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INTRODUCTION
- Chordates are animals in which notochord is present.
- Notochord is a flexible rod-like structure of mesodermal origin, that serves as the principal longitudinal structural element of chordates and gives rise to the spinal cord. Therefore, chordates are also known as vertebrates (possessing a vertebral column)
GENERAL CHARACTERISTICS
- Chordates are characterized by the presence of;
- Notochord
- Dorsal nerve cord
- Paired pharyngeal slits
- Members of phylum Chordata are bilaterally symmetrical.
- They are triploblastic (have three germinal layers) and possess a segmented body.
- They are coelomates and display an organ-system level of organization.
- They consist of a closed circulatory system and have separate sexes.
- Chordates reproduce only by sexual means.
CLASSIFICATION OF PHYLUM CHORDATA
Phylum Chordata consists of three subphyla;
- Urochordata
- Cephalochordate
- Vertebrata
UROCHORDATA
- Animals belonging to this subphylum are known as sea-squirts.
- The typical chordate structures (notochord, dorsal nerve cord and pharyngeal slits) are only present in the larval stage and disappear in the adult stage.
- The notochord is restricted to the tail of the larva.
- The dorsal nerve cord is replaced by a dorsal ganglion in the adult stage whereas the pharyngeal slits in the larval stage give rise to pharynx stigmata in the adult stage.
- The larva is motile and undergoes retrogressive metamorphosis.
FOR EXAMPLE
- Herdmania (Sea Squirt)
- Ascidia
- Ciona
CLASSES OF UROCHORDATA
Urochordata subphylum is further divided into three main classes;
- Larvacea
- Ascidiacea
- Thaliacea
CEPHALOCHORDATA
- Cephalochordates are marine organisms.
- The notochord (extending from anterior to posterior end) and nerve cord in cephalochordate persist throughout life.
- The body of cephalochordates is fish-like with a head and tail and is useful for swimming and burrowing.
- They possess a closed circulatory system.
- Reproduction is sexual and is characterized by external fertilization.
CLASSES OF CEPHALOCHORDATA
Subphylum cephalochordata only consist of a single class;
- Leptocardii
VERTEBRATA
- The members of subphylum vertebrata possess a notochord during the embryonic period. In adults, the notochord is replaced by a vertebral column.
- Therefore, all vertebrates can be termed as chordates but all chordates are not vertebrates.
- These are advanced chordates that have cranium around the brain and possess a high degree of cephalization (formation of head).
- The epidermis consists of an exoskeleton of scales, feathers or hair.
- They possess a closed circulatory system and a ventral, well developed heart that consists of two, three or four chambers.
- All of them are unisexual except the Hagfish which is bisexual.
CLASSIFICATION OF VERTEBRATA
Subphylum vertebrata further consist of two divisions
- Agnatha
- Gnathostomata
AGNATHA
Agnatha consists of existing and extinct jawless chordates such as the hagfish.
Agnatha further consists of two class
- Cyclostomata
CLASS-CYCLOSTOMATA
- Cyclostomata are marine organisms that consist of a circular, sucking jawless mouth and act as ecto-parasites on some fishes.
- The body of cyclostomata is elongated with 6-15 pairs of gill slits for respiration and is devoid of scales or paired fins.
- They possess a closed circulation and a cartilaginous cranium and vertebral column.
- This feature distinguishes them from the rest of the vertebrates.
FOR EXAMPLE
- Lampreys (Petromyzon)
- Hagfish (Myxine)
GNATHOSTOMATA
Gnathostomata are jaw bearing chordates and consist of two super-classes.
- Pisces
- Tetrapoda
SUPER-CLASS PISCES
- These are jaw bearing, true vertebrates that possess specialized organs for excretion, respiration and circulation.
- Pisces consists of aquatic organisms (fish) that reside in fresh, marine and brackish water.
- Their body is spindle shaped or streamlined, enabling them to move rapidly in water.
- Body of such organisms consists of a head, trunk and tail and is covered with waterproof dermal scales.
