What is retrogressive metamorphosis? Give an illustrated account of this phenomenon in Herdmania. Also write a note on its affinities. (IAS 2018/15 Marks)

Retrogressive Metamorphosis:
Please refer to below IAS 2010 question.
Illustrated Account of Retrogressive Metamorphosis in Herdmania
•    Larval Stage (Advanced Stage):
o    Free-Swimming Larva: The tadpole-like larva of Herdmania is motile and shows chordate features.
    Notochord: Present in the tail for structural support.
    Dorsal Nerve Cord: Functions as a primitive nervous system.
    Pharyngeal Gill Slits: Assist in respiration.
    Post-Anal Tail: Helps in swimming.
    Sensory Organs: Simple ocellus (light-detecting) and statocyst (balance organ).
•    Metamorphosis Process:
o    Attachment: The larva attaches to a substrate using adhesive papillae.
o    Degeneration:
    Notochord and nerve cord are reabsorbed.
    Tail is lost.
    Sensory organs disappear.
o    Transformation:
    Body becomes a bag-like structure.
    Pharynx enlarges to form a branchial basket, aiding in filter-feeding.
    Tunic (protective covering) is secreted.
•    Adult Stage (Simpler Stage):
o    Sedentary and immobile.
o    Lacks chordate features like the notochord and tail.
o    Possesses a siphon-based system for water intake and expulsion.
Note on the Affinities of Herdmania
•    Chordate Affinities:
o    The larval stage of Herdmania strongly supports its chordate lineage due to:
    Presence of a notochord.
    Dorsal nerve cord.
    Post-anal tail.
    Pharyngeal gill slits.
•    Non-Chordate Features in Adults:
o    Loss of notochord and nerve cord in adulthood resembles non-chordates.
o    The adult stage is more aligned with invertebrate-like sessile forms.
•    Phylogenetic Importance:
o    Herdmania demonstrates the evolutionary transition from chordates to non-chordates.
o    It highlights the adaptive modifications for a sedentary lifestyle.
Conclusion:
Retrogressive metamorphosis is a fascinating biological phenomenon that highlights the plasticity and adaptability of certain organisms. By undergoing a reversal of their developmental stages, these organisms are able to survive in changing environments and conditions.