Urochordata includes about 2000 fixed and 100 pelagic species. It has been divided into the following three classes:
Class 1. Larvacea (Appendicularia):
1. Small (5 mm long solitary) free-swimming pelagic forms.
2. Neotenic forms which retain the larval form throughout adult life with persistent tail, nerve cord, notochord.
3. Test is temporary and renewed periodically.
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4. Single pair of gill-slits is present opening directly to outside.
5. Anus opens ventrally on the surface of the body.
6. Sexes are united, i.e., hermaphrodite.
7. No metamorphosis.
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Order 1. Endostylophora:
1. House bilaterally symmetrical, with separate inhalant and exhalant apertures.
2. Pharynx with endostyle.
Examples: Oikopleura, Appendicularia.
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Order 2. Polystylophora:
1. House bilaterally symmetrical with single aperture.
2. Pharynx with endostyle and four longitudinal ciliated rows.
Example- Kowalevskia.
Class 2. Ascidiacea:
1. Fixed or free-swimming marine forms.
2. Simple or compound, solitary or colonial.
3. Adults are never provided with a locomotory appendage or tail and have no trace of notochord and nerve cord.
4. Test is permanent, well developed and thick.
5. Branchial sac is large and well developed with its walls perforated by numerous gill- slits. Atrium opens dorsally by atriopore.
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6. Sexes united. Larva free-swimming and highly developed and undergoes retrogressive metamorphosis.
7. Colonial forms reproduce by gemmation.
Order 1. Enterogona:
1. Body sometimes divided into thorax and abdomen.
2. Neural gland usually ventral to nerve ganglion.
3. Single gonad lying in or behind intestinal loop.
4. Larva with ocelli and otolith.
Suborder 1. Phlebobranchia:
1. Pharynx with accessory longitudinal vessels.
2. Budding or gemmation is rare.
Examples- Ascidia, Ciona, Phallusia.
Suborder 2. Aplousobranchia:
1. Body elongated with distinct abdominal region.
2. Pharynx with bars, longitudinal vessels and transverse ridges.
3. Budding is common.
Example- Clavelina.
Order 2. Pleurogona:
1. Body compact and not divided.
2. Neural gland dorsal or lateral to nerve ganglion.
3. Gonads two or more embedded in mantle wall.
4. Larva with otolith. Separate eye absent.
Examples- Herdmania, Botryllus, Molgula, Styela.
Class 3. Thaliacea:
1. Free-swimming pelagic forms.
2. Solitary or colonial.
3. Test is permanent thin and transparent, with circular muscle bands.
4. Atriopore posterior.
5. Branchial sac has either two large or many small gill-slits.
6. Sexes united.
7. Tail, nerve cord and notochord absent in adults.
8. Life history exhibits an alternation of generations.
Order 1. Pyrosomida:
1. Free-swimming, pelagic colonial form. Colony closed at one end.
2. Zooids of the colony are embedded in a common test. Branchial apertures of the zooids on the outer side and atrial apertures on the inner side.
3. Muscle bands confined to body ends.
4. Phosphorescent.
5. Free-swimming larval stage absent.
Example- Pyrosoma.
Order 2. Cyclomyaria (= Doliolida):
1. Free-swimming pelagic forms.
2. Body is cask-shaped with branchial and atrial apertures at opposite ends.
3. Test is moderately well developed, never much thickened.
4. Muscle-bands are always complete surrounding the body.
5. Stigmata small, few to many.
6. Hermaphrodite. Life history exhibits an alternation of generation.
7. Sexual generation is always polymorphic.
8. Larva possesses notochord and tail.
Examples – Doliolum, Dolchinia, Doliopsis.
Order 3. Hemimyaria or Desmomyaria (= Salpida):
1. Free-swimming pelagic forms.
2. Body is more or less fusiform with the branchial and atrial apertures nearly terminal and opposite.
3. Test is well developed and transparent.
4. Muscle-bands are always incomplete ventrally.
5. Pharynx communicate freely with the atrial cavity through a large gill-slit, stolon is long.
6. Life history exhibits an alternation of generations. Development occurs inside the pouch of the body and the embryo is nourished by placenta.
7. Tailed larva absent.
Examples – Salpa, Scyclosalpa.