Retrogressive Metamorphosis

Dr.C.V. Narasimha murthy

V.S.U.P.G.C. KAVALI

Nellore District

Retrogressive Metamorphosis

• During metamorphosis, the larva will loose all the chordate characters and attains an invertebrate like form.
• This type of metamorphosis, where highly advanced larval form ends in a lowly organised adult is called retrogressive metamorphosis.

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�Organisation of Herdmania Tadpole larva�

• Herdmania is a hermaphrodite animal.
• Zygote undergoes holoblastic unequal clevage, and develops into blastula. It shows upper micromeres and lower macromeres. By invagination of the macromeres, gastrulation takes place and gastrula is formed.
• This gastrula develops into a tailed larva called Ascidian Tadpole larva.

TADPOLE OF HERDMANIA

Characteristics of �Ascidian Tadpole

• The larva is 3 mm in length. It has short oval body and a long tail.
• This larva shows all the chordate features.

1) The body is covered by thin test.

2) Tail is long and shows a tail fin or caudal fin.

3) Tail is supported by notochord. Hence it comes under urochordata.

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CHARACTERISTICS OF �ASCIDIAN TADPOLE

• On the dorsal side above the notochord hollow nerve cord is present. This nerve cord is enlarged at the anterior end as a cerebral vesicle. In the cerebral vesicle pigmented eye spot is present. Statocyst is also present. They work as sense organs.

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CHARACTERISTICS OF �ASCIDIAN TADPOLE

• On either side of the notochord in the tail region muscles are present which help in locomotion.
• Atrium opens out through atriopore.

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CHARACTERISTICS OF �ASCIDIAN TADPOLE

• On the trunk region digestive system is present. It shows large pharynx with few gills slits. They open into atrium. On the mid ventral floor of the pharynx an endostyle is present.
• Heart is ventral to

the pharynx.

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Characteristics of �Ascidian Tadpole

• On the anterior end of the trunk three adhesive papillae are present. These are very much useful to attach the larva to the substratum
• This Herdmanian tadpole larva shows all chordate characters. This larva undergoes ‘retrogressive’ metamorphosis and develops into adult Herdmania.

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Retrogressive Metamorphosis

• Fixation of the larva:

- The larva swims for some time without feeding.

- It is fixed to a sub- stratum with the help of the adhesive papillae.

- It stands erect with the tail upwards. Then it undergoes retrogressive metamorphosis.

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Changes during metamorphosis

• Notochord, nerve cord, muscles and tail will be reduced.
• The alimentary canal becomes complicated. The pharynx enlarges. The number of gill slits will increase by divisions. The stomach and intestine will grow.

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Changes during metamorphosis

• The nervous system is reduced and the anterior part of nervous system is developed into a small neural ganglion. Neural gland is present attached to it.
• The atrial cavity enlarges into a sac like structure.

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Changes during metamorphosis

• The eyespot and statocyst will completely disappear.
• Gonads develop from mesenchyme

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Changes during metamorphosis

• When these changes are taking place, the region between the adhesive papillae and mouth grows very rapidly.
• At the same time the growth of the dorsal region is stopped. Because of this, the body rotates through 180 angle and mouth is brought to the top.

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