[Biology] Chordates

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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.

 

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