FIG. 21.--Astacus fluviatilis.--Two of the abdominal somites, in vertical section, seen from the inner side, to show x, x, the hinges by which they are articulated with one another ( x 3). The anterior of the two somites is that to the right of the figure.
[Editor's note: illustration is reproduced here approximately twice the size of the woodcut in the book.]
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