68 



THE SUBDIVISIONS OF THE BRAIN. 



margin of the thoracic shield. (Fig. 152, ch.pl.) The tergite of the opercular 

 metamere is a narrow wing plate, still clearly outlined on the lateral margins, but 

 in the middle it is completely fused with the abdominal shield op.pl. The hinge 

 of the cranial buckler comes between the tergites of the chilarial and the opercular 

 metameres. (Fig. 155.) 



The chilarial and opercular neuromeres are completely fused with each other 

 and with the posterior end of the midbrain; and in the adult only the commis- 

 sures and the distribution of the corresponding nerves afford a clue to their 

 identity. (Figs. 65, 66, and 70.) 



In the scorpion the first four abdominal metameres belong to the vagus 

 group. At an early period, there is a pair of rudimentary appendages on each of 

 these four metameres. (Figs. 15 and 16.) In young scorpions, the first two pairs 



op.tt. 



FIG. 56. Cross-section of the mesencephalon of a young Limulus brain. On the right, the section passes 

 through the fourth pedal ganglion, with its gustatory, entocoxal, and pedal nerves, and the ascending roots of the 

 fifth and sixth gustatory nerve roots. On the left, the section passes through the root of the fourth haemal nerves, 

 showing its relation to the commissures and to the great longitudinal tracts. The figure is constructed from 

 methylene blue and von Rath's preparations. The capital letters indicate the same neurones as in Figs. 65 and 66. 



form minute papillae near the unpaired genital opening. The third pair form the 

 pectines, and the fourth, in part, the first pair of lung books. 



The four neuromeres become fused into a dense triangular mass, crowded 

 forward beneath the midbrain. Its posterior end is elevated, producing a pro- 

 nounced hind-brain flexure. (Figs. 40, 43, 54.) The vagus neuromeres are rela- 

 tively narrow, and as the corresponding appendages are crowded toward the 

 middle line, their pedal nerves arise in a characteristic manner from the neural 

 surface, not from the sides. 



The Branchiencephalon. In the arachnids, the anterior vagus meta- 

 meres are characterized by their highly modified tactile or sensory appendages, 

 the posterior ones only being respiratory. In the transitional forms between 

 vertebrates and arachnids, respiratory segments were doubtless added to the 



