On the basis of ontogeny and adult morphology, an interpretation of the arrangement of optic neuropiles and fibre connexions of the Crustacean compound eye is presented. In the embryo of phyllopods and decapods, the ommatidia, the lamina ganglionaris, and the medulla externa are developed synchronously from a common medial proliferation zone. As this zone persists in all investigated adult Crustacea that possess compound eyes, such a derivation of the mentioned structures is taken to be universal within the group. The direction of growth of the lamina ganglionaris is parallel with the row of ommatidia, the growth direction of the medulla externa is perpendicular to it and parallel with the long axis of the eyestalk. This arrangement is more or less retained in most adult non-Malacostracan Crustacea, and the axons of fully developed neurons pierce the optic neuropiles and leave and enter on the neuropile side. As a result, there is no chiasma in the non-Malacostracan groups.