134 LARVAL OCELLI AND THE PARIETAL EYE. 



While this is taking place the ends of the anterior liver lobes unite in front 

 of the cephalic lobes, thus apparently isolating that part of the head containing 

 the ocelli, from the neural portion containing the olfactory organs and cephalic 

 lobes. (Figs. 149, 151.) 



At this stage, the surface contours of the forehead cannot be clearly dis- 

 tinguished. But during the early trilobite stages, after boiling in caustic potash, 

 a distinct suture is visible on the cephalo-thoracic shield, marking the boundaries 

 of the primitive procephalon. (Fig. 1^2, pr.c.) 



This suture quickly disappears, and in all subsequent stages the only part 

 of the primitive fore-head visible on the haemal surface is a narrow patch bearing 

 the ocelli. (Fig. 155.) 



Appearance of the Placodes. — We may now confine our attention to the later 

 stages of the parietal eye. 



After the trilobite stage, one pair of ocellar placodes form the lateral walls 

 of a terminal dilatation, that may be called the ecto-parietal eye. (Fig. 102, D.) 

 Their cells become invested with black pigment and they take on the character of 

 typical visual cells. (Fig. 94, l.ec.p.e.) 



The other pair form the walls of a second median dilatation that we shall 

 call the endo-parietal eye, en.p.e. It lies below the surface, and on the proximal 

 side of the ecto-parietal eye. Its cells are unlike the usual retinal cells in shape, 

 arrangement, and pigmentation; but they are provided, temporarily, with plate- 

 like visual rods or rhabdoms. 



Nerves. — In young Limuli, three to four inches long, four nerve fascicles may 

 be seen at the distal end of the eye tube, one for each retina of the ecto-parietal 

 eye, and two for the unpaired endo-parietal eye. (Figs. 94, loi, A.) 



In the middle section of the tube, the four nerves unite to form a common 

 layer of fibers outside the epithelial walls of the tube. Toward its proximal end, 

 the nerve fibers separate from the epithelial walls of the tube and again divide 

 into four fascicles, or two pairs of roots, the larger pair ending in two conical 

 ganglia on the hasmal surface of the olfactory lobes, the smaller one in two smaller 

 ganglia situated a little farther back. (Fig. 51, ey.r^, ey.r'-.) 



Thus the evidence afforded by the infoldings on the cephalic lobes, the struc- 

 ture of the terminal sac, of the eye tube and the four nerve roots, show that the 

 "unpaired eye" of Limulus is formed by the partial fusion of two separate pairs 

 of ocelli. 



Structure of the Retinas. — From the earliest larval stages, the difference in 

 structure between the endo- and ecto-parietal eyes is very striking. The ecto- 

 parietal retinas contain, besides numerous indifferent cells, well defined ommatidia 

 consisting of from five to seven cells with the visual rods arranged in star-shaped 

 rhabdoms near their outer ends. (Fig. 94.) The visual cells contain a relatively 

 small amount of reddish-brown pigment, and little, or none, of the white pigment. 

 The endo-parietal eye, in young Limuli three to four inches long, is a thick-walled, 

 pear-shaped vesicle lying well below the surface and almost inaccessible to light. 



