Urochordates:

  • They are marines known as sea squirts
  • They are solitary or colonial
  • Fixed or free-swimming
  • Numerous gill slits are present
  • The body is covered by a cuticle called a tunic.
  • only in Larval Stages notochord is present.
  • Dorsal tubular nerve cords are present in the larval form while degenerate in the form of the small ganglion in adults.

Maintainance functions:

Body wall:

FIgure: Body Wall of Urochordates
Tunic:

  • The tunic is composed of proteins, various salts, and cellulose, mesodermally derived tissues like blood vessels
  • Tunics have a root-like structure called stolons, stolons help to anchor tunicate to the substrate. Individuals are connected with the colony by Tunic.
  • The muscles help in changing the shape of adults during metamorphosis.
  • Muscles act against the electricity of the tunic and the hydrostatic skeleton that seawater confined to the internal chamber creates.


Nervous System:

  • The nervous system of the tunicate is located on the wall of the pharynx
  • It forms a nerve plexus
  • It has a single ganglion located on the wall of the pharynx between the oral and atrial openings
  • For coordinating bodily functions this ganglion is not vital. 
  • Tonics are sensitive to a variety of mechanical and chemical stimuli. The receptor for these senses are distributed over the body wall, especially around the siphon
  • The internal structure of the urochordates has a very large pharynx and a cavity called the atrium
  • The atrium surrounds the pharynx dorsally and laterally.
  • The pharynx of tunicates starts at the oral siphon
  •  It is constant with the rest of the digestive tract.
  •  There are tents in the oral cavity of the neck that prevents large objects from entering the neck.
  • Numerous pharyngeal slits called stigmas are present, the cilia associated with stigma caused water to circulate in the pharynx through the stigma and into the surrounding artia and water leaves the tunicates through atrial siphons

Digestive system:

  • The digestive system starts from the pharynx and ends with the anus near the atrium endostyle  is a ventral ciliated that forms a mucus sheet
  • Cilia move the mucus sheet dorsally along the pharynx
  • Food particles are brought into the oral siphon within current water and trapped in mucus sheet and passed dorsally
  • Food is incorporated into a string of mucus that’s why ciliary action moves into the next region of the gut
  • Digestive enzymes are secreted in the stomach and most absorption occurs around the wall of the intestine
  • The discharge of water from the anus carries digestive wastes out of the atrial siphon.

Respiration:

  • The tunic is located at the base of the heart and neck.
  • The pharynx also functions in gas exchange, gaseous exchange as water circulates through tunicate

Circulatory System:

  • The tunic is located at the base of the heart and neck.
  • One vessel of the heart runs anteriorly under the endostyle and the other runs posteriorly to the digestive organs and gonads
  • The blood flow to the heart is not unidirectionally,
  • Peristaltic contractions of the heart may propel blood in one direction for a few beats and then the direction is reversed
  • Tunicate blood plasma is colorless it contains various kinds of ameobide cells

Excretion:

  • Ammonia is released into the water which passes through the neck and is excreted.
  • In diffusion ameobide cells of the circulatory system accumulates uric acid in the intestinal loop.
  • The pyloric glands outside the intestine also act as excretions.

Reproduction and development:

  • Tunicates are monoecious having both male and female sex organs or gametes present in a single individual
  • Gonads are located near the loop of intestine
  • External fertilization occurs
  • Gonads open out near the atrium siphon
  • Mostly self-fertilization occurs but sometimes also cross-fertilization occurs
  • After the fertilization development starts and tadpole-like larvae are formed having the 5 basic or unique characteristics of chordates
  • Metamorphosis occurs in larvae after a brief free-swimming larval period
  • The larvae do not feed anything during this period
  • After this, the larvae are attached to any substrate with the help of adhesive papillae
  • The outer epidermis shrinks during metamorphosis and notochord are removed. They are organized into adults tissues. The internal structure rotates at 180 degrees. This brings the oral siphon against the adhesive papilla. It also turns the digestive system into a U shape.

Sub-Phylum Cephalo chordates

(Kephalo, head + chords, cord)

  • They have all the 5 basic characters e.g Notochords, post-anal tail Pharyngeal slits, tubular nerve cords, and endostyle.

Example: Lancelets

  • Cephalo-chordates generally consist of 2-genera, one is branchiostoma and the other is Asymntetron.
  • Both have about 45 species
  • They are distributed throughout the world's oceans, in shallow clear water.
  • They are small tadpole-like animals about 5 cm in length
  • They are elongated, laterally flatted, and nearly transparent
  • They have a streamlined shape, but they are weak swimmers
  • Most of their time is spent in the filter-feeding position.
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