ffn.7. 



Fig. 25. — Astaciis fluviatiUs. — The central Dervous Bystem seen from 

 above (nat. size), a, vent ; an, antennary nerve ; a'n, antennulary 

 nerve; t, circumoesophageal oommissTires ; gn. 1, supracEsophageal 

 ganglion ; gn. 2, infraoesophageal ganglion ; gn. 6, fifth thoracic 

 ganglion ; gn. 7, last thoracic ganglion ; gn. 13, last abdominal gang- 

 lion ; ceg, oesopha-gus in cross section ; on, optic nerve ; sa, sternal 

 artery in cross section ; sgn, atomatogastric aerve. 



