Nervous System

Adults ascidians lack a nerve cord (Ruppert et al.,2004). The cerebral ganglion, a neural tube remnant, is a hollow “brain” located in the connective tissue between the siphons. The inhalant siphon and musculature are supplied by nerves arising from the anterior end of the ganglion and the posterior end supplies nerves to the rest of the body (Ruppert et al.,2004).

The tunic vessels may help nerve impulses travel though the zooid and are responsible or initiating vessel contractions and coordinate retractions of the zooids (Ruppert et al.,2004). The tunic blood vessels are lined with epidermis which does not contain neurons, but are still capable of conducting impulses along the epithelium. It is possible that tunicate epithelial tissue may have a rhythmic electrical activity and conductive ability similar to that of the heart (Ruppert et al.,2004).

Below the cerebral ganglion is the neural gland, a hollow, blind sac, also derived from the embryonic neural tube (Ruppert et al.,2004). Nerves are absent from this gland. A ciliated duct, another neural tube remnant, extends from the neural gland and opens into the pharynx. This opening is a large, often complexly coiled, ciliated funnel known as the dorsal tubercle (Ruppert et al.,2004). Cilia in the duct create an incurrent flow causing water to enter the gland before entering the branchial blood vessels. Cilia on the dorsal tubercle stop large particles entering the duct and phagocytes lining the gland remove bacteria and other particles from the water (Ruppert et al.,2004). Together the dorsal tubercle, ciliated duct, and neural gland maintain the fluid volume of the blood (Ruppert et al.,2004).

Sensory organs are absent in adults, but sensory cells are abundant on the siphon surfaces, the buccal tentacles, and in the atrium. These cells most likely play a role in controlling the water current passing through the pharynx (Ruppert et al.,2004).