- 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)
- Chordates are characterized by the presence of;
- 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;
- 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.
- Herdmania (Sea Squirt)
CLASSES OF UROCHORDATA
Urochordata subphylum is further divided into three main classes;
- 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;
- 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 consists of existing and extinct jawless chordates such as the hagfish.
Agnatha further consists of two class
- 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.
- Lampreys (Petromyzon)
- Hagfish (Myxine)
Gnathostomata are jaw bearing chordates and consist of two super-classes.
- 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 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.
- Pristis (saw fish)
- Trygon (stingray)
- 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.
- Marine water- Hippocampus (sea horse)
- Fresh water- labeo (rohu)
- 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:
- 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
- Buffo (toad)
- Rana (frog)
- 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.
- Testudo (turtle)
- Vipera (viper snake)
- 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.
- Corvus (crow)
- Pavo (peacock)
- 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.
- Felis (cat)
- Panthera tigris (tiger)
- Arihant’s handbook of biology. Animal Kingdom. Page no: 43-73.
- NCERT biology; textbook for class 11. Animal Kingdom. Page no: 46-62.