What is retrogressive metamorphosis? Explain.

Hint: Some of the urochordates like herdmania possess eyespot, statocyst, a long tail and a well developed noto-chord in the larval stage. But as they grow into the adult form the eyespot and statocyst disappear and the tail and notochord get reduced.

Complete answer:
It is a biological process by which an animal physically develops after birth through cell growth and differentiation. The transformation of a maggot into an adult fly and of a tadpole into an adult frog are examples of metamorphosis.
'Retrogressive metamorphosis' means degenerative changes wherein an active larva transforms into a sedentary adult. For example, in Urochordata, the larva bears all advanced characters of Chordata but after metamorphosis, the adult loses its chordate characters. Thus, here the larva exhibits advanced characters and during metamorphosis, retrogression of characters occurs, such type of metamorphosis is called retrogressive metamorphosis. Let us consider an example of Herdmania, a Urochordate.
Changes during Metamorphosis in Herdmania (Urochordata) are:
-The notochord, nerve cord muscles, and tail will be reduced. All these structures help the larva to swim freely in the water. But they are not useful to adults.
-The alimentary canal becomes complicated. The pharynx enlarges in size. The number of gill slits will increase by divisions.
-The stomach, gonads, and intestine will grow.
-The nervous system is reduced.
-The atrial cavity enlarges into a sac-like structure.
-The eyespot and statocyst will completely disappear.

Note: Progressive metamorphosis is when an organism increases in complexity and develops more advanced characteristics over the course of time. Retrogressive metamorphosis is the opposite in which the larval stage has complex organs but disappears or reduced in the adult stage.