That the lateral eyes of Scorpio have a single cell-layered or " monostichous " ommatidium like that of Limulus.
[How the inversion of the nerve-end-cells and their connexion with the nerve-fibres is to be reconciled with the condition found in the adult, or with that of the monostichous eye, has not hitherto been explained.] (From Korschelt and Heider.) The great pericardial sinus is strongly developed in both animals.
The monomeniscous eye is rarely provided with a single layer of cells beneath its lens; when it is so, it is called monostichous (simple lateral eye of Scorpion, fig.
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.).