PROTOPLASMIC MOVEMENTS 23 



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 or 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 thus play an important part, some- 

 times as in protozoa and larval forms of invertebrates, in loco- 

 motion, sometimes as in the ciliated cells of various ducts, in 

 creating currents in the surrounding media. 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 

 movement. These are rarely found in higher animals save as 

 the motile organs of spermatozoa, but are characteristic of many 

 unicellular animals and of many plant zoospores and motile 

 gametes. 



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, in higher animals at least, is usually the 

 sole means of locomotion from place to place; in these animals 

 the muscles usually connect movable joints with fixed parts of 

 the skeleton. The majority of muscles are under the control 

 of the organism and can be moved at will. These, the volun- 



