114 THE KIDDLE OF THE UNIVEKSE. 



III. — Other organisms which float in water — for 

 instance, many of the radiolaria, siphonophora, kteno- 

 phora, 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 infu- 



soria, 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 



