Chap, xii.] NER vo us SYS TEM OF AR THR OPODA . 405 



(" archicerebrum" of Lan- 

 kester). Such an arrange- 

 ment is found also among 

 annulate worms. 



In the greater number 

 of the Arthropoda we 

 not only see that the nerve 



trunks lie intprnally to the 



muscular l^prj^ of the 



body wall, but also that 

 the cerebrum is no longer 

 primitive, but has other 

 ganglionic cells used with 

 it ; or, to use the words of 

 Rathke, as applied to the 

 developing scorpion, the 

 brain is " ^qrnp rt s Q 4 ol 

 severa.1 pairs of 



lying firm behind 



Nor is this kind 

 of fusion con tined to the 

 brain ; a longitudinal sec- 

 tion of part of the nerve 

 cords of a crayfish shows 

 that the ganglionic cells in 

 a segment have become 

 closely united together, 

 while, at the same time, 

 the cords are still distinct. 

 Nor is this all ; while 

 Apus has a distinct gang- 

 lionic enlargement in 

 every segment of its body, 

 we find that in higher 

 forms various ganglia be- 

 come connected together, 

 until at last, in the common 



-41- oc 



Fig. 174. Diagram of the Ante- 

 rior Portion of the Nervous 

 System of Apus, showing: the 

 " archicerebrurn " (c), and the 

 Ganglia of the Lateral Cords. 

 (From Laukester, after Zad- 

 dach. ) 



x, Frontal nerves ; oc, optic nerves ; 

 CE, oesophagus ;. 1, nerve for first 

 antenna;2, nerve for second ditto : 

 lid, nerves for mandible ; six, for 

 maxilla; M/J, for maxilliped ; T 1, 

 for flrst thoracic appendage. 



