CHAPTER III 



THE ORGANIZATION OF THE PROTOZOA EXTERNAL FORM 

 AND SKELETAL STRUCTURES 



A UNICELLULAR organism of any kind is a more or less minute mass 

 or corpuscle of the living substance, protoplasm, containing 

 usiially other substances, fluid, solid, or even in some rare instances 

 gaseous, in greater or less amount substances which are either 

 the product of its own vital activity or have been taken up into 

 the body from without. As will be shown in more detail in the 

 next chapter, protoplasm is a substance or complex of substances 

 which, considered in the aggregate, exhibits the physical properties 

 of a viscid fluid. Some samples of protoplasm may be less, others 

 more fluid, but the essentially fluid nature of the whole mass of 

 protoplasm composing the cell-body is very obvious, as a rule, in 

 the case of Protozoa. 



A drop of a fluid substance, when suspended in another fluid with 

 which it is not miscible, tends immediately, under the action of the 

 physical laws of surface-tension, to assume the geometrical form in 

 which the surface is least in proportion to the mass ; that is to say, 

 it tends to become a perfect sphere, except in so far as this tendency 

 may be altered or modified by the contact or pressure of other 

 bodies, or by the operation of other forces or conditions which 

 oppose the action of surface-tension. 



The sphere may therefore be regarded as the primary form of 

 the living cell the form, that is to say, Avhich the organism tends 

 to assume under the influence of physical forces when not checked 

 or inhibited in their operation by other factors. A great many 

 Protozoa exhibit the spherical form in a striking manner, especially 

 those species which float more or less freely in the water, such as the 

 Heliozoa (Fig. 3) and Radiolaria (Fig. 13). But the majority of 

 Protozoa depart more or less widely from the primitive spherical 

 form, for reasons which must be considered in detail. 



In the first place, departure from a spherical form may be merely 

 temporary, the result of vital activity producing altered conditions 

 of surface-tension. In order that a drop of fluid may assume a 

 spherical form as the result of surface-tension, its surface must be 



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