Fig. 25.—Astacus fluviatilis.—The central nervous system seen from 
above (nat. size). a, vent; an, antennary nerve; a’n, antennulary 
nerve; ¢c, circumcesophageal commissures ; gn. 1, supracesophageal 
ganglion ; gn. 2, infracesophageal ganglion ; gn. 6, fifth thoracic 
ganglions; gn. 7, last thoracic ganglion ; gn. 13, last abdominal gang- 
lion ; @s, cesophagus in cross section ; on, optic nerve; sa, sternal 
artery in cross section ; sgn, stomatogastric nerve. 
