Subphylum Urochordata: Characteristics Classification Examples 

Characteristics Features of Subphylum Urochordata: 

• They live in marine water, almost present in all part of the world and in deep ocean as well as in shallow coastal areas. 
• They can live in solitary state, sometimes large number of Urochordate live in groups in solitary state, some of them show colonial form. 
• They are generally sedentary, fixed to the oceanic bottom to solid substratum, some of them show pelagic free swimming stage. 
• In size they show great variation, some of them are microscopic while some members reach around few centimetres. 
• Development maybe direct or indirect, in indirect development most of the time tadpole like tailed larva present, the larva have Notochord limited to tail region. 
• The larva undergoes retrogressive metamorphosis and in adult most of the advanced features absent, in adult notochord absent, brain absent only nerve ganglion present. 
• In most of the cases body is covered by tunic membrane, a sac like covering made of tunicine polymer. 
• Two tube like structure present, one is atrial siphon and another one is branchial siphon, the opening of atrial siphon is known as atrial aperture or cloacal aperture while the opening of branchial siphon is known as branchial aperture or mouth opening. 
• Body cavity is represented by atrial cavity which have ectodermal lining and open outside through atrial pore. 
• Pharynx is large contain branchial sac which have endostyle and perforated due to presence of gill slits, number of gill slits vary species to species. 
• The test is highly vascularized, the vascularized test give the animal a pinkish appearance and the vascular ampullae present on the test give red tints over the pinkish background of test. 
• Respiration occur through the gill slits present on the pharyngeal wall and through the highly vascularized test.  
• Open type of blood vascular system observed in tunicata and the tubular heart present on the ventral side of the body. 
• In their blood a special cells vanadocytes present which are able to extract vanadium from the ocean and store inside the cell. 
• Excretion occur through neural gland, nephrocytes and through pyloric gland. 
• They are hermaphrodite but self fertilization is very rare, the male and female reproductive system do not attain mature at same time so generally cross fertilization occur. 
• Generally they reproduce through sexual reproduction but in some cases they also show asexual mode of reproduction through the process of budding. 

Classification of Subphylum Urochordata: 

According to the classification scheme of Storer and Usinger Subphylum Urochordata have three classes and each class is divided into orders and suborders. 

Class 1: Ascidiacea: 

• They show solitary as well as colonial form and they generally sedentary, remain fixed to the bottom of ocean. 
• Body is covered by thick permanent tunic covering, makes the outer surface wrinkled. 
• On dorsal side atrial cavity open through atrial pore. 
• The pharynx is large and perforated due to presence of large number gill slits. 
• They are hermaphrodite, development is indirect, tadpole like free swimming larval stage present in their life cycle. 
• The tadpole larva undergoes retrogressive metamorphosis and finally form adult stage which lacks Notochord, nervecord and brain. 

Order 1: Enterogona:  

• Body have two conspicuous part upper thorax part and lower narrow abdomen part. 
• Neural gland present, it situated at ventral side of nerve ganglion. 
• Only one hermaphrodite gonad present inside the intestinal loop. 
• In their life cycle a well developed laraval stage present with ocelli and otolith as sense organs. 

Suborder 1: Phlebobranchia: 

Their pharynx has internal longitudinal vessels and some important members of suborder Phlebobranchia are Ciona, Ascidia. 

Suborder 2: Aplousobranchia: 

Pharynx have longitudinal vessels and they reproduce through the asexual mode of reproduction through the process of budding, Clavelina is an important member of this suborder. 

Order 2: Pleurogona: 

• Body is not divided into several parts, body is compact. 
• Neural gland present on dorsal or lateral side of the nerve ganglion. 
• They have two or more hermaphrodite gonads present in the mantle wall in embedded state. 
• Development include larval stage, have otolith sense organs but separate eyes absent. 

Important members of this order are Herdmania, Botryllus, Molgula. 

Class 2: Thaliacea: 

• They live in solitary as well as colonial state, adult stage show pelagic free living stage as compared to sedentary other members of Urochordata. 
• Tunic is very thin and transparent, in some cases it is so thin that the internal organs are visible from outside. 
• The mantle have circular muscle bands which encircle the body and the muscular band are visible from outside through the transparent tunic. 
• Atrial cavity open outside through the atrial pore at posterior region of the body. 
• Alternation of generation is common and in asexual stage stolon responsible for budding process. 

Order 1: Pyrosomida: 

• They live in a tubular colonial form, one end of the colony is closed, they form symbiotic association with some bacteria and famous for producing strong light, show bioluminescence. 
• Zooids are embedded in common test to form cylindrical colonial form, pharyngeal wall have tall large number of gill slits. 
• Development stage do not include any tailed larva, reproduction occur through budding. 

