THE DIENCEPHALON OF ARACHNIDS AND VERTEBRATES. 63 



they happen to lie. The tectum opticum, for example, really belongs to the third 

 forebrain neuromere, in front of the diencephalon, and except as a matter of 

 convenience, cannot be classified as part of the mesencephalic neuromeres. 



(See p. 157.) 



Summary. Thus we have in the diencephalon of vertebrates a remarkable 

 combination of special characters; viz. a sharp cranial flexure; a funnel-like depres- 



FIG. 51. Forebrain of a young Limulus, haemal surface. A. A single optic fibre, showing the arrangement of 

 its principal branches. Methylene-blue preparation. Camera outlines. 



sion in the floor, with voluminous nerve centers (lobi inferiori) on its side wall; 

 the presence of important nerve tracts arising from the cerebellum, the olfactory 

 and gustatory organs, and that converge toward a common center in the in- 

 fundibular region; the presence of a membranous sac that contains the remnants 

 of sensory cells, and a special set of neuro-muscular reflexes. Each of these 

 characters is without parallel elsewhere in the brain of vertebrates. They indi- 

 cate that this particular region either has some very unusual part to play in cere- 



