THE DIENCEPHALON. 



57 



of the egg. The angle of this bend lies behind the cheliceral neuromere, which, 

 therefore, faces forward, connecting the forebrain, now on the haemal surface of the 

 egg, with the thoracic neuromeres on the neural surface. (Figs. 43-46.) 



In practically all adult arachnids, the chelicerae move forward to the very an- 

 terior end of the head and lie close together in front of the rostrum and stomo- 

 daeum, instead of behind them, as in the earlier stages. The result is that the 

 cheliceral nerves, instead of arising from the sides of the brain, like all the other 



,,pec. 



pa.ey.r-- 



vg.n. 



ol.l.-' 



st.n. 



ol.co. 



FIG. 47. Sagittal sections of brain models. A, young scorpion; B, Limulus; C, a hypothetical brain, combining 

 the principal characters of the brain of Limulus and scorpion, and with the parts in the position they are supposed 

 to occupy in a primitive vertebrate. 



nerves to the appendages, arise from the median, neural surface, and point 

 cephalad and neurad. (Fig. 40.) 



In Limulus, the cheliceral neuromere is less conspicuous in the older stages 

 because it is partly covered by the posterior, lobes of the hemispheres, which grow 

 back over it. (Figs. 37, 38, 47 and 48.) As the cheliceral neuromere moves 

 forward, it unites so intimately with the third neuromere of the forebrain that it 

 is difficult to distinguish the boundaries between them. Both neuromeres help 



