TYPE GBUBTACEA. 



379 



on, a pair of ganglia occurring in eacli metamere througliout 

 the body. Such a condition as this is found only in em- 

 bryonic stages, and even there not always perfectly. The 

 ganglia representing the preantennulary metamere fuse with 

 the cerebral, as do also the antennulary, and in higher forms 

 the antennary ganglia, there being thus formed a complex 



Fig. 169.— DtAGRAM of Nervous 

 System op Ckostacban. 

 ce = cerebral ganglion. 

 g^ = second " 



g^ = antennulary " 

 g' = antennary " 

 mn = mandibular " 



mx', ftiQ? = maxillary ganglia. 

 (B = oesophagus. 

 W = first thoracic ganglion. 



Fig. 170. — Nervous System op 

 (,A) AN IsopoD, Asellus, and 

 (B) A Brachyuban Decapod, 

 Maja (after Milhe-Bdwards). 



cerebrum, which, in contrast to the simple cerebrum (archi- 

 cerebrum) of the Annelida, may be known as a syncerebrum. 

 The remaining ganglia may remain perfectly separate, the 

 connectives joining the more anterior ones usually being 

 much shortened, or a greater or less number of them may 

 fuse. Thus in the Crayfish the ganglia of the three posterior 



