28 CELLS AND EXTRACELLULAR ELEMENTS. 



of active vibratory or undulatory movements. They are 

 composed of protoplasm like that of the cell-body, and their 

 motility is of the same nature as that of protoplasm generally. 

 When they are comparatively short and thickly crowded on 

 the surface, or a part of the surface, of the cell, they are called 

 cilia; when they are long and occur singly or only in small 

 numbers on the cell, they are called flagella. They occur not 

 only on low organisms, like bacteria and infusoria, but are of 

 wide distribution and great importance in the human body; 

 e. <?., where cilia occur extensively on ciliated epithelium, and 

 flagella on the spermatozoa. 



The motion of cilia is vibratory, and, as they act in unison, 

 a wave-like motion results, which, acting from a fixed surface, 

 as in mammalia, carries particles along the surface or creates 

 currents in the fluids adjacent. 



The motion of flagella is undulatory, and when attached to 

 free cells this movement rapidly propels the cells through the 

 fluid in which they occur, acting in the same manner as the 

 tail of a tadpole or fish. Flagella therefore are a means for 

 rapid locomotion of spermatozoa, monads, and other free- 

 swimming cells provided with them. 



Other cell- processes : Besides motile processes like those just 

 described, some cells possess permanent and non-motile pro- 

 jections, which may be very various in form and size, coarse 

 or fine, long or short, branching or very irregular. Good ex- 

 amples are furnished by nerve-cells, bone-cells, connective- 

 tissue cells. 



Forms of cells : Cells vary widely in shape, according to 

 the situation in which they occur and the purposes they serve. 

 The typical shape is perhaps spheroidal or oval. The leuko- 

 cytes, which are typical active cells, are spherical when at 

 rest. Many cells which when mature are of diverse shapes 

 are spheroidal when young. In a differentiated multicellular 

 organism there is a great diversity of form in different locali- 

 ties. On exposed surfaces they become flattened or disk- 

 shaped ; in other situations they are spheroidal, or columnar, 

 or elongated, or branching, sheath-like, or tubular, or alto- 

 gether irregular. But however diverse their form, their 

 fundamental type is identical throughout ; they are all definite 

 protoplasmic bodies, usually nucleated. 



