70 THE ORIGIN OF VERTEBRATES 
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 
Fria, 29.—DraGRam 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. 
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. 
Thus we see that eyes can be placed in different categories, ¢.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 
