[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.