THE NERVES OF THE METENCEPHALON. 



lOI 



recognized on the median margin of each appendage. About the time of hatching, 

 the genital ducts are carried forward and unite to form an unpaired opening, 

 between the remnants of the first pair of abdominal appendages and the second. 

 During stage G, the brain may be dissected out, and the arrangement of neuro- 

 meres and some of the nerves observed (Fig. 54). The first two neuromeres are 

 crowded forward and are overlapped by the posterior margin of the hindbrain. 

 This produces a sharp haemal flexure in the brain, at the dividing line between the 



Fig. 74. — D, Section through the basal lobe of the third thoracic appendage of an embryo scorpion; E, section of one 

 of the segmental sense organs on the outer margin of the thoracic appendages. See Figs, 15 and 16. 



thoracic and vagus region; the first two vagus neuromeres are thus partly concealed, 

 in surface views, under the overlapping hindbrain. The vagus nerves, during 

 these early stages, are small and cannot be followed with certainty. In the adult 

 scorpion, they present an interesting condition. The nerves are now divided 

 into two groups, one containing all the neural nerves, the other all the haemal. 

 (Fig. 42.) This is largely due to the constriction which is such a characteristic 



' pit ^1715 p ent 



Fig. 75. — Side view of the endocranium, brain, neural arches, and associated muscles of an adult Limulus; semi- 

 diagrammatic. 



feature of the vagus region in arthropods. All the paired organs in this 

 vicinity, such as genital ducts and appendages, are drawn toward a median 

 position, hence in the adult the corresponding nerves take their origin from the 

 neural surface of the cord, near the middle line. 



The Neural Nerves. — One small nerve, supplying the sexual ducts and papil- 

 lae, probably represents the nerve of the second pair of rudimentary appendages 

 CFig. 42, v^). The nerve to the pectines has three roots, the first one g.n. forming 



