• Phylum –  CHORDATA (Dorsal tubular nerve cord, gill-slits and notochord present)
  • GroupACRANIATA (No head, cranium or brain)
  • SubphylumUROCHORDATA (Marine. Body covered by a thick test. Notochord present only in larval tail)
  • Class – LARVACEA (Test with circular muscle bands. Free-living, pelagic)
  • Genus – Oikopleura


Oikopleura is widely distributed in open sea.


Oikopleura lives in upper levels of the sea and swims by contractions of the bent tail. Some are brilliantly pigmented (coloured) with orange or violet and when abundant, may colour the water. It is a neotenous and planktonic tunicate.


  • Oikopleura is minute and microscopic measuring about 2 mm.
  • Animal lives in a ‘house or test’ made by secretion from a special part of the skin, called as oikoplastic epithelium. Body wall contains test and ectoderm.
  • It has larva-like appearance comprising body, tail and persistent notochord.
  • Body narrows towards mouth and expands posteriorly dorsoventrally. The caudal appendages have undergone torsion of 90°.
  • Tail is a broad structure, which rests at an angle to the rest of the body. It has notochord, nerve cord, broad tail fin and 7 to 12 striped muscle cells.
  • Movement of the tail produces water current. Food particles are filtered by an elaborate arrangement in the house. Water enters into house by a pair of posterior filtering windows and is passed through filter pipes in front of mouth. Minute flagellates are sucked by mouth.
  • Dorsal lamina and tentacles are absent. Otocyst present.
  • Mouth leads into alimentation comprising of pharynx, oesophagus, stomach and intestine. Pharynx has gill-slits opening to outside. The food consists of microorganisms. Sense organs, oral gland, otocyst, tail ganglous, atrial opening muscle bonds epipharyngeal ridge, otocyst, epipharyngeal ridge.
  • Protandrously hermaphroditic, kidney-shaped two testis. Left testis encloses, ovary.
  • Internal space filled by haemocoelomic lacunae having blood circulation by a transport heart.


The general structural organization resembles with a typical ascidian tadpole and it can be very safely concluded that these forms have arisen from tunicates by the acceleration of the fate of development of alimentation and reproductive systems, so that the metamorphosis and the adult stage are omitted and it shows phenomenon of neoteny or paedomorphosis. The anatomy of the animal suggests that larvaceans are not like the ancestral chordates but are neotenic descendants of the sessile ancestors resembling Urochordata.


The animal contains peculiar body with persistent tail having notochord, etc. and above features, hence it is Oikopleura.




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