PHYLUM PROTOZOA 55 



tion, some consider the Sarcodina as the most primitive class of 

 the phylum. It remains as a possible explanation that the 

 Sarcodina represent the simplifying effects of a regressive 

 evolution. In their vegetative stages, the Sporozoa show con- 

 vincing evidences of rhizopod affiliations through the presence 

 of pseudopodia and in their general organization. Sporozoa 

 probably had their rise through some ancestral form of our 

 present-day Sarcodina. 



In the Ciliophora is encountered the highest morphological 

 and physiological differentiation found in Protozoa. Without 

 doubt this subphylum stands at the apex of protozoan evolution. 

 The Suctoria have been given off as a side branch from the main 

 ciliate stem, as evidenced by the fact that suctorians, in their 

 development, pass through a ciliated larval stage. 



Questionable Protozoa. — In discussing the Mastigophora, 

 attention was called to the fact that many of the Phytomastigina 

 possess characters which point to plant affinities as well as animal 

 relationships. The problem of assigning such forms to either 

 plant or animal kingdom depends upon establishing definite 

 criteria for separating these two kingdoms. The zoologist con- 

 fronts a much more difficult task when he comes to consider some 

 of the extremely minute bodies which have been named as pos- 

 sibly the causal agents of certain diseases. In some such 

 instances there is yet no ground for proof that the bodies under 

 discussion are actually hving cells. Thus the Negri bodies 

 found in the nervous system of animals suffering from hydro- 

 phobia are thought by some to be degeneration products of 

 diseased cells, while others believe them to be protozoan cells. 

 Similarly, for many diseases, minute bodies which in their 

 behavior resemble Protozoa more closely than bacteria have been 

 observed, though they defy actual assignment to any known 

 group of the Protozoa. The bodies associated with Rocky 

 Mountain spotted fever, trench fever, and typhus fever, so very 

 minute that they pass through filters, are considered as Protozoa 

 by some workers but there is no general agreement as to their 

 classification. 



As pointed out previously, the spirochaetes have been shuffled 

 back and forth between the Protozoa and the bacteria because 

 of the lack of decisive characters that would compel them to be 

 lined up with one group or the other. This problem of the rela- 

 tionships of the lower plants and animals is far from settled. 



