14 PROTOZOA AND DISEASE 



conclusion that certain bodies I have described in sarcoma and other 

 pathological tissues probably belonged to this group. 



Whatever information there may be with regard to the suctoria 

 proper is not made very prominent in such text-books of biology as 

 I have at hand. I have read that suctorians living within the nucleus 

 of some other protozoon have been mistaken for and described as 

 new and peculiar nuclear structures, and when the mistake has been 

 pointed out, there has been some amusement, but, as far as I am 

 aware, no prominence has been given in biology to the study of the 

 features that distinguish intranuclear suctoria from nucleoli. 



The ordinary mode of reproduction in ciliates, as mentioned 

 in Part I., is by mitotic division into two. It was shown by 

 Maupas in 1889 that, after a certain number of such divisions, 

 conjugation with mutual interchange of nuclear elements must 

 occur if senile degeneration and death are to be prevented. This 

 mode of life has been compared to that of the metazoa a sexual 

 phase followed by a long series of asexual divisions being likened to 

 the impregnation of metazoan ovum, followed by the long series of 

 somatic cell-divisions that build up and maintain the metazoan body. 

 There is one important difference between the life of this protozoon 

 and that of a metazoon. Apart from accidents, there is no necessary 

 death of individual infusorians, whilst in the metazoa individual 

 death is a necessity. This distinction, however, is not absolute, as has 

 been shown by Calkins, who concludes : ' There is a fundamental 

 difference in the protoplasmic elements which go to make up 

 the body of a protozoon, one of which is to be compared with the 

 somatic cells of metazoa, the other with germ-cells : the one con- 

 nected with vegetative functions of metabolism, the other with repro- 

 duction ; the one may give out, and so lead to " physiological death " 

 (Hertwig), or it may be restimulated ; the other may give out, and 

 so lead to " germinal death " of the race.' 1 



Encystment. Ciliates have also the property of secreting a 

 chitinous envelope in various circumstances, whether as a pre- 



1 G. N. Calkins, ' Studies on the Life-History of Protozoa,' Journal of Experi- 

 mental Zoology, vol. i., No. 3, 1904. 