Pyrosoma is a single genus of this order and is famous for its strong light, they produce stronger light as compared to other marine bioluminescent organisms. 

Order 2: Doliolida: 

• They have a cylindrical body shape which is not generally observed in other groups of Urochordata. 
• The mantle have 8 circular muscle bands which observed from outside through the transparent tunic. 
• Development is indirect, have laraval stage with tail. 

Doliolum and Doliopsis are genera of order Doliolida. 

Order 3: Salpida: 

• They have special body shape, body is cylindrical or prism shaped. 
• Mantle have circular muscle bands but they are not form complete ring, they are incomplete at ventral side. 
• Pharynx is connected to the atrial cavity through large sized gill slits. 
• Development phase do not include a tailed larval stage. 

Salpa, Scyclosalpa are important genus of the order Salpida. 

Class 3: Larvacea: 

• They have free swimming pelagic larva stage with notochord in tail, brain, nervecord. 
• Atrial cavity and atrial aperture do not present. 
• They are hermaphrodite, neoteny observed, metamerism absent. 
• Test or house is temporary which after some time renew. 

Order 1: Endostylophora: 

Their temporary house or test show bilateral symmetry and have separate inhalent and exhalent aperture. Pharynx have endostyle, some important members of order Endostylophora are Oikopleura and Apendicularia. 

Order 2: Polystylophora: 

Test or temporary house show biradial symmetry and have only one aperture pharynx do not have any endostyle,  Kowakvskia is an important member of the order Polystylophora. 

Examples of Subphylum Urochordata:  

• Ascidia: They are solitary sedentary Urochordata generally found in groups attached to the rocky substratum. They show great similarity with Herdmania, body is covered by wrinkled test, attached to the solid substratum by broad base. Body is cylindrical and have tadpole like larval stage which undergoes retrogressive metamorphosis and finally become sessile adult. 
• Ciona: They are cylindrical sedentary Urochordate have cylindrical siphon and commonly known as tube sea-squirt. Body is covered by thin transparent test, the broad base attach the animals to the solid substratum. The mantle have longitudinal and circular muscle bands, dorsal tubercle is horseshoe shaped. Heart is V shaped and single hermaphrodite gonad present inside the intestinal loop. 
• Clavellina: It is a colonial Urochordate live in clusters, each individual zooids are connected to the stolon. The elongated body is covered by transparent test, the body is divided into two parts, upper thorax part and lower abdomen part. Clavellina is viviparous, development of zygote occur inside the atrial cavity and the free swimming larval goes outside through atrial pore. 
• Botryllus: It is generally found in ship bottom, live in compound state. Individual zooids arranged in radial pattern to form a star like arrangement, several such star like arrangement live together in a common gelatinous test.  

• Molgula: They are sedentary rounded tunicata live in groups to different types of solid substratum. The dorsal tubercle is C shaped and the thick test have sand particles. This tunicata show viviparity and single hermaphrodite gonad show protandry. 
• Pyrosoma: They live in cylindrical tube like colony with one end closed. This tunicata is known for its great bioluminescence, among the all bioluminescent organisms in ocean they produce stronger light.  
• Doliolum: It is a free-swimming cosmopolitan pelagic tunicata, they show alternation of generation with solitary sexual gonozooid stage and colonial asexual oozooid stage. The gonozooid are barrel shaped, the atrial and branchial aperture open at two ends of the cylindrical body. Mantle have 7-8 complete circular muscle bands, after fertilization free swimming tadpole like larval stage and form asexual oozooid stage. The oozooids almost similar to the gonozooid but gonad absent and have stolon.
• Salpa: Salpa is a marine cosmopolitan Urochordate show alternation of generation with solitary asexual oozooid stage and colonial sexual gonozooid stage. The oozooids are pism shaped, the branchial and atrial aperture open at two opposite ends. The circular muscle bands do not form complete ring, on ventral side it is incomplete. The sexual gonozooid show viviparity and the embryo nourished by placenta. 

• Oikopleura: It is a free swimming pelagic tunicata generally live in common temporary test or house. This house or test is not made of tunicine polymer as that of other members of tunicata. The animal present inside the test secret the outer egg shaped test or house.

Reference Urochordata Classification Morphology Characteristic Features

Examples of Urochordata: Salpa, Doliolum

Detailed Information on

Characteristics Features of Subphylum Urochordata 

Classification of Subphylum Urochordata

Examples of Subphylum Urochordata: Clavellina, Salpa, and Doliolum

Examples of Subphylum Urochordata