In this article we will discuss about the classification of subphylum urochordata.
1. Mostly sessile, marine, non-metameric, largest group of protochordates with a cellulosic body covering, the tunic.
2. The pharynx is greatly enlarged, saclike, perforated with numerous lengthwise slits, stigmata.
3. A deep groove, the endo-style, below the pharynx extends down the length of the pharyngeal chamber.
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4. A dorsal nervous system, notochord, gill slits and brain-eye develop in the free-swimming tadpole larval stage.
5. The larval structures degenerate during retrogressive metamorphosis and are not found in adult.
The urochordata have been grouped in three classes:
(A) Class Ascidiacea:
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1. Sessile, marine ascidians in-habitat from rocky shore to ocean depths, but most species live in shallow coastal waters. Distribution polar to the equator seas.
2. The pharynx is large with persistent gill-slits.
3. The compound ascidians have a common external aperture, the cloaca, into which their ex-current siphons discharge.
4. The compound ascidians reproduce asexually by gemmation and also produce eggs and sperms for sexual reproduction.
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5. Development in most species proceeds up to the larval stage within the atrium, then the young emerge.
6. Most ascidians are attached (Botrylus, Ciona), but some are free-living (Polycarpa) on sand or mud.
Order Enterogona:
1. The body may be divided into two regions.
2. Single gonad located in or behind the intestinal loop.
3. The neural gland usually situated ventral to the nerve ganglion.
4. The larva bears two sense organs.
Examples: Ascidia, Ciona, Phallusia, Clavelina, etc.
(B) Class Thaliacea:
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1. Free-swimming, transparent, tunicates, sometimes simple, sometimes colonial, without caudal appendage in the adult.
2. The test is a permanent structure.
3. The muscular fibres of the body wall are arranged in complete or interrupted ring-like bands or held diffusely.
4. The mouth and atriopore are situated at opposite ends of the body.
5. The pharynx has either two large—or many small—stigmata leading into atrial cavity which communicates with the exterior by the atrial aperture.
6. There is usually an alternation of generations.
7. Tailed larval stage may or may not be present.
Order 1. Doliolida (Cyclomyaria):
1. Hermaphrodite, generally found in phytoplanktonic zone in tropical and subtropical seas, usually up to a depth of 200 metres.
2. The solitary sexual individual or gonozooid is barrel-shaped, with transparent and delicate test, and eight completely encircling muscle bands (hence the name Cyclomyaria).
3. The oral and atrial apertures are at opposite extremities.
4. The oral aperture leads into a wide pharynx occupying at least the anterior half of the body.
5. Only the posterior wall of the pharynx is usually perforated by stigmata.
6. An endostyle and peripharyngeal band occurs but there is no dorsal lamina.
7. The heart lies immediately posterior to the endostyle in the mid-ventral line.
8. The nervous system composed of dorsal ganglion, lying in the third or fourth inter-muscular zone.
9. The larva has a tail with notochord, in the asexual stage or ‘nurse’. Gradually the tail aborts and two processes, one posterodorsal known as cadophore and the ventral one stolon, grow out from the body of the larva. Zooids develop from cadophore.
Examples: Doliolum, Doliolopsis, Dolioletta, Dolioloides, etc.
Order 2. Pyrosomida:
1. Luminescent, colonial tunicates of warmer seas found up to a depth of 500 metres. The colony consisting of numerous individuals forming a gelatinous tube.
2. The zooids of a colony are seemingly independent of one another.
3. Body cavities two, the pharynx and atrium.
4. The gut and gonads. He near the midline; the heart and stolon are situated directly behind the endostyle.
5. Musculature weak.
6. The pharynx is divided. The prebranchial buccal cavity lies anterior to peripharyngeal bands. The branchial sac has the endostyle extending throughout its length.
7. Development is direct, without tailed larval stage, and takes place within the body of parent.
Examples: Pyrosotna, Pyrostremma, etc.
Order 3. Salpida (Desmomyaria):
1. Planktonic, prism-shaped or cylindrical, transparent tunicates, 8-190 mm in size, occurring abundantly in almost every sea, down to 200 metres.
2. Forward propulsion is by contraction of muscle bands ejecting water posteriorly.
3. The pharynx is reduced to its median, dorsal and ventral walls, the lateral wall being absent.
4. Occur in two forms—oozooid and blastozooid:
a. Oozooid:
The oozooid or solitary form develops from egg. It is bilaterally symmetrical and has a budding stolon but without gonad. Both branchial and atrial apertures are wide and situated at the opposite ends of the body; test is relatively thin and delicate.
Gill slits are represented only by a pair of large apertures on either side of a single median gill bar. In the dorsoanterior region occurs a horseshoe-shaped eye. A single large tentacle, the so-called languet, extends dorsally into the buccal cavity in front of the peripharyngeal bands. Reproduction asexually.
b. blastozooid:
The blastozooid or aggregate symmetrical form develops from a bud; gonad—single ovary and single testis; no stolon. Blastozooids remain connected in groups initially, but become separate later in life. Development is direct and takes place within the body of parent. A placenta is formed.
Examples: Salpa, Cyclosalpa, Octacnemus, etc.
(C) Class Larvacea (Appendicularia):
1. Transparent, free-swimming, oceanic tunicates with a globular body and permanent tail supported by a skeletal notochord.
2. Most are very small in size, with a trunk, rarely more than 1 mm long and a tail generally not more than three times the length of the trunk.
3. The test is a relatively large envelope, formed from the secretion of ectoderm and frequently thrown off and renewed.
4. The pharynx has only two stigmata leading directly to exterior.
5. Peribranchial or atrial cavity absent.
6. Development without metamorphosis and budding never occurs.
Order Copelata:
1. Pelagic small tunicates with persistent notochord and only two gill apertures.
2. The gill apertures and anus open directly to the exterior.
3. The mouth is at the front end and the anus on the ventral surface.
4. Lacks dermis, and muscles are usually absent in the region of the body.
5. Hermaphrodite, single or double ovary between two testes, the male organ maturing first (protandrous).
Examples: Oikopleura, Fritillaria, Stegosoma, Megalocercus, Koivalevskia, etc..