The =compound eyes of Crustacea= resemble those of Insects in that they are composed of a very large number of similar elements or “ommatidia,” more or less isolated from one another by pigment. Each ommatidium consists typically of a corneal lens (Fig. 103, _c_), secreted by flat corneagen cells (_c.g_) below; beneath the corneal lens is a transparent refractive body called the “crystalline cone” (_cr_), which is produced by a number of cells surrounding it called the “vitrellae” (_v_). Below the crystalline cone comes the “rhabdom” (_rh_), produced and nourished by “retinulacells” (_r_). The rhabdom is a transversely striated rod, constituting the true sensory part of each ommatidium, and is in connexion at its lower end with a nerve-fibre (_n_), passing to the optic ganglion. The rhabdoms rest upon a membrane (_f_) called the “membrana fenestrata.” Each ommatidium is isolated from its fellows which surround it by a complete cylinder of pigment, part of which is especially crowded round the crystalline cone, and is known as “irido-pigment” (_ip_), while the part which surrounds the rhabdom is called “retino-pigment” (_rp_). When the pigment is arranged in this way, as in Fig. A, only those rays of light which strike an ommatidium approximately at right angles to the corneal surface can be perceived, since only these can reach the top of the rhabdom; the others pass through the crystalline cones obliquely, and are absorbed by the cylinder of pigment surrounding each ommatidium, so that they neither reach the rhabdom of the ommatidium which they originally entered, nor can they penetrate to the rhabdom of neighbouring ommatidia. This gives rise to what is known as “mosaic vision,” that is to say, each ommatidium only perceives the rays of light which are parallel to its long axis, and in this way an image is built up of which the various points are perceived side by side by means of separate eye-elements. The distinctness and efficiency of this mode of vision depends chiefly upon the number of ommatidia present, and the completeness with which they are isolated from one another by the pigment. Now this form of vision, depending as it does upon the absorption of a great number of the light-rays by pigment, and the transmission of only a limited number to the sensory surface, is only possible when there is a strong light, and there is no need for economising the light-rays. The most important discovery was made by Exner,[117] that the majority of animals with compound eyes had the power of so arranging the pigment in their eyes as to enable them to see in two ways. In bright light the pigment is situated as in Fig. 103, A, so as completely to isolate the rhabdoms from one another (day-position); but in the dusk the pigment actively migrates, the irido-pigment passing to the surface (B) near the tops of the crystalline cones, and the retino-pigment passing interiorly to rest on