Urochordates (tunicates/ascidians) exhibit retrogressive metamorphosis where free-swimming tadpole larva loses chordate features (notochord, tail, nerve cord) to become sessile adult. For Q.30—”Sequence: A. non-feeding free swimming larva, B. epidermis shrinks/notochord regresses, C. larva settles by adhesive papilla, D. 180° internal reorganization”—correct answer is option 2: A, C, B, D.
Option Analysis
Option 1: C, B, D, A
Settling (C) before larva exists (A) is impossible—metamorphosis begins with free-swimming tadpole stage.
Incorrect; reverses developmental chronology.
Option 2: A, C, B, D
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A: Non-feeding tadpole larva swims (24-36 hrs), chordate features present (notochord, dorsal nerve cord, tail).
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C: Attaches head-first to substrate via anterior adhesive papillae (sensilla).
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B: Tail, notochord, nerve cord regress; epidermis contracts; larval organs phagocytosed.
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D: 180° rotation repositions gut; pharynx enlarges (gill slits multiply); atrial cavity forms; heart develops.
Correct; standard ascidian metamorphosis (Herdmania, Ciona).
Option 3: A, B, C, D
Tail regression (B) before attachment (C) wrong—larva must settle first, triggering degeneration signals.
Incorrect; settling induces retrogressive changes.
Option 4: D, A, B, C
Reorganization (D) before larva (A) defies logic—adult features develop post-larval settlement.
Wrong; internal rotation follows regression.
Metamorphosis Stages Table
| Stage |
Event |
Key Changes |
| A |
Free-swimming larva |
Tadpole: notochord, tail, nerve cord, eyespot, otolith |
| C |
Settlement |
Adhesive papillae → permanent attachment (head down) |
| B |
Regression |
Tail/notochord absorbed; nerve ganglion forms |
| D |
Reorganization |
180° rotation; pharynx → branchial basket; gonads mature |
Retrogressive: Larva more “chordate-like” than adult; tunic (cellulose) secreted post-settlement.
Exam Relevance
CSIR NET/GATE Chordata: Distinguish progressive (frog) vs. retrogressive (urochordate) metamorphosis. Larva proves tunicate-chordate link. Thyroid hormone (TH) triggers settlement.
