II. In the case of many freely moving protists (especially the amoeba, the heliozoon, and the rhizopod) the stimuli from without produce on every spot of the unprotected surface of the unicellular organism external movements which take the form of changes of shape, and sometimes changes of place (amoeboid movement, pseudopod formation, the extension and withdrawal of what look like feet); these indefinite, variable processes of the protoplasm are not yet permanent organs. In the same way, general organic irritability takes the form of indeterminate reflex action in the sensitive plants and the lowest metazoa; in many multicellular organisms the stimuli may be conducted from one cell to another, as all the cells are connected by fine fibres. III. Many protists, especially the more highly developed protozoa, produce on their unicellular body two little organs of the simplest character--an organ of touch and an organ of movement. Both these instruments are direct external projections of protoplasm; the stimulus, which alights on the first, is immediately conducted to the other by the psychoplasm of the unicellular body, and causes it to contract. This phenomenon is particularly easy to observe, and even produce experimentally, in many of the stationary infusoria (for instance, the _poteriodendron_ among the flagellata, and the _vorticella_ among the ciliata). The faintest stimulus that touches the extremely sensitive hairs, or _cilia_, at the free end of the cells, immediately causes a contraction of a thread-like stalk at the other, fixed end. This phenomenon is known as a "simple reflex arch." IV. These phenomena of the unicellular organism of the infusoria lead on to the interesting mechanism of the neuro-muscular cells, which we find in the multicellular body of many of the lower metazoa, especially in the cnidaria (polyps and corals). Each single neuro-muscular cell is a "unicellular reflex organ"; it has on its surface a sensitive spot, and a motor muscular fibre inside at the opposite end; the latter contracts as soon as the former is stimulated. V. In other cnidaria, notably in the free swimming medusæ--which are closely related to the stationary polyps--the simple neuro-muscular cell becomes two different cells, connected by a filament; an external _sense-cell_ (in the outer skin) and an internal _muscular cell_ (under the skin). In this _bicellular reflex organ_ the one cell is the rudimentary organ of sensation, the other of movement; the connecting bridge of the psychoplasmic filament conducts the stimulus from one to the other.