PROTOZOA 145 



characteristics; the larger stationary macrogamete is the 

 female. Vorticella is not as specialized in this particular 

 as some of the flagellates and sporozoans (Volvox, page 140; 

 Plasmodium, page 141). 



GENERAL FACTS ABOUT PROTOZOA 



Protozoa are remarkable among animals because their 

 entire bodies consist of single cells. Some of them are the 

 most complex animal cells known, others are the most 

 simple. Metabolism in many protozoans is extremely rapid. 

 One species, Didinium nasutum, can devour other proto- 

 zoans that are ten times its own size, the body becoming 

 stretched so that it forms a thin covering for the food, yet in 

 an hour or two it is ready to eat another meal of similar 

 size. 



It is often said that "the Protozoa are immortal," and 

 this may indeed be true of most species. When a protozoan 

 cell is about to die of old age, it may become young again by 

 conjugation or by fusing with another cell, but must, of 

 course, sacrifice its identity by so doing. This is not true, 

 however, of protozoans in which there is a differentiation 

 into somatic (body) and germ (reproductive) cells, (Volvox, 

 Plasmodium). In these, as in many-celled animals, the 

 "body" must die and only the germ cells may help to form 

 a new generation. The statements in regard to the necessity 

 for conjugation in order to attain youth would probably not 

 be accepted by Professor L. L. Woodruff, who has reared a 

 ciliate (Paramcecium) for more than five thousand genera- 

 tions without conjugation. In a recent paper (1916) he 

 says: "an individual of Paramoecium is self-sufficient to 

 reproduce indefinitely without recourse to conjugation." 

 If Woodruff's point of view is accepted, it may be necessary 

 to revise completely the current ideas in regard to the pro- 

 duction of youth, or it may be found that only a few proto- 

 zoans are able to multiply indefinitely by fission without 

 dying of old age. Perhaps the latter alternative may be 

 looked upon as the more probable for among many-celled 



