Question

Retrogressive metamorphosis is seen in
A. Balanoglossus
B. Branchiostoma
C. Herdmania(Urochordata)
D. All of the above

Answer 1

Hint: Metamorphosis is a biological process through which an animal evolves externally after birth or hatching, involving, via cell growth and differentiation, a noticeable and relatively sudden transition in the body structure of the animal.

Complete Answer:
Advanced characters of an early form are lost in the adult in retrogressive metamorphosis. Retrogressive metamorphosis is an attribute of urochordata in which an aquatic, free swimming larva with notochord transforms into an adult sessile, non chordate. The complete loss of advanced characters present at the larval stage involves this form of metamorphosis.

Some urochordates have avoided this route by neotene to retrograde form, i.e. their larvae do not display any changes associated with metamorphosis, and paedogenesis, i.e. sexual reproduction, is subsequently accomplished through mature larval form.

The ancestors of other chordates are known to be descendants of such urochordates.
Ascidia, Salpa and Doliolum are typical examples of Urochordata. The branchiostoma is a Cephalochordate, and the hemichordate animal is Balanoglossus.

Herdmania's tadpole larvae are just 1-2 mm long when they hatch from the egg. It does not eat and thus has only 3 hours of survival during which it has to swim around in search of an appropriate attachment substratum. For its free swimming nature, the larva requires advanced features, which is therefore important for the population to be distributed to distant places that the sedentary adult has no means to do.

Herdmania is converted into a bag-like sedentary animal attached by a foot to the rock when the metamorphosis is over and has branchial and atrial openings for water inlet and outlet, respectively. With a large number of stigmata for philtre feeding, the pharynx becomes immensely swollen and the digestive system is well developed. Other advanced chordate characters of the larva, however, degenerate into basic structures, which is why it is called retrogressive metamorphosis.

The correct Answer is option (C) Herdmania(Urochordata).

Note: During metamorphosis, changes that happens are
- With the help of chin warts, Larva sticks to the ground, head downward and tail up.
- Between the chin warts (adhesive papillae) and the mouth, rapid growth occurs, with virtually no growth on the opposite side of the body.
- The body begins to move in such a way that the mouth eventually migrates to the upper side, due to rapid development on one side.
- In the process, the pharynx enlarges and the number of stigmas increases.