152 COMPARATIVE ZOOLOGY. * 
CHAPTER XVII; 
HOW ANIMALS MOVE. 
1. Muscle.—The power of animal motion is vested in 
protoplasm, cilia, and muscles. The simplest forms of 
life, as the structureless Sponge and Ameba, move by 
the contraction and extension of the protoplasm of which 
they consist. In nearly all animals we find with the mi- 
croscope myriads of little hair-like cilia, which are inces- 
santly vibrating. They are seen on the outside of Infu- 
soria, serving as paddles for locomotion; they fringe the 
gills of the Oyster, creating currents for respiration ; and 
they line the passage to our lungs to expel the mucus. 
The cause of ciliary motion is unknown; no muscle or 
nerve has been traced to them. Water seems to be a nec- 
essary condition. 
But muscular tissue is the great motor agent, and exists 
in all animals from the Coral to Man. The power of con- 
tractility, which in the Ameeba is diffused throughout the 
body, is here confined to bundles of highly elastic fibres, 
called muscles. When a muscle contracts, it tends to bring 
Fig. 121.—A Contracting Muscle. 
its two ends together, thus shortening itself, at the same 
time increasing in thickness. This shrinking property is 
excited by external stimulants, such as electricity, acids, 
alkalies, sudden heat or cold, and even a sharp blow; but 
