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Chordates

Introduction:

  • The term chordata originated from the two Greek words i.e 'Chorda' meaning ‘a thick string’ and 'Ata' meaning 'to have' and the overall meaning of chordata is animals having notochord.
  • Thus, the animals belonging to phylum chordates have notochord at any stage of their lifespan.
  • According to the data, 90 - 95% animals present on the Earth are non-chordates while the rest of the 3 - 5% animals are chordates.
  • The animals belonging to phylum chordata is mainly characterised due to the presence of following cardinal characters which are found in either embryonal stage or throughout the life -
    - Notochord
    - A dorsal hollow nerve cord
    - Paired, lateral pharyngeal gill slits
    - Post-anal tail

Topics covered

  • Characteristics of Chordata
  • Classification of Chordata
  • Subphylum Acraniata/Protochordata
  • Subphylum Vertebrata
  • Super class pisces
  • Class Amphibia
  • Class Reptilia
  • Class Aves
  • Class Mammalia

Characteristics of Chordata
 

Detailed explanation
 

Fundamental characteristics of phylum Chordata
 

All animals belonging to phylum Chordata possess following Cardinal Characters -
 

1. Notochord/Chorda dorsalis -

  • Notochord is a solid rod-like structure which is present just below the nerve cord and above the alimentary canal.
  • It is situated at the dorsal surface and extended from the anterior to posterior end of the body.
  • It is mesodermal in origin and forms a primary endoskeleton which provides the support to the body.
  • In Protochordates, the notochord is not replaced by a vertebral column.
  • In vertebrates, notochord is replaced by backbone or vertebral column in adults.

2. Dorsal Hollow Nerve Cord -

  • In the organisms belonging to phylum Chordata, the central nervous system (CNS) is located at the dorsal surface of the body.
  • A single, hollow, tubular nerve cord is found beneath the body wall and just above the notochord.
  • Nerve cord is ectodermal in origin.

3. Pharyngeal Gill Slits -

  • In phylum chordata paired lateral gill clefts are present in the walls of pharynx for respiration in the both embryonic and the adult stage.
  • In aquatic chordates (pisces) and lower chordates, pharyngeal gill clefts are found in the both embryonic and the adult stage.
  • In terrestrial chordates, gill clefts are found only in the embryonic stage and are absent in adults.
  • The main respiratory organs are lungs in adults.

4. Anal tail

  • Tail is reduced or absent in chordates but if it is present then it’s found in the post anal part of the body.

Other characters of phylum Chordata -
 

Level of organisation

  • Organ-system level of organisation.

Symmetry

  • Bilaterally symmetrical.

Germ layers

  • Triploblastic animals

Segmentation

  • Metameric segmentation present

Body plan

  • Tube-within-tube body plan.

Cephalization

  • Cephalization is present which is defined as the differentiation of a definite head at the anterior end.
  • The sense organ, nervous tissue (brain) and food catching organs are present at the anterior end.

Coelom (body cavity)

  • Eucoelomates
  • Enterocoelom is present.

Digestion

  • Digestive system is complete.

Respiration and excretion

  • Specialised organs are present for respiration and excretion.

Circulatory system

  • Closed circulatory system is present.
  • Ventral heart.

Reproductive System

  • Reproduction is sexual
  • They are mostly unisexual i.e sexes are separate in them.
  • Gonads are present in them with gonoducts.

Non-chordate vs chordate
 

Non-chordate Chordate
Notochord is present Notochord is absent
Central nervous system is ventral and solid Central nervous system is dorsal and hollow
Gill slits are absent in them. Gill slits are present in the pharynx at embryonic and adult stage
Tail is absent Tail is present at some stages of life
Heart is usually absent but if present then it is dorsally situated Heart is ventral
RBC’s are absent and respiratory pigments like haemoglobin are present in the blood plasma RBCs are usually present, containing respiratory pigment like haemoglobin

 

Classification of chordates

chordates

 

  • Phylum chordata is divided into 2 groups, on the basis of cranium, -

a. Acraniata/Protochordata (lower chordates)

  • It is divided into two sub phyla -
    1. Urochordata (e.g- Herdmania)
    2. Cephalochordata (e.g- Amphioxus)

b. Craniata/Eurochordata (higher chordata)

  • Craniata includes the sub - phylum Vertebrata which is further categorised into 2 divisions,on the basis of jaws -

a. Agnatha (jawless vertebrates)

  • It is further divided into two classes -
    1. Ostracodermi (extinct)
    2. Cyclostomata (e.g : Petromyzon, Myxine)

b. Gnatha (jawed vertebrates)

  • It is further divided into two super classes -

1. Pisces (true fishes)

  • It is further divided into three classes -
    (i)Placodermi (fossils) - eg : Climatius
    (ii)Chondrichthyes (cartilaginous fishes) - e.g : Scoliodon
    (iii)Osteichthyes (bony fishes) - e.g : Labeo

2. Tetrapoda (usually having four limbs)

  • It is further divided into four classes -
    (i) Amphibia
    (ii)Reptilia
    (iii)Aves/ Birds
    (iv)Mammalia

Frequently Asked Questions: FAQs
 

Q1. What is cephalization ?
Ans :

  • Cephalization is present which is defined as the differentiation of a definite head at the anterior end.

Q2. differentiate between Chordates and non-chordates ?
Ans :

 

Non-chordate Chordate
Notochord is present Notochord is absent
Central nervous system is ventral and solid Central nervous system is dorsal and hollow
Gill slits are absent in them. Gill slits are present in the pharynx at embryonic and adult stage
Tail is absent Tail is present at some stages of life
Heart is usually absent but if present then it is dorsally situated Heart is ventral
RBC’s are absent and respiratory pigments like haemoglobin are present in the blood plasma RBCs are usually present, containing respiratory pigment like haemoglobin


Q3. What are the characteristics of phylum Chordata ?
Ans :

  • They possess four cardinal characteristics which are found in either embryonal stage or throughout the life -
  • Notochord
  • A dorsal hollow nerve cord
  • Paired, lateral pharyngeal gill slits
  • Post-anal tail

Q4. What is notochord ?
Ans :

  • Notochord is a solid rod-like structure which is present just below the nerve cord and above the alimentary canal.

Q5. What is the main respiratory organ of chordates in the adult stage ?
Ans :

  • Lungs

Q6. What is the origin of notochord ?
Ans :

  • The notochord is mesodermal in origin.
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