18 BIOLOGY OF THE PROTOZOA 



which have been the stepping stones in this evolution 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 distinguish with strict accuracy either of these groups from the 

 others. Haeckel (1866) undertook to avoid the difficulty by com- 

 bining 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 treatises on Protozoa, some types 

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

 and on Bacteria. In this connection the greatest difficulty lies in 

 the separation of one group of the flagellated Protozoa from the 

 unicellular algae. We are still tied firmly to the old tradition that 

 animals move and plants are quiescent, and a chlorophyll-bearing 

 organism, if actively moving, is ipse facto an animal. Were I to 

 advocate this as the main distinction between animals and plants, 

 there would be, undoubtedly, a storm of protests from all biologists. 

 And yet, what other characteristics do chlorophyll-forming organ- 

 isms have to justify us in claiming them as animals? At the present 

 time there is a double taxonomic system, one botanical, the other 

 zoological for these questionable forms, and these systems are 

 widely different. We can avoid the resulting confusion by adopting 

 one or the other system of classification. My own conviction is 

 that zoologists should follow the historical precedent furnished in 

 the last century by the elimination from Protozoa of filamentous 

 algae, desmids and diatoms, and now transfer to the botanists the 

 entire aggregate of so-called Protozoa in which the ability to form 

 chlorophyll is a characteristic. (See also p. 412.) 



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 some questionable cases, e. g., Mago- 

 sphaera, these colonial aggregates resemble tissues of Metazoa in 

 their structural appearance, but tissue cells are dependent upon 



