CHAPTER IV. 

 DERIVED ORGANIZATION. TAXONOMIC STRUCTURES. 



Although fundamentally important in vital functions, the 

 various granules and structures which have been described can 

 hardly be regarded as obvious or visible characteristics of Protozoa. 

 Careful study, involving elaborate technical methods, is necessary 

 to reveal the parts they play, and for some, at least, even this has 

 not yet yielded positive results. 



The visible characteristics, those we see upon casual examination 

 with a microscope— form, color, movement, shells, tests, stalks, etc. 

 —are secondary in importance in respect to the ultimate vital activi- 

 ties. It is in connection with these, however, that the Protozoa 

 are best known and the peculiar fascination which they have for the 

 microscopist is mainly due to these obvious features. The outer 

 structures which please the eye, or the motile organoids which cause 

 the fascinating endless variety of movements, represent the out- 

 come or product of the activities going on between the various 

 constituent elements of the protoplasm. Some of them are neces- 

 sary for the continued life of the organism, some are useful in one 

 way or another, but not absolutely necessary, and some, e. g., the 

 scalloped cuirass of Entodinium or the fantastic forms of many 

 sapropelic types, have no obvious reason for being. These structures 

 represent the completed derived organization and furnish the obvious 

 characteristics upon the basis of which the Protozoa are classified. 



In some types of Protozoa, even on superficial examination, it 

 is evident that the aggregate of substances making up the protoplasm 

 is differentiated into an external zone and an internal, medullary 

 part. The external portion is usually called ectoplasm, the inner 

 part endoplasm. The ectoplasm is that part of the protoplasm 

 which comes in direct contact with the environment. It is the 

 part through which food substances must pass into the organism 

 and through which the waste matters of destructive metabolism, 

 as well as undigested food, must be voided to the outside; it is the 

 part which first receives external stimuli of various kinds, and it is 

 the part which gives rise to the more easily visible portions of the 

 locomotor structures, and to the specializations for support and 

 protection. 



Acting thus as a medium of exchange between the living proto- 

 plasm and the external world, the ectoplasm has become modified 



