Urochordata

Although the traditional classification is generally followed, evidence suggests that Ascidiacea is an artificial group of paraphyletic status. The new classification should be as follows:

Class – Stolidobranchia

Class – Aplousobranchia

Class – Phlebobranchia

Class – Thaliacea

Class – Appendicularia (Larvacea)

Class – Sorberacea (may belong in Ascidiacea or as a taxon of its own).

The tunicates are marine, filter-feeding animals such as sea squirts (class Ascidiacea), which show affinities with other chordates. Adult sea squirts are sessile, globular animals, with prominent incurrent and excurrent siphons and many of them grow in colonies.

The body of adult is occupied by a very large pharynx with numerous gill slits or stigmata that act as a sieve for filtering food. Water current entering through the incurrent siphon enters the pharynx and passes out through the gill slits, leaving food particles trapped in the mucus coated pharynx.

A groove in the pharynx called the endostyle secretes mucus that traps the particles and conveys them through the dorsal lamina into the digestive tract. The movement of mucus is caused by the action of cilia. Water leaves the atrium, which is a sac surrounding the pharynx, by way of the excurrent siphon. Thus the gill slits in tunicates serve as filter feeding apparatus and do not have respiratory function as the primary job.

The sea squirt larva is a free-swimming animal resembling a tadpole. The head contains a brain hollow nerve cord and sense organs, a small pharynx and digestive tract, and a ventral heart. Incurrent and excurrent openings are located on the top of the head.

The tail is a muscular appendage with a notochord that functions as a swimming organ. When the larva is ready to undergo metamorphosis it attaches to an object head downward. The tail, notochord, and nerve cord degenerate, the pharynx enlarges, and the other organs shift in position. The incurrent and excurrent openings develop siphons. This is called retrogressive metamorphosis.

HERDMANIA

Species of Herdmania

H. armata · H. claviformis · H. colona · H. coutieri · H. curvata · H. fimbriae · H. grandis · H. insolita · H. japonica · H. kiiensis · H. litoralis · H. mentula · H. momus (Red Throated Ascidian) · H. pallida · H. polyducta · H. subpallida.

Tunicates are suspension feeders that collect planktons from water. They have two openings in their body cavity: a branchial or oral siphon and the other atrial siphon. The branchial siphon is used to draw food and water into pharynx, while the ex-current or atrial siphon expels wastes and water.

Planktons get trapped in mucus secreted from the endostyle of pharynx, which is covered by ciliated cells, which force the trapped planktons to pass down through the dorsal lamina to the oesophagus. Their gut is short U-shaped and the anus opens to the atrial siphon that leads to outside environment.

Tunicate blood is particularly interesting. It contains high concentrations of the metal vanadium and vanadium-associated proteins as well as higher than usual levels of lithium. Some tunicates can concentrate vanadium of up to one million times that of the seawater. Specialized cells can absorb heavy metals, which are then deposited in the tunic.

Most tunicates are hermaphrodite. There is a single male and female gonad in the loop of intestine. The eggs are kept inside their body until they hatch, while sperms are released in water from where they enter the other individuals through the incurrent water.

Tadpole like larval form appears like a primitive chordate, with a notochord, pharyngeal gill slits, dorsal hollow nervous system and a postanal tail. The larval stage ends when it finds a suitable rock to affix to. The larval form is not capable of feeding and is a mechanism for dispersal.

Many physical changes occur in the larval body which is called retrogressive metamorphosis, in which the hollow nerve cord is replaced by a solid ganglion. Sense organs, notochord, tail and muscles of tail also disappear. However, pharynx enlarges and digestive system becomes functional. Once grown, adult develops a thick covering called a tunic, to protect their barrel-shaped bodies.