- The tail acts as a locomotory organ and enables the fish to propel across water. The fins act as appendages and maintains balance while swimming.
- They comprise of 4-7 pairs of gills for respiration which may be naked or covered with an operculum.
- They possess a closed circulation.
- Their Endoskeleton is made up of either cartilage or true bones.
CLASSES OF PISCES
Super-class Pisces is further divided into two classes;
- Chondrichthyes
- osteichthyes
CLASS-CHONDRICHTHYES
- Chondrichthyes are marine animals with a streamlined body.
- Notochord in such organisms persist throughout life.
- Mouth is ventrally located.
- Gill slits are separate and without operculum.
- The skin is tough and covered with minute placoid scales.
- Heart of such organisms is two chambered and consist of one auricle and one ventricle.
FOR EXAMPLE
- Pristis (saw fish)
- Trygon (stingray)
CLASS-OSTEICHTHYES
- This class consists of both marine and freshwater fish with a streamlined body.
- Mouth is terminally located in these organisms.
- They possess four pairs of gills which are covered by an operculum.
- The skin of osteichthyes is covered with cycloid scales.
- Heart is two chambered i.e. consists of one auricle and one ventricle.
FOR EXAMPLE
- Marine water- Hippocampus (sea horse)
- Fresh water- labeo (rohu)
SUPER-CLASS TETRAPODA
- Tetrapod literally means an animal with four feet.
- Tetrapods are primarily terrestrial while some are also aquatic.
- They have two pairs of pentadactyl limbs (having five fingers or toes) for locomotion.
- Their body is covered by an exoskeleton of epidermal hair, feathers or scales.
- Lungs are the main respiratory organs in Tetrapods. Amphibians are an exception and may respire by gills, skin or lungs.
- They possess 3-4 chambered heart
CLASSES OF TETROPODA
- They are divided into 4 classes:
- Amphibia
- Reptilia
- Aves
- Mammals
CLASS-AMPHIBIA
- Amphibians can inhabit both terrestrial and aquatic environments
- Most amphibians have two pairs of limbs and the body is divided into a head and trunk. Tail may be found in some amphibians.
- Amphibian’s skin is moist.
- The eyes are covered by eyelids and tympanum represents the ear.
- They possess a cloaca, which is a common chamber into which the urinary tract, alimentary tract and reproductive tract opens.
- Respiration in amphibians can occur through gills, lungs or skin.
- They are cold-blooded animals with a three chambered heart
FOR EXAMPLE
- Buffo (toad)
- Rana (frog)
CLASS-REPTILIA
- Reptiles are mostly terrestrial animals with a dry cornified skin, epidermal scales or scutes.
- They display a creeping or crawling mode of locomotion.
- Reptiles are cold blooded animals with a three-chambered heart. Crocodiles however possess a four chambered heart.
- Reptiles such as snakes and lizards shed their scales as skin cast.
FOR EXAMPLE
- Testudo (turtle)
- Vipera (viper snake)
CLASS-AVES
- Aves (birds) are characterized by the presence of feathers and possess the ability to fly except some flightless birds.
- The forelimbs are modified into wings while their hind limbs are covered with scales and are used for walking.
- Aves have the streamlined body which reduces the resistance during flight.
- They have a bony endoskeleton and the long bones are hollow with air cavities.
- They are warm-blooded with a four-chambered heart.
FOR EXAMPLE
- Corvus (crow)
- Pavo (peacock)
CLASS-MAMMALIA
- Members of this class are characterized by the presence of mammary glands, which are milk-producing glands through which young ones are nourished
- Mammals are highly developed, mostly terrestrial animals with a much advanced brain.
- Their Body is usually covered with hair and possesses external ears or pinna for hearing.
- Respiration occurs with the help of lungs.
- They are warm blooded and possess a four-chambered heart.
FOR EXAMPLE
- Felis (cat)
- Panthera tigris (tiger)
REFERENCES
- Arihant’s handbook of biology. Animal Kingdom. Page no: 43-73.
- NCERT biology; textbook for class 11. Animal Kingdom. Page no: 46-62.