— Tunicates (Herdmania, Ascidia, Pyrosoma)
General Characters
Larvae contain all chordate characteristics while adults only exhibit pharyngeal pouches.
Marine animals. Some are solitary, some are colonial.
Sessile as adults but motile during larval stages.
Possess all chordate characteristics as larvae. Upon adulthood, they settle upon a hard substrate and undergo dramatic metamorphosis. Tail, Notochord, Muscle Segments and Nerve Cord disappear.
-Notochord is present in tail of larvae, disappears in adult.
-Dorsal tubular nerve cord in larvae is replaced by dorsal ganglion in the adult.
-Larvae undergoes retrogressive metamorphosis.
Adult body is degenerate, sac-like, unsegmented and without paired appendages.
Adult body is covered by an outer envelope (test), composed of fibres of Tunicin, embedded in a mucopolysaccharide matrix.
-Tunic encloses a basket like pharynx, which is perforated by Gill slits. They open into an ectoderm lined cavity.
-Forms a protective jacket around the body and also acts as an accessory respiratory organ and a receptor organ.
Filter feeders: Plankton is trapped in a sheet of mucus and cilia later directs the food-laden mucus to the stomach.
Incurrent branchial siphon and ex-current atrial siphon form entrance and exit portals for water that circulates through the body. Branchial siphon opens into pharynx.
Coelom is not recognisable.
Alimentary canal is complete. Large Pharynx with several pairs of gill slits. Ciliary feeder.
Heart is simple, tubular and ventral. Flow of blood is periodically reversed. Vanadocytes in blood extract vanadium from sea water.
Hermaphrodite — reproduce both sexually and asexually.
External cross fertilisation, takes place in ocean.
Open blood vascular system.
Excretion by neural gland, pyloric gland and nephrocytes.
Simple structures as aggregated cells with nerve endings represent sense organs. Tangoreceptors (touch), Photoreceptors (light), Rheoreceptors (water currents), Thermoreceptors (temperature), Chemoreceptors (chemicals).