200 



ZOOLOGY 



The 



parasitic 

 ameboid 

 Protozoa 



Filterable 

 viruses 



The 



disease- 

 causing 

 flagellates 



suffice. Even so, however, they departed little, as a 

 rule, from the primitive protozoan structure, and some 

 of them would hardly be recognized as parasites if taken 

 out of their proper environment. The evolution has 

 been largely physiological, a change in abilities and reac- 

 tions rather than in outward form or obvious structure. 



2. Among the Rhizopods we find ameboid species, 

 known as Entam&ba, inhabiting the alimentary canal. 

 One of these types is the cause of dysentery, a disease 

 especially prevalent and fatal in warm countries. The 

 cause of rabies or hydrophobia, long in doubt, is now be- 

 lieved to be an ameboid protozoan, which establishes 

 itself in the nervous system of the victim. A protozoan 

 has also been connected with smallpox, while the exist- 

 ence of other disease-producing, amiba-like organisms is 

 inferred rather than certainly known. In the case of 

 yellow fever, for example, the virus or organism cannot 

 be seen, nor can it be isolated from a liquid by means of 

 filtration. It belongs to a class of filterable viruses, rec- 

 ognized only by their effects. The list of such disease- 

 producing but invisible creatures is being increased as 

 new observations are made, and from the close analogy 

 between their effects and those due to Protozoa we infer 

 that they probably belong to this group. Possibly some 

 day microscopical technique will be improved suffi- 

 ciently to enable us to see and study the structure of 

 these infinitesimal beings, but at present it appears im- 

 possible to combine an image sufficiently large for vision 

 with adequate illumination. 



3. The flagellate Protozoa or Mastigophora include 

 the genus Trypanosoma, which is the cause of some of 

 the most serious diseases known. Trypanosomes are 

 long, pointed animals, with an undulating membrane 

 along the side, the margin of which extends as a flagel- 



