=The simple, or single-lensed eye (ocellus).=—Morphologically the simple eye is a modified portion of the ectoderm, the pigment enclosing the retinal cells arising from specialized hypodermal cells, and covered by a specialized transparent portion of the cuticula, forming the corneal lens. The apparatus is supplied with a nerve, the fibres of which end in a rod or solid nerve-ending, as in other sensory organs. As seen in the ocellus of Dyticus (Fig. 260), under the corneal lens the hypodermis forms a sort of pit, and the cells are modified to form the vitreous body (vitrella) and retina. Each retinal cell (_re_) is connected with a fibre from the optic nerve, contains pigment, and ends in a rod directed outwards towards the lens. The cells at the end of the pit or depression are, next to the lens, without pigment, and, growing in between the retina and the lens, fill it up, and thus form a sort of vitreous body. The ocellus appears to be a direct heirloom from the eyes of worms, while the many-facetted compound eye of the crustaceans and of insects is peculiar to these classes. The compound eye of the myriopod Scutigera differs structurally in many respects from the compound eye of insects, and that of Limulus still more so. It should be observed that in the young nymph of Ephemera, as well as in the semipupa of Bombus, each of the three ocelli are situated on separate sclerites. In Bombus the anterior ocellus has a double shape, being broad, transversely ovate, and not round like the two others, as if resulting from the fusion of what were originally two distinct ocelli. The ocelli are not infrequently wanting, as in adult Dermaptera, in the Locustidæ, and in certain Hemiptera (Hydrocora). In Lepidoptera there are but two ocelli; in geometrid moths they are often atrophied, and they are absent in butterflies (except Pamphila). =The compound or facetted eye (ommateum).=—The facetted arthropod eye is wonderfully complex and most delicately organized, being far more so than that of vertebrates or molluscs. The simplest or most primitive facetted eye appears to be that of Lepisma. As stated by Watase, the compound eye of arthropods is morphologically “a collection of ectodermic pits whose outer open ends face towards the sources of light, and whose inner ends are connected with the central nervous system by the optic nerve fibres.” The facetted eye is composed of numerous simple eyes called _ommatidia_, each of which is complicated in structure. The elements which make up an _ommatidium_ are the following: (1) The facet or cornea, which is a specialized portion of the cuticula; and (2), the crystalline lens or cone; (3), the nerve-ending or _retinula_, which is formed out of the retinula cells and the _rhabdom_ or rod lying in its axis; and (4) of the pigment enclosing the lens and rod; the last three elements are derived from the hypodermis. The single eyes are separated from each other by pigment cells.