BRANCHIOSTOMA-NERVES, OCELLI

The  Peripheral nervous system includes paired nerves from nerve cord.The two pair arising from cephalic vesicle are called cerebral nerves. They lack ventral roots. They are sensory in nature and convey impulses from receptors of snout, oral hood and buccal cirri to the nerve cord.The nerves arising from nerve cord behind the cerebral vesicle are called spinal nerves. Each pair of spinal nerves actually includes separate dorsal and ventral nerve roots .Dorsal nerve root is both sensory and motor and passes out to skin between myotomes. The ventral nerve root is motor and opposite to the myotome .The dorsal and ventral roots of a pair do not originate at the same level and the dorsal root of one side lies opposite the ventral root. The dorsal root is single the ventral root is made of several branches. Nerves of amphioxusare primitive and non-myelinated is not covered by a sheath of myelin in the vertebrates.
OCELLI-These are photoreceptors or light-sensitive organs on the ventro-lateral sides of nerve cord. They are originated in different directions and help the animal in burrowing in sand or in swimming spirally to perceive light from all directions. The eye spot or ocelli is made up of two cells an outer pigment cell or melanocyte and inner photosensitive cell with a striated apicalborder which serves as a lens. The photosensitive cell sends a fine nerve fibre to the nerve cord.