It may happen that the part invaginated is the optic sense-plate itself, as would be the case if in the former figure, instead of C, the part B was modified to form a sense-plate. This will give rise to an eye of a character different from the former (Fig. 29). The optic nerve-fibres now lie between the source of light and the retinal end-cells, the layer A as before forms the cuticular lens, and its hypodermal cells elongate to form the corneagen; there is no pre-retinal layer, but, on the contrary, a post-retinal layer, C, called the tapetum, and, as is seen, the light passes through the retinal layer to the {70}tapetum. The cuticular surface of the retinal cells forming the rods or bacilli is directed towards the tapetal layer away from the source of light, and the nuclei of the retinal cells are pre-bacillary in position, in contradistinction to the upright eye, where they are post-bacillary. The retinal end-cells are devoid of pigment, the pigment being in the tapetal layer. Such an eye, in contradistinction to the former type, is an eye with an _inverted retina_; but still the same law holds as in the former case--the optic nerve-fibres enter at the nuclear ends of the cells, and the rods are formed from the cuticular surface. In these eyes the pigmented tapetal layer is believed to act as a looking-glass; the dioptric apparatus throws the image on to its shiny surface, from whence it is reflected directly on to the rods, which are in close contact with the tapetum. A similar process has been suggested in the case of the mammalian lateral eye, with its inverted retina. Johnson describes the post-retinal pigmented layer as being frequently coloured and shiny, and imagines that it reflects the image directly back on to the rods. [Illustration: FIG. 29.--DIAGRAM OF FORMATION OF AN INVERTED SIMPLE RETINA. The arrow shows the direction of the source of light in this as in the preceding figure. In both figures the cuticular rhabdites are represented by thick black lines.] Thus we see that eyes can be placed in different categories, _e.g._ those with an upright retina and those with an inverted retina; also, according to the presence or absence of a tapetum, eyes have been grouped as tapetal or non-tapetal. All the eyes considered so far are called simple eyes, or ocelli; and a number of ocelli may be {71}contiguous though separate, as in the lateral eyes of the scorpion. They may, however, come into close contact and form one single, large, compound eye. Such ocelli, in a very large number of cases, retain each its own dioptric apparatus, and therefore the external appearance of the compound eye represents not a single lens, but a large number of facets, as is seen in the eyes of insects. Owing to these differences, eyes have been divided into simple and compound, and into facetted and non-facetted.