18 BIOLOGY OF THE PROTOZOA 



which have been the stepping stones in this evohition have been 

 and are still in progress among all types of unicellular things, so 

 that no artificial definition of Bacteria, of Protozoa, or of Algae will 

 accurately distinguish either of these groups from the others. 

 Haeckel (1866) undertook to avoid the difficulty by combining all 

 unicellular forms under the common name Protista, but this is, 

 obviously, only another name for the aggregate and an artifice for 

 concealing the real difficulties which we should like to overcome. 

 Minchin (1912), on the ground of structural characters, would 

 distinguish Protozoa from Bacteria by the assumption that the 

 latter are not of "cellular grade" because of the absence in many 

 Bacteria of a typical cell nucleus. Here again, however, the old 

 difficulty shows its head for in this sense, many well-recognized 

 Protozoa are not, while many Bacteria are, of cellular grade (see 

 Dobell, 1911). The problem after all has mainly an academic inter- 

 est, and the chief practical value to be gained by its solution would 

 be to set the limits of a text-book or monograph. We may reason- 

 ably expect to find therefore, in an^' treatise on Protozoa, some types 

 which with equal right should be included in works on lower plants 

 or on Bacteria. 



It is less difficult to distinguish between Metazoa and Protozoa; 

 the occurrence of a gastrula stage in the development of a question- 

 able form is sufficient to place it unmistakably with the higher 

 animals. Protozoa, indeed, are often associated in cell aggregates 

 called colonies, the individual cells being held in place by proto- 

 plasmic connections, by stalk attachments, or by fixation in a com- 

 mon gelatinous matrix. In many cases these colonial aggregates 

 resemble tissues of metazoa in their structural appearance, but tissue 

 cells are dependent upon other parts of the animal for fulfilment 

 of their vital activities while every cell of a colonial protozoon may 

 be self-sufficient and independent, and differentiation among them 

 is limited, at most, to reproducti\'e and somatic cells {e. g., Volvox 

 globator, Pleodorina illinoisensis, and their close relatives). 



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 



