There is much reason to believe that odorous bodies affect crayfish; but it is very difficult to obtain experimental 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 {115} nature, developed on the under side of the outer branch of the antennule, play the part of an olfactory apparatus. Both the outer (fig. 26 A. _ex_) and the inner (_en_) branches of the antennule are made up of a number of delicate ring-like segments, which bear fine setæ (_b_) of the ordinary character. The inner branch, which is the shorter of the two, possesses only these setæ; but the under surface of each of the joints of the outer branch, from about the seventh or eighth to the last but one, is provided with two bundles of very curious appendages (fig. 27, A, B, C, _a_), one in front and one behind. These are rather more than 1‐200th of an inch long, very delicate, and shaped like a spatula, with a rounded handle and a flattened somewhat curved blade, the end of which is sometimes truncated, sometimes has the form of a prominent papilla. There is a sort of joint between the handle and the blade, such as is found between the basal and the terminal parts of the ordinary setæ, with which, in fact, these processes entirely correspond in their essential structure. A soft granular tissue fills the interior of each of these problematical structures, to which Leydig, their discoverer, ascribes an olfactory function. It is probable that the crayfish possesses something analogous to taste, and a very likely seat for the organ of this function is in the upper lip and the metastoma; but if the organ exists it possesses no structural peculiarities by which it can be identified. {116} There is no doubt, however, as to the special recipients of sonorous and luminous vibrations; and these are of particular importance, as they enable the nervous machinery to be affected by bodies indefinitely remote from it, and to change the place of the organism in relation to such bodies. * * * * * Sonorous vibrations are enabled to act as the stimulants of a special nerve (fig. 25, _a′n_) connected with the brain, by means of the very curious _auditory sacs_ (fig. 26, A, _au_) which are lodged in the basal joints of the antennules.