General Characteristics Of Urochordata .

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🐚 Urochordata Overview

Urochordates, commonly called "sea squirts," undergo a dramatic transformation in their life cycle. Their chordate traits are more prominent during the larval stage, while adults resemble invertebrates more than chordates.

🐣 Larval Characters

  • The elongated larva is called an ascidian tadpole.
  • Notochord is confined to the tail region—hence the name Urochordata.
  • A dorsal hollow nerve cord extends the length of the body.
  • Pharyngeal gill slits are present.
  • Larvae are highly active with efficient swimming tails.

🧍 Adult Characters

  • Body is enclosed in a tunic made of tunicin (protein) and a cellulose-like polysaccharide.
  • Adults are sessile, attached to marine substrates.
  • Incurrent (branchial) siphon and excurrent (atrial) siphon direct water flow.
  • Pharynx forms a branchial basket.
  • Sensory tentacles around the incurrent siphon filter incoming water.
  • Hermaphroditic; reproduce both sexually and asexually.

🧬 Classification of Urochordata

📘 Class Ascidiacea

  • Brightly colored marine forms.
  • Can be solitary or colonial.
  • Adults are sessile; larvae are planktonic and non-feeding.
  • Adult body is sac-like, covered in a tunic.
  • Most larval chordate traits disappear after metamorphosis.
  • Nervous system in adults reduces to a nerve ganglion.
  • Examples: Ascidia, Ciona, Herdmania

🌊 Class Thaliacea

  • Free-living pelagic urochordates.
  • Transparent, thin tunic.
  • Have muscle bands around the body wall.
  • Incurrent and excurrent siphons located at opposite ends.
  • Possess a few pharyngeal gill slits.
  • Show polymorphism and alternation of generations.
  • Examples: Salpa, Doliolum

🧪 Class Larvacea (Appendicularia)

  • Tiny planktonic marine forms found globally.
  • Adults resemble ascidian tadpoles (retain larval traits).
  • Body divided into trunk (contains organs) and tail (flat, used for locomotion).
  • Movement via muscle bands acting on notochord.
  • Possess a tubular nerve cord.
  • Mostly monoecious, often protandrous.
  • Examples: Oikopleura, Appendicularia.


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