154 COMPARATIVE ZOOLOGY. 



CHAPTEK 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. 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, J) 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 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 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. No structure 

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



