II. Many protists, particularly unicellular algæ of the groups of diatomacea and desmidiacea, accomplish a kind of creeping or swimming motion by _secretion_, by ejecting a slimy substance at one side. III. Other organisms which float in water--for instance, many of the radiolaria, siphonophora, ktenophora, and others--ascend and descend by altering their _specific gravity_, sometimes by osmosis, sometimes by the separation or squeezing-out of air. IV. Many plants, especially the sensitive plants (mimosa) and other papilionacea, effect movements of their leaves or other organs by _change of pressure_--that is, they alter the strain of the protoplasm, and, consequently, its pressure on the enclosing elastic walls of the cells. V. The most important of all organic movements are the _phenomena of contraction_--_i.e._, changes of form at the surface of the organism, which are dependent on a twofold displacement of their elements; they always involve two different conditions or phases of motion--contraction and expansion. Four different forms of this plasmatic contraction may be enumerated: (_a_) Amoeboid movement (in rhizopods, blood-cells, pigment-cells, etc.). (_b_) A similar flow of protoplasm within enclosed cells. (_c_) Vibratory motion (ciliary movements) in infusoria, spermatozoa, ciliated epithelial cells. (_d_) Muscular movement (in most animals). The elementary psychic activity that arises from the combination of sensation and movement is called _reflex_ (in the widest sense), reflective function, or _reflex action_. The movement--no matter what kind it is--seems in this case to be the immediate result of the _stimulus_ which evoked the sensation; it has, on that account, been called stimulated motion in its simplest form (in the protists). All living protoplasm has this feature of irritability. Any physical or chemical change in the environment may, in certain circumstances, act as a stimulus on the psychoplasm, and elicit or "release" a movement. We shall see later on how this important physical concept of "releasing" directly connects the simplest organic reflex actions with similar mechanical phenomena of movement in the inorganic world (for instance, in the explosion of powder by a spark, or of dynamite by a blow). We may distinguish the following seven stages in the scale of reflex action: I. At the lowest stage of organization, in the lowest protists, the stimuli of the outer world (heat, light, electricity, etc.) cause in the indifferent protoplasm only those indispensable movements of growth and nutrition which are common to all organisms, and are absolutely necessary for their preservation. That is also the case in most of the plants.