64 



PRINCIPLES OF ANIMAL BIOLOGY 



Flagella may beat regularly in one plane, or they may have a rotary 

 motion which may involve the whole flagellum or only the free end of it, 

 or the movement may be irregular, involving any portion of the flagel- 

 lum. Because of the diversity of these movements, a diversity which 

 may be observed within a few minutes in the same specimen, it seems 

 clear that the structure of the flagellum is not the same as that of the cilium. 

 It has been shown for Euglena (Fig. 31) that the flagellum is composed 

 of an elastic peripheral layer within which are several contractile threads 

 extending throughout the length of the flagellum. The movement is 

 due to the contraction of these threads.. The return stroke is due to 

 the elasticity of the outer layer or to the contraction of certain of the 

 threads, or both of these factors may aid in the production of certain 

 types of movement. The movement of the organism when propelled 



A 



Fig. 31. — Euglena. A, outline showing flagellum {fl). B, the fibrils of the flagellum. 

 original; B after Dellinger in Journal of Morphology.) 



(A 



by one or more of these organelles is jerky or erratic. There is rarely 

 the gliding movement which is characteristic of animals which employ 

 ciha in locomotion. Flagella are the normal organs of locomotion in 

 the Mastigophora or Flageflata, one group of the Protozoa. They are 

 also the organs of locomotion in some plant cells. Flagella occur rarely 

 as cell organs in higher forms. They occur singly on the endoderm cells 

 of the Ccelenterata, and in Porifera (sponges) each cell of the radiating 

 tubes (see Fig. 32) is provided with a flagellum and a protoplasmic collar 

 surrounding the base of the flagellum. In these two groups of metazoan 

 animals the flagella serve to create currents of water which circulate 

 through the cavities of the animals. Cilia occur much more frequently 

 as cell organs than do flagefla, for in almost every group of animals some 

 species may be found which have ciliated surfaces. Both ciha and 

 flagella are protoplasmic structures whose functioning depends upon the 

 contractility of protoplasm. 



