THE OPTIC GANGLIA. 155 



In the phyllopods and arachnids, they are generally drawn upward, so that 

 they lie on, or over, the neural surface of the brain. 



Sections and surface views of scorpion embryos show how this is done. 

 (Figs. 15, 16, 18, 41, 103, 104.) It will be seen that as the palial folds advance, the 

 optic ganglia move upward and inward till they lie on the neural surface of the 

 forebrain neuromeres, the parietal eye ganglion lying in front of the lateral eye 

 ganglion. In this position they give us the clueto the interpretation of the optic 

 centers in vertebrates for, clearly, one represents the ganglion habenula, the other 

 the tectum opticum, or the roof of the midbrain. (Figs. 43, 44.) 



Parietal Eye Ganglia. — We have already shown that the two pairs of gan- 

 glia, from which the roots of the parietal eye nerves arise, are represented in 

 petromyzon by a four lobed ganglion habenula. (Fig. 104, B.) The latter 

 occupies the same relative position as in the scorpion, but a very different one from 

 that in Limulus. However, the difference is more apparent than real, because the 

 anterior roots of the ganglia habenulee are directed downward and forward 

 toward the olfactory lobes, showing not only the direction in which the ganglia 

 have been shifted, but that their original point of union with the brain is the 

 same as it is in Limulus. (Compare Fig. 47, A, B, and C) 



The difference in position of the parietal eye ganglia, in scorpion and Limulus, 

 is due to the fact that in Limulus the eye is drawn forward and hgemally by the 

 extraordinary size of the cephalic shield, and by the rapid growth of the hemi- 

 spheres, which have grown up behind the epiphysis, instead of in front of it as in 

 all other arthropods. (Compare Figs. 43, 44, 46, and 47.) 



Lateral Eye Ganglia. — The characteristic shape of the optic ganglia is well 

 seen in large-eyed insects, as Vespa, where they consist of three principal lobes. 

 (Fig. 107, A.) I. A proximal one, that is roughly spherical; 2. a median one, 

 consisting of an immense concave crescentic disc, and 3. a long narrow one, 

 extending around the distal margin of the middle lobe. Each lobe contains a 

 mass of felted fibers of the same general shape as the lobe, and is covered, on its 

 outer surface, by a thick layer of ganglion cells. 



The hemispherical middle lobe is the most conspicuous one, and the one 

 which, by its contour and dimensions, reflects most accurately the variations in 

 the eye to which it belongs. This significant fact has also been observed in the 

 ocelli of Acilius, where each one of the six pairs of eyes has a special shape, or some 

 peculiarity in the arrangement of retinal cells, which is accurately repeated in the 

 size, form, and structure of the neuropile core of the corresponding ganglion. 



Minute Structure in Limulus. — In Limulus, the ganglia have a similar con- 

 figuration to those of insects. (Figs. 37-39, 51,66.) The inner lobes (Figs. 51 and 52) 

 contain large association neurites, op.g.^ and op.g.*; the two outer ones, the central 

 ends of the optic nerve fibers, op.f., and two relays of optic neurones, op.g.''^ and 

 op.g.' 



Each of the two outer lobes is a disc-shaped mass of fibers, one surface covered 

 with a thick layer of nerve cells, the other bare. In certain cases, successfully 



