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, h) 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 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. Ifo structure 

 can, however, be seen in thevn with the microscope. No 

 * See Appendix. 



