Sense organs in Asterias

Sense organs in Asterias

The senses are poorly developed and the sense organs in Asterias comprises of the neurosensory cells, terminal tentacles and the eyes.

Neurosensory Cells:

These are present scattered throughout the body surface, but are quite abundant in the suckers of the tube feet, in the ambulacral regions, in the epider­mis, at the base of pedicellariae and spines. Each neurosensory cell has a slender fusiform body, the proximal end of which is connected with sub-epithelial nerve plexus and the distal end goes up to the cuticle. These neurosensory cells are tactile in func­tion and act as chemoreceptors.

Terminal Tentacles:

These tentacles are regarded as tactile receptors. They help to survey the environment during locomo­tion.

Eyes:

The major sensory organs are the simple, pigmented eye spots situated one at the tip of each arm, on the oral side of the so- called terminal tentacle. The radial nerve cord ends at the base of the terminal tentacles as a sensory pad or cushion. This cushion is termed as the eye because it bears numerous photoreceptors in the form of cup-like pig­mented ocelli or eye pits

Externally, each ocellus, is covered by the cuticle, beneath which is present a lens-like transparent thickening formed by the epider­mis. This may be absent in some cases. The wall of the cup is composed of red pigmen­ted cells and retinal cells.

Each retinal cell ends in a small clear and highly refractile bulb, projecting into the cavity of the pit. A trans­parent gelatinous tissue fills the cavity of the eye pit. The number of ocelli per eye may be about 80-200 and the number may even increase with age.