Retrogressive metamorphosis is a special type of metamorphosis in which a well-developed, advanced larva changes into a simpler, less-developed adult. It is seen in ascidians (sea squirts), which are urochordates, such as Herdmania. In most animals metamorphosis is progressive — the simple larva develops into a more advanced adult. But in ascidians the opposite happens: the free-swimming, tadpole-like larva, which has typical chordate features, loses many of these features and becomes a simple, fixed (sessile) adult. Because the adult is more primitive than the larva, this is called retrogressive (backward) metamorphosis.
Retrogressive metamorphosis is the change of an advanced larva into a simpler adult.
It is seen in ascidians (urochordates) like Herdmania.
The larva is free-swimming and tadpole-like with chordate features.
During metamorphosis the tail, notochord and nerve cord are lost.
The adult becomes simple, sessile (fixed) and plant-like.
It is the opposite of progressive metamorphosis (as in frogs).
It proves that ascidians are true chordates, shown by their larva.
Metamorphosis means the change from a larva into an adult. It is of two kinds: • Progressive metamorphosis: the larva is simpler and the adult is more advanced (as in frogs). • Retrogressive metamorphosis: the larva is advanced and the adult becomes simpler.
In ascidians like Herdmania, the larva is a free-swimming, tadpole-like form that has important chordate characters such as a notochord, a dorsal hollow nerve cord and a tail. During metamorphosis, the larva settles down and loses most of these advanced features, becoming a simple, sessile (fixed) adult. Since the organisation goes from advanced to simple, it is called retrogressive metamorphosis.
When the ascidian larva (the 'tadpole larva') changes into the adult, the following changes take place:
Features that are lost: • The tail is lost (absorbed). • The notochord disappears. • The dorsal nerve cord is reduced to a small nerve ganglion. • The tail muscles, sense organs (like the eye spot and balancing organ) are lost.
Features that develop or increase: • The larva attaches itself to a rock or surface and becomes sessile. • The pharynx with gill slits enlarges greatly for filter feeding. • A test (tunic) covers the body. • The atrium and other adult organs develop.
Thus the advanced, chordate-like larva becomes a simple, plant-like, fixed adult.
In short, retrogressive metamorphosis is important because the simple adult hides the animal's true chordate nature, which is revealed only by its larva.
Retrogressive metamorphosis is a type of metamorphosis in which a well-developed, advanced larva changes into a simpler, less-developed adult. It is seen in ascidians (sea squirts) such as Herdmania, where the free-swimming, tadpole-like larva loses its chordate features and becomes a simple, fixed adult. Because the change is from advanced to simple, it is called retrogressive.
Retrogressive metamorphosis is seen in ascidians (sea squirts), which are urochordates. A common example is Herdmania. Its free-swimming larva has chordate characters like a notochord, nerve cord and tail, which are lost when it changes into the simple, sessile adult.
During retrogressive metamorphosis, the larva loses its tail, notochord, dorsal nerve cord (which reduces to a ganglion), tail muscles and sense organs. At the same time, it attaches to a surface and becomes sessile, the pharynx with gill slits enlarges for filter feeding, and a protective test (tunic) covers the body. The advanced larva thus becomes a simple adult.
It is significant because the ascidian larva shows clear chordate features (notochord, nerve cord, gill slits) even though the adult does not. This proves that ascidians are true chordates and shows their relationship with higher chordates. It also supports the idea that the ancestors of vertebrates may have been free-swimming, larva-like chordates.
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