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COMPARATIVE ZOOLOGY. 
CHAPTER XYII. 
HOW ANIMALS MOVE. 
1. The power of animal motion is vested in protoplasm, 
cilia, and muscles. The power of contractility is one of 
the ultimate physiological properties of protoplasm, like 
sensibility and the power of assimilation. Protoplasma- 
animals, like the Amoeba and Rhizopoda, move by the 
contractility of their protoplasm, as also may the germs 
of higher animals upon the yolk of the egg. The proto¬ 
plasm may be extended into projections called pseudopodia, 
by whose contraction the animal may move (Fig. 186). 
Infusoria, and nearly all higher animals, possess cilia 
(Fig. 188). These are microscopic hairs (Fig. 2, b) which 
have the power of bending into a sickle-shape and straight¬ 
ening out. As they bend much faster than they straight¬ 
en, and as they all w r ork together, they can cause motion 
of the animal, or may serve to produce currents in the 
water, the animal remaining at rest. They are seen on 
the outside of Infusoria, and of very many embryos of 
higher animals, serving as paddles for locomotion; they 
fringe the gills of the Oyster, creating currents for respi¬ 
ration; and they line the passage to our lungs to expel 
the mucus. Flagella (Fig. 189) are a sort of long cilia, 
which are thrown into several curves when active, resem¬ 
bling a whip-lash, whence their name. Both cilia and fla¬ 
gella seem to be wanting in Arthropods. 
The cause of ciliary motion is unknown. Their one¬ 
sided contraction is their property, as the straight con¬ 
traction of the muscle-fibre belongs to it. Ho structure 
can, however, be seen in them with the microscope. Ho 
