INTRODUCTION 19 



other parts of the animal for fulfilment of their vital activities while 

 every cell of a colonial protozoon may be self-sufficient and inde- 

 pendent, and differentiation among them is limited, at most, to 

 reproductive and somatic cells (e. g., Epistylis, Zoothamnium and 

 other vorticellids) . 



While the single protozoon is to be compared structurally with a 

 single isolated unit tissue cell of a metazoon as a bit of protoplasm 

 differentiated into cell body, or cytoplasm, and nucleus, it is a 

 very different unit physiologically. In its vital activities it should be 

 compared, not with the unit tissue cell, but with the entire organism 

 of which the tissue cell is a part. All animal organisms perform 

 the same fundamental vital activities of nutrition, excretion, irri- 

 tability with movement and reproduction, which are fundamental 

 attributes of living animal protoplasm. In the higher types of 

 Metazoa these primary activities are performed by complex organ 

 systems, nutrition for example, involving not only the digestive 

 system but the muscular, nervous, circulatory and respiratory 

 systems as well. Each organ has its particular part to play in the 

 economy of the whole and each cell is differentiated for the purpose 

 of its specialized function. Tissue cells, therefore, are physiologic- 

 ally unbalanced cells since they are preeminently specialized for 

 secretion, or contraction, or irritability, etc. Division of labor in 

 a physiological sense here reaches its highest expression. 



In the lower Metazoa the organ systems are less highly special- 

 ized; fewer organs are present to perform the same fundamental 

 vital activities and the tissue cells have relatively more kinds of 

 work to do for the organism as a whole. Thus the supporting and 

 covering cells of a coelenterate combine the functions of respiration, 

 irritability, muscular contraction, excretion and circulation with 

 the primary functions of an epithelium. Each of them is more 

 nearly balanced physiologically than a single cell of the higher 

 types, but it still needs the activities of other cells, and the organism 

 is again the sum-total of all its cellular parts. 



In the protozoon, finally, we find a cell which is physiologically 

 balanced ; it is still a cell and at the same time a complete organism 

 performing all of the fundamental vital activities within the con- 

 fines of that single cell. Whitman, in his essay on "The Inadequacy 

 of the Cell Theory" (1893), clearly expressed the inconsistencies in 

 the common use of the designation "cell" for this variety of struc- 

 tures, and later writers, notably Gurwitsch (1905) and Dobell (1911), 

 have followed in a similar vein. 



As organisms the Protozoa are more significant than as cells. In 

 the same way that organisms of the metazoan grade are more and 

 more highly specialized as we ascend the scale of animal forms, so 

 in the Protozoa we find intracellular specializations which lead to 

 structural complexities difficult to harmonize with the ordinary 



