Subphylum: Urochordata

• Body is unsegmented and usually lacks tail, covered by a test or tunic composed largely of tunicin, allied to cellulose. Appendages absent. (Tunicin is called animal cellulose)
• The notochord is only present in the tail of the larva and disappears in the adult.
• Coelom absent; digestive tract complete
• Respiratory system has two to numerous gill-slits in the pharyngeal wall. Respiratory pigment rich in both Sulphuric acid and Vanadium.
• Circulatory system open; simple tubular ventral heart. It Reverses the direction of its heart beats periodically.
• Nervous system is represented in the adult by a single dorsal ganglion.
• Larva has a hollow nerve cord.
• Excretion is carried on by nephrocytes, pyloric gland or neural gland.

Fig. Herdmania : External features in left view

• Sexes united, always cross and external fertilization; asexual reproduction by budding; some forms exhibit alternation of generation.
• Larva undergoes retrogressive metamorphosis.
• The larva is known as Ascidian tadpole e.g., Herdmania (Sea squirt), Salpa, Doliolum. Pyrosoma (Bioluminiscent) is free swimming while adult is fixed.