COMPARISON WITH VERTEBRATES. 157 



The proximal lobe consists of two parts; an anterior one, composed of large 

 neurones, sending their main fibers into the olfactory lobes, and collaterals into 

 the second lobe of the optic ganglion. (Figs. 51 and 52, op.g. 4 ); and a posterior 

 part consisting of a spherical mass of somewhat smaller neurones, sending their 

 main fibers backward into the longitudinal tract of the brain (op. f. 3 ) , and their 

 collateral branches into the second lobe of the ganglion (op.g. 3 ). 



The optic ganglia are united with other parts of the brain by the following 

 tracts : a, a distinct bundle of fine fibers that passes without interruption through 

 the optic ganglion into the crura, and run the whole length of the brain (Figs. 51 and 

 66 op.tr.)] b. a tract uniting the ocellar ganglia with the second lobe of the lateral 

 eye ganglia (oc.tr.) ; c. a commissural tract through the olfactory lobe, formed by 

 the neurones of the fourth lobe (op.f.^^d. a longitudinal tract formed by the neu- 

 rones of the third optic lobe, and which extend backward the whole length of the 

 crura (op.f. 3 ) ; e. and finally, an important tract, the source of whose fibers is 

 unknown extending from the second lobe of the optic ganglion into the cerebral 

 hemispheres (Fig. 52, op. tr. H.). 



IV. COMPARISON WITH VERTEBRATES. 



We have already shown that the four ganglia of the parietal eye in Limulus are 

 comparable with the four lobed ganglia habenulae of the cyclostomes. There is 

 also a striking resemblance between the lateral eye ganglia of arachnids and the 

 optic lobes of vertebrates, especially when we make due allowance for certain 

 peculiarities of position and structure. 



We may best explain the origin of the optic lobes of vertebrates on the 

 assumption that the optic ganglia of some form like Limulus have been bent 

 backward and upward onto the neural surface of the brain, in the manner shown 

 in Figs. 44, 57, 58, and 108. In this position, which is similar to the one 

 they occupy in the scorpion and other arachnids, they have the same general 

 form and the same relation to the rest of the brain that the optic lobes have in 

 vertebrates; the three lobes of the optic ganglia corresponding respectively to the 

 colliculus, the tectum, and the torus. 



It will be observed that in this new position (Fig. io8,5), the general contour 

 of each of the ganglia is retained, but the general appearance of the whole series 

 is greatly disguised, owing to the change of curvature of the second and third 

 lobes, and to the diagonal movement of the ganglia in a caudad and neurad 

 direction. (Fig. 46.) Both the distal and proximal ends of the series are fixed at 

 the optic commissure, the original point of attachment of the ganglia to the basal 

 lobes of the forebrain. The convergence of the fiber tracts of the optic lobes 

 toward the optic chiasma shows that the optic lobes belong primarily to the hemi- 

 sphere neuromeres, and that their position in vertebrates is a secondary one. 



