It is curious and instructive to see the trouble that excellent writer, Treviranus[202] takes, to find out in the lower animals, such as _infusoria_ and _zoophyta_, which movements are voluntary, and which are what he calls automatic or physical, _i.e._ merely vital. He founds his inquiry upon the assumption that he has to do with two primarily different sources of movement; whereas in truth they all proceed from the will, and the whole difference consists in their being occasioned by stimuli or by motives, _i.e._ in their having a brain for their medium or not; and the stimulus may again be merely interior or exterior. In several animals of a higher order--crustaceans and even fishes--he finds that the voluntary and vital movements, for instance locomotion and respiration, entirely coincide: a clear proof that their origin and essence are identical. He says p. 188: "In the family of the _actinia_, star-fishes, sea-urchins, and _holothuriæ_ (_echinodermata pedata Cuv._), it is evident that the movement of the fluids depends upon the will of the animals and that it is a means of locomotion." Then again p. 288: "The gullet of mammals has at its upper end the pharynx, which expands and contracts by means of muscles resembling voluntary muscles in their formation, yet which do not obey the will." Here we see how the limits of the movements proceeding from the will and of those assumed to be foreign to it, merge into one another. _Ibid._, p. 293: "Thus movements having all the appearance of being voluntary, take place in the stomachs of ruminants. They do not however always stand in connection with the ruminating process only. Even the simpler human stomach and that of many animals only allows free passage to what is digestible through its lower orifice, and rejects what is indigestible by vomiting." [202] Treviranus, "Die Erscheinungen und Gesetze des Organischen Lebens," vol. i. pp. 178-185.