Biology Sikshya

   

   

    Gr.,oura=tail, L.,chorda=cord or string

• They are exclusively marine and cosmopolitan.

• They may be solitary or colonial. Some are pelagic.

• Triploblastic animals i.e., body is derived from 3 embryonic germ layer (ectoderm, mesoderm, and endoderm).

• Body is bilaterally symmetrical,

• Body is covered by a tunic or test which is mainly composed of Tunicine (similar to cellulose).Hence the name Tunicata is given to it.

• Body segmentation is absent.

• Organ-system grade of body oraganisation is seen.

• Coelom is usually absent. Instead of it an ectoderm lined atrial cavity is present which opens to outside by an atrial aperature.

• Adults are sessile, but larvae are free-swimming.

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• Notochord is present at the tail region.Tail is only present in larvae and not in adult. That means, tail is absent in adult.

• Large pharynx with endostyle and two to many gill-slits are present.

• A dorsal atrial aperature and a terminal branchial aperatures are usually present.

• They are the filter feeders.

• Respiration takes place through test or gills.

• A complete alimentary canal is found.

• Excretion occurs by neural glands, pyloric glands and nephrocytes.

• Blood vascular system is open type. A ventral tubular heart is found. Vanadocytes are present which extract vanadium from sea water.

• In larvae a dorsal, hollow nerve cord is found, but in adult get reduced into a single nerve ganglion.

• Reproduction is mostly sexual, but some reproduces asexually (budding) too.

• Development is indirect, Retrogressive metamorphosis occurs.

• Fertilization is external but cross fertilization is the rule.

• Most of the Tunicates are monoecious.