Retrogressive metamorphosis of ascidian tadpole larva | Retrogressive metamorphosis | Metamorphosis

Metamorphosis of Ascidia:
Metamorphosis of tadpole larva in Ascidia is a two-way process. As the majority of the organs of the larva are lost, the process has been termed as retrogressive. But at the same time some of the structures further develop, which may be considered as progressive.
Changes in the larva following fixation:
Retrogressive changes:
1. The length of the tail is considerably shortened and finally the tail disappears.
2. The notochord becomes coiled, disorganized and disappears with the tail.
3. The nerve cord becomes restricted to the trunk region and gradually reduced to a solid ganglion.
4. 90° shifting of the mouth occurs from the point of attachment due to rapid growth of the region between adhesive papillae and mouth and also suppression of growth of the original dorsal side.
Progressive changes:
1. The pharynx enlarges in size and the number of stigmata increases.
2. Atrium enlarges.
3. Velum appears.
4. Neural gland differentiates.
5. Reproductive organs gradually develop.

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