CHAPTER XI 

 SYSTEMATIC REVIEW OF THE PROTOZOA : THE SARCODINA 



As stated in Chapter I., the Protozoa are commonly divided into 

 four principal classes. Of these, two namely, the Sarcodina and 

 Mastigophora may be regarded as the more primitive groups, 

 comprising- the main stock of less specialized and typical forms 

 from which the other two classes have been evolved. The Sporozoa 

 are an assemblage of exclusively endoparasitic forms exhibiting 

 clearly the modifications and adaptations induced by, or necessary 

 for, their particular mode of life ; and it is practically certain that 

 the Sporozoa are not a .homogeneous class showing mutual affinities 

 based upon a common ancestry, but that one section of the group 

 is a specialized offshoot of the Mastigophora, the other of the 

 Sarcodina, and that the two sections are united only by characters 

 of convergence due to the influence of a similar mode of life. The 

 Infusoria, on the other hand, are a specialized group in which great 

 complexity of organization has been attained ; they are the highest 

 class of the Protozoa, and furnish examples of the most extreme 

 degree of structural differentiation of which a unicellular organism 

 is capable. 



While there is but little difficulty, as a rule, in defining the classes 

 Sporozoa and Infusoria, or in assigning members of these groups 

 to their proper systematic position, the case is different, very often, 

 when we have to deal with the other two classes. The verbal 

 distinction between them is based chiefly on the use of the word 

 " adult ": Sarcodina are Protozoa which have no permanent organs 

 of locomotion in the adult condition, but move by means of pseudo- 

 podia extruded from the naked protoplasmic body ; Mastigophora, 

 on the other hand, bear organs of locomotion in the form of flagella 

 in the adult condition, whether the protoplasmic body is naked 

 and amoeboid or corticate and of definite form. In both classes the 

 youngest stages may be flagellate ; if, in an amoeboid form, the 

 flagella are retained in the adult, the organism is classed in the 

 Mastigophora ; if lost in the Sarcodina. 



The word " adult " when applied to the Metazoa has a meaning 

 which can be defined clearly, as a rule, by the criterion of sexual 



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