These are not considered eyes because they lack enough structure to be considered an organ, and do not produce an image. Non-compound eyes
Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustaceans and cubozoa. Pit eyes
Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eyes". They are small, comprising up to about 100 cells covering about 100 µm. The directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material.Pit vipers have developed pits that function as eyes by sensing thermal infra-red radiation, in addition to their optical wavelength eyes like those of other vertebrates (see infrared sensing in snakes). However, pit organs are fitted with receptors rather different from photoreceptors, namely a specific transient receptor potential channel (TRP channels) called TRPV1. The main difference is that photoreceptors are G-protein coupled receptors but TRP are ion channels. Spherical lens eye
The resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered—hence increasing the resolution obtainable.
They occur at the end of injured nerve fibres as a form of ineffective, unregulated nerve regeneration; it occurs most commonly near a scar, either superficially (skin, subcutaneous fat) or deep (e.g., after a cholecystectomy). They are often very painful. Synonyms include scar neuroma, amputation neuroma, or pseudoneuroma. Mortons neuroma (a mononeuropathy of the foot) is another example of the more general usage of the term neuroma. Some prefer the term "Mortons metatarsalgia", thus avoiding the term neuroma and its association with tumors. Etymology
The stem neuro- originates from the Greek word for nerve (νεῦρον), while the suffix -oma (-ωμα) denotes swelling. The stem does not imply that neuromas necessarily arise from neurons; neuromas generally arise from non-neuronal nerve tissues. The word was originally used to refer to any nerve tumor, but its meaning has evolved. == References ==