Subphylum Urochordata

Subphylum Urochordata (or Tunicata) includes animals such as Herdmania, Salpa and Doliolum, commonly called tunicates or ascidians. They are exclusively marine. The soft body is covered by a protective covering called the test or tunic, made of a substance called tunicine.

The notochord is present only in the tail of the free-swimming larva, which is the reason for the name Urochordata (uro = tail); it is lost when the larva undergoes metamorphosis into the sessile or drifting adult form.

The pharynx bears numerous gill slits, arranged in two longitudinal rows, through which respiration occurs. The alimentary canal is complete and either straight or U-shaped. The circulatory system is closed but simple in structure, and excretion is carried out with the help of a structure called the glomerulus. Nervous tissue is embedded within the epidermis on both the dorsal and ventral sides of the body, rather than forming a distinct internal nerve cord in the adult. …