PROTOPLASMIC MOVEMENTS 23 



the source of energy probably lies in the chemical processes 

 which are transpiring all of the time and in all parts of the 

 protoplasmic substance. In another type of movement, termed 

 ciliary movement, the main liberation of energy is apparently 

 confined to one region of the cell and to specialized parts of the 

 protoplasm of that cell. Manifestations of the liberated energy 

 are expressed solely by such specialized portions or by out- 

 growths from them. These outgrowths, known as flagella and 

 cilia, are minute whip-like processes of the cell which undulate 

 in the surrounding medium or lash it like an oar. Flagella are 

 usually single or at most, few in number, but cilia are numerous 

 and their beating moves the cells with considerable rapidity 

 if they are free, or creates currents in the surrounding medium if 

 the cells are fixed. Cilia play an important part, sometimes 

 as in protozoa and larval forms of invertebrates, in locomotion, 

 sometimes as in the ciliated cells of various ducts, for creating a 

 current in the surrounding medium. Thus the ciliated cells of 

 the trachea, sweep particles of dust, mucus, etc., to the outside. 

 For this purpose the stroke of the cilia is upward, and is much 

 stronger than the recovery. Flagella have an entirely different 

 type of motion, acting with a cork-screw or sculling move- 

 ment. These are rarely found in higher animals save as the 

 motile organs of spermatozoa, but in the unicellular animals 

 they are common. 



Muscular Contraction. The most highly specialized type of 

 movement of living protoplasm is undoubtedly muscular 

 contraction. This is limited to special cells of fibrous nature 

 which are usually bound together in bundles thus forming 

 muscles. Upon irritation a stimulus is transmitted by a nerve 

 to the muscle cells and contraction results. In this contraction 

 the bulk of the muscle cell remains the same but the form changes, 

 the muscle bundle becoming shorter and thicker (Fig. n). 

 Muscular action is usually the sole means of locomotion from 

 place to place in higher animals at least, and they usually 

 connect some movable joint with a fixed part of the skeleton. 

 The majority of muscles are under the control of the organism 

 and can be moved at will, these, the voluntary muscles, are 



