154 
COMPARATIVE ZOOLOGY. 
CHAPTER XVII.* 
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. Protoplasmic 
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. Protoplasm 
may be extended into projections called pseudopodia , by 
whose contraction the animal may move (Fig. 185). 
Infusoria, and nearly all higher animals, possess cilia 
(Fig. 188). These are short microscopic threads of proto¬ 
plasm (Fig. 2, b) which have the power of bending into a 
sickle-shape and straightening out. As they bend much fast¬ 
er than they straighten, and as they all work 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 locomo¬ 
tion ; they fringe the gills of the Oyster, creating currents 
for respiration ; and they line the passage to our lungs to 
expel the mucus. Flagella (Figs. 187, 189) are a sort of 
long cilia, which are thrown into several curves when ac¬ 
tive, resembling a whip-lash, whence their name. Both 
cilia and flagella seem to be wanting in Artnropods. 
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. No structure 
can, however, be seen in them with the microscope. No 
* See Appendix. 
