CHAPTER XX* 



HOW ANIMALS MOVE 



i. The power of animal motion is vested in proto- 

 plasm, cilia, and muscles. The power of contractility 

 is one of the fundamental physiological properties of 

 protoplasm, like sensibility and the power of assimila- 

 tion. Protoplasmic animals, like the amoeba and Rhiz- 

 opoda (Figs. I, 213), move by the contractility of their 

 protoplasm, as also may the embryos of higher animals 

 upon the yolk of the egg. Protoplasm may be extended 

 into projections called pseudopodia, by whose contrac- 

 tion the animal may move. 



Infusoria, and nearly all higher animals, possess cilia 

 (Figs. 9, 1 1 ). These are short microscopic threads of 

 protoplasm which have the power of bending into a 

 sickle shape and straightening out. As they bend much 

 faster 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 embryos of very many higher animals, serving as 

 paddles for locomotion ; they line the channels in the 

 gills of the oyster, creating currents for respiration ; 

 and they cover the walls of the passages to our lungs to 

 expel the mucus. Flagella (Figs. 4, 5, 6) are a sort of 

 long cilia, which are thrown into several curves when 

 active, resembling a whiplash, whence their name. Both 

 cilia and flagella seem to be wanting in arthropods. 



* See Appendix. 

 363 



