Two distinct forms of the Arthropod eye are observed - the monomeniscous (simple) and the polymeniscous (compound).
But they seem to point to a community of origin of Hexapods and Crustacea in regard to the complicated ommatidia of the compound eye, and to a certain isolation of the Arachnida, which are, however, traceable, so far as the eyes are concerned, to a distant common origin with Crustacea and Hexapoda through the very simple compound eyes (monostichous, polymeniscous) of Limulus.
The lateral eyes of Crustacea are polymeniscous, with highly specialized retinulae like those of Hexapoda, and unlike the simpler compound lateral eyes of lower Arachnida.
The later eyes are polymeniscous, with specialized vitrellae and retinulae of a definite type peculiar to this grade.