🐠 Introduction

The Phylum Chordata includes animals possessing a notochord, a dorsal hollow nerve cord, and pharyngeal gill slits at some stage of development. These features mark a significant evolutionary advancement from invertebrates to vertebrates.

🧬 Distinguishing Features of Chordates

  1. Notochord: A flexible rod-like structure providing support. Present during embryonic stage or throughout life.
  2. Dorsal Hollow Nerve Cord: Lies above the notochord; develops into brain and spinal cord.
  3. Pharyngeal Gill Slits: Paired openings for respiration or feeding. These features are present at least during some stage of their life cycle.

General Characteristics

  • Symmetry: Bilaterally symmetrical.
  • Germ Layers: Triploblastic.
  • Coelom: Coelomate.
  • Level of Organisation: Organ-system level of organisation.
  • Circulatory System: Closed with ventral heart.
  • Post-anal Tail: Presence of a post-anal tail (an extension of the body beyond the anus). Aiding locomotion in some.
  • Endoskeleton: Usually present and made of bone or cartilage.

🧩 Classification of Chordata

Chordates are divided into three subphyla:

1. Urochordata (Tunicata)

  • Marine, sessile adults; free-swimming larvae with chordate features.
  • Notochord present only in larval tail (hence 'Uro-chordata').
  • Body enclosed in a tunic (cellulose-like covering).
  • Example: Ascidia, Salpa, Doliolum.

2. Cephalochordata

  • Marine, burrowing forms.
  • Notochord extends through entire body (head to tail).
  • Example: Branchiostoma (Amphioxus).
  • Show all chordate features throughout life.

3. Vertebrata (Craniata)

  • Possess a notochord during the embryonic period.
  • The notochord is replaced by a cartilaginous or bony vertebral column (backbone) in the adult.
  • Thus, All vertebrates are chordates but all chordates are not vertebrates.
  • Have a ventral muscular heart with two, three, or four chambers.
  • Possess kidneys for excretion and osmoregulation.
  • Have paired appendages which may be fins or limbs.
  • Distinct head with brain enclosed in a cranium.
  • Highly developed organ systems.
  • Example: Fish, Amphibia, Reptilia, Aves, Mammalia.

Subphylum Vertebrata is further divided into two Divisions:

  • Agnatha (Jawless vertebrates)
  • Class: Cyclostomata
  • Gnathostomata (Jawed vertebrates)
  • Superclass: Pisces (bear fins)
    • Class: Chondrichthyes (Cartilaginous fishes)
    • Class: Osteichthyes (Bony fishes)
  • Superclass: Tetrapoda (bear limbs)
    • Class: Amphibia
    • Class: Reptilia
    • Class: Aves
    • Class: Mammalia (These classes will be discussed in the next section).

🧠 Key Differences: Non-Chordates vs Chordates

Feature Chordates Non-chordates
Notochord Present (at least in some life stage) Absent
Central Nervous Sys. Dorsal, hollow, and single Ventral, solid, and double
Pharyngeal Slits Present Absent
Heart Ventral (if present) Dorsal (if present)
Post-anal Tail Present Absent
Circulation Closed Open (usually)
Example Fish, Frog, Human Prawn, Earthworm

💡 Questions and Answers

Q1. Name the three diagnostic features of chordates.

A1. Notochord, dorsal hollow nerve cord, and pharyngeal gill slits.

Q2. Which subphylum of chordates shows notochord only in larval stage?

A2. Urochordata.

Q3. Which subphylum retains notochord throughout life?

A3. Cephalochordata (e.g., Amphioxus).

Q4. What is the difference between Urochordata and Cephalochordata?

A4. Urochordata: Notochord only in tail (larva); Cephalochordata: Notochord extends from head to tail.

Q5. Which structure in vertebrates replaces the notochord?

A5. Vertebral column (spinal column).

Q6. What are the four fundamental characteristics shared by all chordates at some point in their life?

A6. The four key diagnostic features of Phylum Chordata are:

  1. Presence of a notochord.
  2. Presence of a dorsal, hollow nerve cord.
  3. Presence of paired pharyngeal gill slits.
  4. Presence of a post-anal tail.

Q7. How do Urochordates and Cephalochordates differ with respect to the notochord?

A7.

  • Urochordata (Tunicates): The notochord is present only during the larval stage and is confined to the tail region. It is lost in the adult.
  • Cephalochordata (Lancelets): The notochord extends from the head to the tail region and persists throughout the entire life of the animal.

Q8. What is Branchiostoma commonly known as, and which subphylum does it belong to?

A8. Branchiostoma is commonly known as Amphioxus or Lancelet. It belongs to the subphylum Cephalochordata.