AFFINITIES OF UROCHORDATES

Published in Zoology
Friday, 07 July 2017 17:08
The typical tunicate looks like a non-chordate animal. If the life history of such animal is studied, the larval form reveals the chordate characters of that animal.

In 1816, Lamarck and Cuvier placed these animals in one class ‘Tunicata’. Allis described a compound Ascidian “Botryllus’ in 1756. KowalevskSi. Worked on the development of the Ascidian and placd them in tunicates after observing the chordate features.
 
Urochordates Resemblance With Chordate:
 
  1. Presence of dorsal tubular nerve cord.
  2. Presence of Notochord.
  3. Well developed pharynx with gill-slits.
  4. Presence of endostyle on the ventral side of the pharynx.
  5. Presence of atrium around pharynx.
  6. Presence of post - anal tail, with tail fin.
 
Because of these chordate features tunicates are included in chordates group.

Urochordates Resemblance With Amphioxus:
 
  1. Presence of notochord.
  2. Presence of dorsal tubular nerve cord.
  3. Presence of large pharynx with gill slits.
  4. Presence of atrium and atriopore.
  5. Presence of muscle band.

Thus, the Urochordates show close relation with Cephalochordates.
 
Urochordates Differ With Other Chordates:
 
  1. Presence of retrogressive metamorphosis.
  2. Absence of segmentation.

Because of these characters zoologists included these animals in a separate sub-phylum Urochordates.

During recent years many zoologists regarded the tunicates as primitive and ancestral forms to chordates as a whole.
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