The formation of visual faculty can be defined into three grades overall the
Primary blind trilobites liberated no kind of visual system
within the scope of their evolution. With the species of secondary blind
trilobites, fully functional eyes developed during their previous evolution were
reduced an finally lost during further evolution. Such evolutionary trends were
never curious and occured in various trilobites orders. Possible coming from
light conditions prevalented species bentic e. g. dimmed or even lack of light
in deep- water bentic, where trilobites have gradually lost their visual systems
without suffering an adaptive disadvantage in relationship to an advantage over
deep- water bentic life. The third grade finally representing the viewing
trilobites with eyes.
At the latter's grade
schizochroal, holochroal and abathochroal eyes are to be differentiated.
Basically the eyes of trilobites consisted of an assembly of many lenses,
material based of calcite (crystalline structure). Calcite, chemically
considered as a calcium carbonate (CaCo3), is clear as crystal at
maximum purity with an high birefringence. It's presumed the respective
receptors were Inside of the single prisms (derived from the construction of the
compound eyes of today's Arthropods).
Schizochroal eyes are merely to be found at some representatives of the
Order Phacopida. This kind of eyes are consisting about 700 single lenses, each
covered by a separate corneal membrane (cornea) and seperated from it's
neighbours lenses by a tunic membran (sclera).
Compared to this type, one holochroal eye consist of a close packing of
up to over 15000 hexagonal outlined biconvex lenses. A single corneal
layer covers all of the lenses. Holochroal eyes have no sclera.
The abatochroal type of eyes is only to be found in a few
trilobites of Suborder Eodiscina (Kobayashi, 1939). This type consits of up to
circa 70 little single lenses, each with it's own cornea. The corneal membrane
doesn't extend inside the eye, and it ends at the edge of the lens. The thin
sclera is ending in lenses depth.