302 



MOVEMENT IK LOWER ANIMALS 



the boat is tied. In such case, the result is that the 

 water, and not the boat, is moved. In many of the lower 

 animals the digestive cavities are lined with such ciliated 

 cells, and so the currents of water which they create 

 cause the food, floating in the water, to be brought within 

 reach by the currents set up. In our bodies, the walls 

 that line the bronchial tubes are 

 lined with such ciliated cells, and 

 here they serve to free the surface 

 of the membrane from particles of 

 dust which might produce harm if 

 they were allowed to remain. In 

 fact, it is only in the case of the 

 protozoans and a few of the young 

 stages of the lower animals that cilia 

 are used to move the body of the 

 animal itself. In all other forms 

 their function is the setting up of 

 currents or the removal of particles. 

 Cilia and flagella are to be con- 

 sidered, then, as merely a modified portion of protoplasm 

 in which the power of rhythmic contraction and expan- 

 sion has been specially developed. They are chiefly 

 interesting, therefore, as indicating the manner in which 

 protoplasm can become developed into a motile organ. 



Muscular movement. — Muscle cells differ from ordinary 

 cells in that they contain certain modifications of pro- 

 toplasm called fibrillse. These fibrilla; are thin threadlike 

 fibers embedded in the fluid protoplasm of the cell, and may 

 be variously arranged as to direction, but are always par- 

 allel to one another. In certain instances these fibrillsB 

 are divided into segments by cross lines or strise, and these 



Fig. 135 — Ciliated epl- 

 tlielial cells liulug the 

 bronchial tubes; a, basal 

 membrane ; 6, spherical 

 epithelial cells; c, cili- 

 ated cells ; rf, cilia. 



