132 LARVAL OCELLI AND THE PARIETAL EYE. 



anterior edge of the lateral eye ganglion, representing the sense organ of the 

 third segment, ol.o.; and the compound eye placode itself, about opposite the 

 chehcerje, and belonging to the fourth segment. I.e. 



The compound eyes arise just behind the true cephalic lobes; apparently they 

 are not represented in the scorpion or in spiders, or in the embryonic cephahc 

 lobes of those insects that undergo a metamorphosis. 



During the following stages, the two pairs of ocellar tubes unite in the median 

 line in front of the olfactory lobes, forming a single median tube or epiphysis, 

 directed forward, below the skin. (Fig. 142, e.p.) Its distal end is dilated and 

 contains, as shown by its structure in the later stages, four ocellar placodes, two 

 paired and two practically unpaired ones; its posterior end opens on the surface 

 of the head by an oxaX pore situated just in front of the hemispheres, an. p. 



Meantime the two paired olfactory placodes move mesially and a new un- 

 paired olfactory placode appears just in front of the pore of the eye tube. The 

 compound eyes migrate in the opposite direction, toward the posterior heemo- 

 lateral surface of the thorax. 



We may harmonize these conditions with those in Acilius by assuming 

 that the two pairs of ocelli of the second segment are the only larval ocelli of the 

 acilius type retained in Limulus; and that the three olfactory placodes and the 

 compound eyes represent respectively the three stemmata and the compound 

 eyes of insects, which do not appear there till the close of the larval life. In 

 other words, in Limulus the secondary, or imaginal, set of eyes, and the primary, 

 or larval set, appear at the same period, the more recent organs being reflected 

 back into the same embryonic period as the more ancient ones. 



While these events are taking place, the palial fold is forming in separate 

 sections, one being directed backward and inward over each lateral eye ganglion 

 op.g.; another over each infolding for the olfactory lobes, ol.l, and a third over the 

 ocellar plates, an.p. 



The margins of all three folds gradually move toward the anterior margin of 

 the hemispheres where they unite to form a common opening, the anterior neiiro- 

 pore. This pore appears to be merely the opening to the united ocellar tubes, 

 but in reality it represents more than that. It is obviously comparable with the 

 anterior neuropore of the scorpion, differing from it only in that it lies farther 

 forward. In both cases, the main opening represents the point toward which all 

 the epithelial overgrowths of the forebrain converge and the last point to be 

 co\'ered by them. This interpretation is no doubt the correct one, for it is clear 

 that the opening offers access, as it does in the scorpion, not only to the eye tubes, 

 but also to the cavities of the olfactory lobes, the spaces between the hemispheres 

 and the palial wall, and the spaces between the under surface of the hemispheres 

 and the floor of the forebrain. (Fig. 47, B.) 



Change of Position. — The distal end of the ocellar tube is at first directed 

 horizontally forward toward the ectoderm that forms the anterior margin of 

 the procephalon. (Fig. 142.) 



