Answer
Metamorphosis means a series of developmental changes in the form or structure of an animal occurring subsequent to birth or hatching. For example the metamorphosis of tadpole to frog.
In retrogressive metamorphosis the larva possesses advanced characters which are lost during the development and the adult is either sedentary or degenerated with primitive characters.
for example, the metamorphosis in the larva of Herdmania
The advanced Chordate characters of the larva-
· There is a rod-like notochord in the tail to which are attached muscle bands for swimming.
· There is a dorsal hollow nerve cord which is enlarged to form brain at the anterior end. A photoreceptor ocellus and a balancing organ, the statocyst are attached to the brain.
· There are only two pairs of gill slits in pharynx but the mouth is closed by a membrane and intestine is rudimentary.
· Endostyle on the ventral side of pharynx is very well developed which functions like thyroid gland and helps in metamorphosis.
· Heart is on the ventral side of gut but is non-functional.
· The larva possesses on the anterior end three ectodermal adhesive papillae which help in firm attachment on the substratum.
Changes during metamorphosis-
· Larva attaches to the substratum with the help of chin warts, head downward and tail up.
· Rapid growth takes place between the chin warts (adhesive papillae) and mouth and almost no growth on the opposite side of body.
· Due to rapid growth on one side, body starts rotating in such a way that mouth gradually migrates to the upper side.
· Meanwhile pharynx enlarges and stigmata increase in numbers.
· Intestine becomes functional and atrial opening is formed on the opposite side of oral aperture.
· Both tail and notochord are gradually absorbed in the body during metamorphosis.
· The hollow nerve cord is reduced into a solid nerve ganglion on the dorsal side.
· Sense organs, namely ocellus and statocyst are lost.
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