54 THE SUBDIVISIONS OF THE BRAIN. 



bral activities now, or did have some such function in the past; but what that 

 function was, or is, or what is the history and the meaning of these parts, neither 

 vertebrate anatomy or physiology gives us the slightest clue. 



But when we compare these conditions with those in the arachnids, their 

 meaning is sufficiently clear. The infundibulum is the passageway for the old 

 stomodaeum, and the latter is the saccus vasculosus. The lobi inferiori are the 

 lateral stomodaeal ganglia; the nerve plexus of the saccus, the stomodaeal nerves; 

 the tween-brain flexure is the one which occurs in arachnids between the supra- 

 and infra-stomodaeal ganglia; the remarkable centralization of fiber tracts in the in- 

 fundibular region is the retention of the ancestral condition seen in arachnids, 

 where fiber tracts from the olfactory and gustatory organs, and from the stomodaeal 

 commissure, converge toward the swallowing centers, or the ganglia that control 

 the neuro-muscular apparatus of the stomodaeum. 



In both vertebrates and arachnids, there is : a. but one passage, or opening, 



op.tr.- - 



FIG. 52. Semi-diagrammatic, longitudinal section of the lateral eye ganglion of a young Limulus. 



in the floor of the brain, and in both cases that passage is of similar relative 

 dimensions; b. it lies just behind the hemispheres and the lateral eye nerve roots; and 

 c. just in front of the anterior end of the notochord; and d. in front of the first 

 pair of somatic motor nerves; e. in both cases, the opening lies approximately 

 between the right and left halves of the fourth and fifth neuromeres; /. in both 

 cases evaginations from the median face of the half neuromeres form special 

 ganglia (stomodaeal ganglia, lobi inferiores), quite unlike any other cranial ganglia; 

 g. in both cases, these ganglia have similar associations with the stomodaeal com- 

 missure (cerebellum) , with the epithelial tube lying in the opening through the brain 

 floor, and with the olfactory and gustatory organs; h. and although of considerable 

 size, the ganglia have no direct nerve connections with any organs external to 

 the brain. 



The diencephalon, therefore, of arachnids and vertebrates, may be defined 

 as one, two, or more, neuromeres surrounding the primitive stomodaeum, and 

 uniting the primitive forebrain (supra-oesphageal ganglion) with the midbrain, 

 or thoracic neuromeres. It lies in the angle of the oldest, and most striking 



