114 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
There is much reason to believe that odorous bodies 
affect crayfish ; but it is very difficult to obtain experi- 
Fie. 26.—Astacus fluviatilis.—A, the right antennule seen from the 
inner side (x 5); B, a portion of the exopodite enlarged ; C, olfactory 
appendage of the exopodite ; a, front view; b, side view (x 300); 
a,olfactory appendages ; au, auditory sac, supposed to be seen through 
the wall of the basal joint of the antennule ; b, setae ; en, endopo- 
dite ; «xv, exopodite ; sp. spine of the basal joint. 
mental evidence of the fact. However, there is a good 
deal of analogical ground for the supposition that- some 
peculiar structures, which are evidently of a sensory 
