FIG. 25.--Astacus fluviatilis.--The central nervous system seen from above (nat. size). a, vent; an, antennary nerve; a'n, antennulary nerve; c, circumoesophageal commissures; gn. 1, supraoesophageal ganglion; gn. 2, infraoesophageal ganglion; gn. 6, fifth thoracic ganglion; gn. 7, last thoracic ganglion; gn. 13, last abdominal ganglion; oes, oesophagus in cross section; on, optic nerve; sa, sternal artery in cross section; sgn, stomatogastric nerve.
[Editor's note: illustration is reproduced here approximately twice the size of the woodcut in the book.]
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