The radula when present comprises several transverse rows of teeth, and each transverse row may have several teeth (polystichous), two teeth (distichous), or one tooth (monostichous).
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.
Lateral eyes also are often present which are monostichous with aggregated lenses (Limulus)or with isolated lenses(Scorpio), or are diplostichous with simple lens (Pedipalpi, Araneae, &c.).