CHAPTER III. 

 STRUCTURAL DIFFERENTIATIONS. 



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 stiKh', 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 ceaseless activities going on between the 

 various constituent elements of the protoplasm. Some of them 

 are necessary for the continued life of the organism, some are useful 

 in one way or another, but not absolutely necessary, and some, 

 e. (]., the scalloped cuirass of Entodinium, have no obvious reason 

 for being. 



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 

 ])art 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 gi\es 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 

 in ways that would be impossible for the endoplasm. In simple 

 cases, as, for example, in Amoeha proieus, it is not strikingly differ- 

 ent from the endoplasm, but in other cases it becomes a complex of 



