144 LARVAL OCELLI AND THE PARIETAL EYE. 



2. The two parietal eye sacs in the cyclostomes not only stand very nearly, 

 in the median plane, but each sac contains a right sensory placode, or retina, 

 separated from a left one by a median groove, or by an unspecialized band of 

 tissue. Thus there are two symmetrical retinal placodes in each parietal eye. 



3. In young lampreys about two inches long, each ganglion habenula is 

 divided into a smaller anterior lobe, united by two nerves with the inner sac, 

 and a larger posterior one, probably united in a similar manner with the outer 

 sac. Thus there are apparently four ganglia corresponding to the four placodes, 

 These facts are incompatible with the assumption that one sac is the right or 

 left mate to the other. 



4. The asymmetry of the ganglia is pronounced. The left anterior lobe 

 ultimately takes up a central position below the inner sac, and remains com- 

 paratively small. The other three lobes become very large, especially the two 

 on the right; but the reason for this unequal development is not apparent, since 

 the nervous connection with the right sac is insignificant. 



5. A comparison with the parietal eye of arachnids (Fig. loi), shows that 

 the inner sac of petromyzon (parapineal eye) corresponds to the endo-parietal 

 eye of Limulus, both sacs agreeing in position, in their lower grade of histological 

 structure, in their innervation, and in their relation to the epiphysis. The outer 

 sac of the lamprey corresponds with the outer one of Limulus, both sacs agreeing 

 in relative position, in being symmetrically bi-lobed, and in the presence of the 

 more highly specialized visual cells and rods. 



The Lenses of the Parietal Eye. 



It will be recalled that in the simple isolated ocelli of insects, the chitenous 

 lens and the thick transparent ectoderm that serves as a vitreous body are parts 

 of the optic cup, or of the lips of the cup. (Figs. 90, 91.) 



When there are well defined lenses to the parietal eye, as in many arachnids, 

 they are formed from isolated thickenings of the ectoderm and of the overlying 

 chiten, wherever the distal end of the eye tube reaches the surface of the head, 

 however remote that point may be from the one where the retinal placodes first 

 appeared. 



In the scorpions, the parietal eye has a highly developed vitreous body and 

 two lenses. (Fig. 105.) 



In Limulus, there are two well developed lenses, one for each retina of the 

 outer sac (Fig. 94). But the inner sac never has over it a true chitenous lens, or 

 any ectodermic thickening which may represent the remnants of a vitreous body, 

 although there may be a tubercle like thickening of the chiten, or a semi-transpar- 

 ent spot. (Fig. 201.) 



In the phyllopods, although the parietal eye is often \ery highly developed, 

 it lies well below the surface, and there is no thickening whatever of the adjacent 

 ectoderm, or of the chiten, to form a lens or vitreous body for them. The fre- 



