In Star fish the nervous system is primitive and diffuse type. It has no ganglia. It is associated with the epidermis. It has nerve fibres.
J. Smith (1937) divided the nervous system into three parts.
1. Oral or ectoneural system: It is called superficial system, as it is present below the epidermis. It is sensory in function. It has nerve ring around the mouth. It supplies nerves to the oesophagus and peristomial membrane . Five large radial nerve cords arise from angles of the nerve ring. They travel to the arms. Each nerve cord terminates in a optic cushion near the base of the tentacle. In a cross section the radial nerve cord is 'V shaped. It is covered by epidermis on its outside. It gives nerves to the tube feet. Below the body wall sub-epidermal nerve plexus is present. Radial nerve cords will unite with it.
2. Aboral (or) entoneural system: The sub-epidermal plexus is thickened on the outer margins of an ambulacral groove and from marginal nerve cords. They travel the whole length of the arms. They give lateral nerves. They form a plexus below the coebmic lining. It is mesodermal in origin. It is motor in function.
3. Deep or hyponeural system: It is developed from the mesoderm and is primarily motor in function. Like ectoneural system, it forms a double nerve ring, which lies aboral to the main nerve ring. In each arm, it gives off a pair of lateral cords, called Lange's nerves. Each Lange's nerve is a plate of nervous tissue. It lies in the outer oral wall of the radial hyponeural sinus. The branches of Lange's nerve go to muscles of the arm.
Poorly developed sense organs are seen in Star fish. They are,
Neurosensory cells and
1. Neurosensory Cells: Certain neurosensory cells occur scattered all over the body surface. A sensory cell is slender and fusiform. It has a nucleus. Thread like process is developed on the outer surface. The cell is connected to the subepidermal nerve plexus by the fibre. These cells are tactile or olfactory in function.
2. Eyes: At the base of each tentacle, one eye spot is present on the oral side. It is the optic cushion of radial nerve cord. It has ocelli. An ocellus is a cup shaped pocket of the ectoderm. It has a lens like transparent thickening formed by the epidermis
The wall of the cup has pigment cells. Among the pigment cells retinal cells are present. Each retinal cell ends in a bulb, which is projecting upto the cavity of the pit. It is connected with a fine fibre to a radial nerve cord. Eye spots are able to detect changes in light intensity.