When an action arises from conditions developed in the interior of an animal’s body, inasmuch as we cannot perceive the antecedent phenomena, we call such an action “spontaneous;” or, when in ourselves we are aware that it is accompanied by the idea of the action, and the desire to perform it, we term the act “voluntary.” But, by the use of this language, no rational person intends to express the belief that such acts are uncaused or cause themselves. “Self-causation” is a contradiction in terms; and the notion that any phenomenon comes into existence without a cause, is equivalent to a belief in chance, which one may hope is, by this time, finally exploded. In the crayfish, at any rate, there is not the slightest reason to doubt that every action has its definite physical {113} cause, and that what it does at any moment would be as clearly intelligible, if we only knew all the internal and external conditions of the case, as the striking of a clock is to any one who understands clockwork. * * * * * The adjustment of the body to varying external conditions, which is one of the chief results of the working of the nervous mechanism, would be far less important from a physiological point of view than it is, if only those external bodies which come into direct contact with the organism[9] could affect it; though very delicate influences of this kind take effect on the nervous apparatus through the integument. [9] It may be said that, strictly speaking, only those external bodies which are in direct contact with the organism do affect it—as the vibrating ether, in the case of luminous bodies; the vibrating air or water, in the case of sonorous bodies; odorous particles, in the case of odorous bodies: but I have preferred the ordinary phraseology to a pedantically accurate periphrasis. It is probable that the _setæ_, or hairs, which are so generally scattered over the body and the appendages, are delicate tactile organs. They are hollow processes of the chitinous cuticle, and their cavities are continuous with narrow canals, which traverse the whole thickness of the cuticle, and are filled by a prolongation of the subjacent proper integument. As this is supplied with nerves, it is likely that fine nerve fibres reach the bases of the hairs, and are affected by anything which stirs these delicately poised levers. {114} [Illustration: FIG. 26.—_Astacus fluviatilis._—A, the right antennule seen from the inner side (× 5); B, a portion of the exopodite enlarged; C, olfactory appendage of the exopodite; _a_, front view; _b_, side view (× 300); _a_, olfactory appendages; _au_, auditory sac, supposed to be seen through the wall of the basal joint of the antennule; _b_, setæ; _en_, endopodite; _ex_, exopodite; _sp_, spine of the basal joint.]