6o THE SUBDIVISIONS OF THE BRAIN. 



walls of the stomodaeum. (Fig. 53, c.) The evaginated part separates from the 

 stomodaeum and, uniting with the adjacent neuromere (cheliceral or antennal), 

 forms the lateral stomodaeal ganglion. In insects, a frontal, or median stomodasal 

 ganglion arises in a similar manner from the anterior median wall of the stomo- 

 daeum. (Fig. 14.) 



Nerves extend backward from the stomodaeal commissure into the labrum, 

 which never receives nerves from any other source. From the deep, or haemal, 

 end of each lateral ganglion, a nerve extends along the side walls of the oesophagus, 

 connected by several transverse bands with a median nerve arising from the frontal 

 ganglion. (Fig. 35.) 



In arachnids, the median stomodaeal nerve seen in the insects is absent, and 

 there are no traces of ganglion cells in the commissure. 



The stomodaeal nerves and ganglia represent a distinct system of nerves that 

 cannot be compared with any others. That it is a very ancient system is shown by 

 its vigorous growth at an early ontogenetic period. The ganglia, nerves, and 

 commissure, form a special system controlling the peristaltic actions of the stomo- 

 daeum in swallowing, grinding, or sucking food. These reflexes may possibly be 

 directly stimulated through the sensory cells in the inner lining of the stomodaeum 

 or in the lips; but, in Limulus at least, an essential condition appears to be an initial 

 stimulation of the gustatory organs in the jaws, or of the olfactory organ. From 

 the gustatory organs, important nerve tracts converge toward a common center 

 in the cheliceral neuromere, and toward the lateral stomodeal ganglion. (Fig. 

 114.) 



Comparison of the Diencephalon of Arachnids and Vertebrates. — 

 When the mouth of the arachnids was shut off from the exterior by the backward 

 overgrowth of the rostrum and of the optic lobes, and by the closing up of the cerebral 

 vesicle, the stomodaeum and the adjacent ectoderm remained in the vertebrates as 

 the epithelial lining of the third ventricle and adjoining chambers; and the opening 

 through the floor of the brain, which served as the passageway for the old cesopha- 

 gus, remained as the infundibulum. The inner end of the stomodaeum, that pro- 

 trudes through the infundibulum, became the sacci vasculosi; the lateral stomo- 

 daeal ganglia, the lobi inferiori; and the stomodaeal commissure, the anlage of the 

 cerebellum. (Figs. 3, 46.) 



The median haemal portion of the cheliceral neuromere, which is the princi- 

 pal center for the olfactory, gustatory, and stomodaeal impulses, corresponds with 

 the hypothalmic region, while the chehceral lobes and the cerebral association 

 cells, ch.H.c, mark the beginning of the thalamus. On the roof of the ventricle, 

 the median ocellus persists as the parietal eye. (Fig. 47, c.) Let us examine 

 these comparisons more carefully. 



The Stomodaeum. — In existing arachnids, the roof of the diencephalic region 

 consists of the epithelium that was produced by the backward migration of the 

 rostrum and the mouth. (Figs. 3, 46, and 47.) Owing partly to the manner in 



