Unit I – Protochordata

📘 Unit I – Protochordata (Class XII)

1) What are Protochordates?

Protochordates are the simplest chordates — organisms that show some fundamental chordate characters (notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail, endostyle) but lack the complex features of true vertebrates. They form the evolutionary link between invertebrates and vertebrates.

PEP mnemonic: Pharyngeal slits, Endostyle/Notochord, Posterior tail + dorsal hollow nerve cord
  • Cephalochordata — e.g., Branchiostoma (Amphioxus)
  • Urochordata — e.g., Ascidia (Sea squirt)

2) Cephalochordata — Branchiostoma

A. Quick overview

  • Small, laterally compressed, fish-like marine animals that burrow in sand.
  • Notochord extends throughout life (cephalo = head).
  • No true brain or heart; blood without RBC.
  • Filter feeders using ciliary mode of feeding.

B. Important structures

  • Oral hood with cirri
  • Wheel organ
  • Pharynx with gill slits (branchial basket)
  • Endostyle
  • Atrium & Atriopore
  • Notochord & Myomeres

C. Ciliary Mode of Feeding (stepwise)

  1. Water entry: Water enters through oral hood, cirri sieve particles.
  2. Pharyngeal filtration: Water passes through pharynx; endostyle secretes mucus.
  3. Mucus trapping: Food particles trapped in mucus sheet.
  4. Ciliary transport: Cilia move mucus-food posteriorly into gut.
  5. Water exit: Clean water exits through gill slits into atrium → atriopore.
  6. Digestion: Food digested in intestine; wastes exit posteriorly.
[Insert labelled diagram of Branchiostoma side view with arrows showing water & food flow]

D. Memory Trick

  • Cilia = Conveyor — imagine conveyor belt moving food from pharynx to gut.
  • Amphioxus = Amphi-ox → simple and fish-like.

3) Urochordata — Ascidia

A. Quick Overview

  • Adults are sessile, bag-like with tunic.
  • Larvae are free-swimming tadpole-like with notochord, dorsal hollow nerve cord, tail.
  • Retrogressive metamorphosis: larva loses chordate features to become simpler adult.

B. Important structures

  • Adult: incurrent/excurrent siphons, pharyngeal basket, endostyle, tunic
  • Larva: tail with notochord, dorsal nerve cord, ocellus, otolith

C. Retrogressive Metamorphosis

  1. Larva swims freely with chordate features.
  2. Attaches to substrate.
  3. Tail, notochord, dorsal nerve cord degenerate.
  4. Adult forms: sessile sac with pharynx and tunic.
  5. Adult is simpler than larva → “retrogressive” metamorphosis.
[Insert two-panel diagram: larva → adult showing retrogressive metamorphosis]

D. Memory Trick

  • “Ascidia forgets backbone”: larva has it, adult loses it.

4) Quick Comparative Summary

  • Cephalochordata: Notochord persists in adult; ciliary feeding; mobile.
  • Urochordata: Larva shows chordate characters; adult sessile; retrogressive metamorphosis.

5) Mini Homework (5–10 minutes)

  1. Draw & label: Branchiostoma side view; Ascidia larva → adult diagram.
  2. Write 1 paragraph: Ciliary mode of feeding in Branchiostoma (use keywords: cilia, endostyle, mucus, gill slits, atriopore).
  3. Write 1 paragraph: Retrogressive metamorphosis in Ascidia.

6) References

  • R. L. Kotpal — Vertebrate Zoology
  • Jordan & Verma — Chordate Zoology
  • Online: University zoology lecture notes