :-Extinct fishes with primitive jaws.
- Class 2. Chondrichthyes :- Cartilaginous fishes. Skin with placoid scales. Gill-slits exposed, no operculum. Example :- Sharks, rays and skates.
- Class 3. Osteichthyes :- Bony fishes. Skin with cycloid or ctenoid scales. Gill-slits covered by an operculum. Example :- Perch, catfish, ganoids and lung-fishes.
Superclass 2. Tetrapoda :- Paired limbs, lungs, cornified skin, and bony skeleton. Amphibious and terrestrial vertebrates.
- Class 1. Amphibia :- Skin moist, soft, glandular and without external scales. Cold-blooded. Example :- Frogs, toads, salamanders.
- Class 2. Reptilia :- Skin dry, with external scales or scutes. Cold-blooded. Example :- Lizards, snakes, turtles and crocodiles.
- Class 3. Aves :- Skin covered with feathers. Forelimbs form wings. Warm-blooded. Example :- Birds.
- Class 4. Mammalia :- Skin covered with hairs. Females have milk glands to suckle young. Warm blooded. Example of Mammals, such as duckbill, bats, moles, dogs, cats, elephant, horse, lion, cow, monkey, buffalo and man.