tadpole larva | development of herdmania

Larva development of herdmania

At the time of development the gastula farm and the archentron open by blastopore later the blastopore closes and develope rudiment of tail. The embryo elongates and farmed tailed larva. The Notochord cells separated from roof of archentron, archentron produce fate of mesoderm as solid bonds about 8 hours after fertilization. The chirion burst and fully farmed larva hatches out to become free swimming.

Tadpole larva

The ascidian larva is called tadpole larva because it appear as small tadpole. It has transparent small and about 1-2 mm long. The body is covered by tunic or test which is secreated by ectoderm the body is divided into head and tail.

Head or trunk 

The trunk is about 3mm long and cylindrical. It's anterior has 3 adhesive papillae in which two dorso lateral and ventromedian. It help for attachment during Metamorphosis. On dorsal side of trunk there is hollow sensory vesicle or brain. It's wall is made of single layer of cells. It has larval sensory organs in which two ocelli and single otocyst which help to man the equilibrium, in front of sensory vesicle there is moss of nerve cells and behind it there is Visceral ganglian alimentary canal is rudimentary which consists of mouth large pharynx, short oesophagus, a swollen stomach and small intestine and rectum the pharynx has well developed endostyle. The perichordium present below the pharynx.

Tail

The larva has powerful transparent tail. It is about 9mm long. It's terminal end is pointed and it has tail fin. The Notochord form the supporting axial single rows of vasculated cells above the Notochord is present hollow nervechord. The test also contains muscle bend.

Retrogressive Metamorphosis

In ascidian larva the Metamorphosis is unique. The larva has advance character which is active and free swimming but after Metamorphosis some characters degenerates. So the Metamorphosis is reffered as retrogressive Metamorphosis.

Retrogressive Metamorphosis of tadpole in involved two types of changes.

Retrogressive changes 

1. Long tail of larva become short and finally disappear.
2. Causal muscle, Nervechord, Notochord disappear by phagocytes.
3. Adhesive papillae disappear completely.
4. Larval sense organs as ocelli and otocyst disappear.
5. Sensory vesicle breakdown and disappear.
6. Anteriror region between point of attachment and mouth shows rapid growth.
7. The mouth shift at 90°c and the branchial and atrial aperture present side of larva.

Progressive changes

1. The trunk become pear shapped and survey for respiration.
2. Adult nural gland and nerve ganglion farmed by nural tube.
3. The body is covered by test and the feeding procees by cilliary water current.
4. The pharynx become blarger and develop blood vessels.
5. The stomach also become enlarge, intestine elongates and liver developes.
6. Circulatory system of Herdmania with heart and pericordium develope and gonads appears from larval mesodermal cells.
7. The tunic or test sperms and covers the body of the animals become thick and vascular.