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
 

Classification of 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 :
 

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.