114 PATTEN. [Vot. II. 
VI. was invaginated, the large clear spot, 6, (Fig. 62) came to 
lie on the ventral and posterior edge of the optic cup (Figs. 8 
and 9, 6). The cells of this sense organ become more and more 
bent and elongated as the cup closes, until they finally form a 
great tongue of cells projecting into the space between the cor- 
neagen and the retina (Figs. 71 and 72, ¢ z.c.). As it increases 
in length, it reduces the optic cavity to an oval space in the 
dorsal and anterior portion of the eye. The cavity is finally 
completely obliterated by the increase in length of the corneagen 
cells (Fig. 73). 
The free ends of the cells belonging to the sixth sense organ 
bear inverted retinal rods, the same as those belonging to the 
upright ones, except that they are less regular in shape. Cross 
sections show that they are cylindrical with a clear central 
portion in which runs an axial nerve. The nuclei are at first sit- 
uated near the free ends of the inverted cells, and are somewhat 
larger and differently stained from those in the remainder of the 
retina. When the pigment, which is scattered through these cells 
in coarse granules like that in the iris, is removed, they appear 
almost colorless, with here and there a few coarse granules. 
Figs. 71-74, represent semi-transverse sections of the head, 
at right angles to the nerve z VI., Fig. 9, and parallel to 
the dark area of eyes III. and I, Fig. 8. When the sections 
are cut in most other cross planes, the retina appears per- 
fectly symmetrical, its periphery being bent over and contin- 
uous with a very thin membrane in the same way that the 
left-hand edge is in Fig. 73. But if any of these cross sections 
should pass a little posterior to the median plane of the eye, 
the tongue of inverted retinophorz would be seen, which might 
readily be mistaken for the cut end of a large number of nerve 
fibres, such as one finds in retinas with inverted cells, as in 
Pecten for example. 
That we should have eyes with both upright and inverted 
retinophore, as in eye V., is remarkable and, so far as I know, 
without parallel. The condition in eye VI. is still more ex- 
traordinary, for there only a comparatively small cluster of rod- 
bearing cells is inverted, while all the rest are upright. 
The sense organ, 5, has either united with 1-4 to form the 
floor of the optic cup or disappeared; at any rate after invagina- 
tion it is no longer distinguishable. 
