Photoreceptors
Photoreceptors are concerned in absorption of light by photosensitive pigments. The chemical change involved produces the impulse concerned in the nerve cells. The pigments involved are carotenoid derivatives containing retinine (vitamin A aldehyde) combined with a protein (opsin). Rhodopsin is one of the general pigments. Simple eyes are extremely common; extremely differentiated compound eyes have evolved in many arthropods, and well differentiated camera type eyes, as comparable to vertebrate eyes occur in cephalopods. You will see among several non-chordates a variety of eyes ranging between these extremes. Simplest eyes in Platyhelminthes and annelids are in the form of pigment spots containing sensory cells associated with pigment cells. They serve differentiation of light from darkness. In several polychaetes they reach higher level of organization. The cup may often secrete a lens. The cephalopod eye is the result of highest elaboration of vesicular eye. Actually the eye in Octopus reaches the structural complexity of that of vertebrate eye, resembling it in considerable details. Though, the cephalopod retina is direct type with sensory ends of the receptor cells directed towards the source of light whereas the retina of vertebrate eye is inverted with the sensory ends turned away from the source of light.