What are Urochordates ?

Introduction:

The Urochordata are also called tunicates, Tunicata, and they are also commonly called sea squirts.  They are called tunicates because having tunics in their surroundings and got the name sea squirts due to the way of pushing the water out from their bodies by exhalant siphon. The tunic is a material that is very similar to cellulose and it protects the body of an animal from predators. They belong to the class Chordata but the adult ones don't have a backbone. The body of an adult tunicate is very simple, which is like a sac having two openings (siphons), one for the entrance of water and the other for exit. When water is entered into this sac-like body it is filtered and they extract their food out of it.  The larva of the tunicates is free swimming having all the derived characteristics of chordates i.e.a dorsal hollow nerve cord, notochord, a post-anal tail, and pharyngeal slits.

 In many tunicates, the larva at the tadpole stage will swim for some time and it finally attaches to a hard substratum and it loses its tail and capability of locomotion. Its nervous system also disintegrates. They are the common marine invertebrates having more than  3,000 species. Some of the tunicates are entirely deep-sea and known as salps they generally have barrel-shaped bodies and are mostly and extremely abundant in the open ocean.

Scientific Classification:

Kingdom:     Animalia

Phylum:       Chordata

Clade:          Olfactores

Subphylum:  Tunicata

Feeding Behaviour:

The pharynx of tunicates is shrouded by tiny and small hairs that are formed from ciliate cells, these appendages permit the consumed food to pass down through to the esophagus. The digestive system of these animals is “U-shaped”, the anus is a simple hollow tube that extends into the outer environment.

These are filter-feeding animals,  probably feeding by taking hundreds of liters of water each day through the mouth. Then this water is allowed to pass through the pharynx where tiny food particles are filtered and finally the water is discharged through the exhalant siphon. Due to the constant beating of the cilia water current is allowed to pass through the mouth.

Water is also pushed out of the atrial cavity or coelom by the muscular contractions of the tunic if the tunicate is in some threat.  Mucous is released by some particular cells and is transferred across the pharynx by beating of cilia and due to this motion, it is transferred into the digestive system. where finally food is digested along with this mucous. The small particles of phytoplankton, zooplankton, etc, are entangled in a continuous moving layer of mucous.

Evolution:

Tunicates are soft-bodied animals that's why they have very little fossil record except for the hard mineral particles that are called “spicules”, which are found in the tunics of some species of these animals. A single ancestry lineage of the class Ascidiacea, the ascidiaceans, or possibly a lineage of ascidian-like tunicates that diverged prior to the same ancestor of the Ascidiaceans, presumably gave rise to the further two classes. The embryonic “thaliaceans” show signs of having emerged from attached colonies. The “pyrosomes”, which are similar to the colonies of some ascidians, class “ascidea” which are evidently branched off first in the class “Thaliacea” and may not even be associated with the “dolioloids” and salps.

 The “Appendicularians” is likely to evolve from an additional specific tunicate that attains sexual maturity before the metamorphosis stage. This development causes the loss of the adult stage of tunicates which is called paedomorphosis, having some juvenile characteristics in the adult stage. Within the Ascidiaceans, the common ancestor is commonly thought to have been a solitary organism that did not reproduce by budding. 

The base of this theory is that many members of class ascidea, the ascidians do not produce bud and the different types of budding that occurs in them represent separate groups suggesting independent origins. Evolution in this group has concerned numerous elaborations of complex colonies, making themselves become smaller and simpler organisms in structure. There is a distinctive trend toward their parental care, particularly when they live in the form of colonies.