FIG. 27.--Astacus fluviatilis.--A, the auditory sac detached and seen from the outside ( x 15); B, auditory hair ( x 100); C, the distal extremity of the same more highly magnified. a, aperture of sac; as, auditory setæ; b, its inner or posterior extremity; n n', nerves; r, ridge.
[Editor's note: illustration is reproduced here approximately twice the size of the woodcut in the book.]
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