xi THE ELEMENTS OF STRUCTURE 193 



cesses occur at different parts of the body, are discharged 

 by special sets of cells, among which the labour of life 

 has been divided. The life of the Protozoon is like that 

 of a one-roomed house which is at once kitchen and 

 work-room, nursery and coal-cellar. The life of the 

 Metazoon is like that of a mansion where there are special 

 rooms for diverse purposes. 



Protozoa multiply by dividing into two daughter-units 

 (fission), or by budding, or by dividing repeatedly within 

 a cyst or envelope so that numerous minute units result 

 (spore-formation). Apart from these modes of multi- 

 plication, there is a frequent recurrence of conjugation, 

 which is remotely analogous to fertilisation in higher 

 animals. Two Protozoa, equal or unequal in size, 

 similar or dissimilar in appearance, may unite to become 

 one (total conjugation), or two apparently similar in- 

 dividuals may be closely apposed, exchange portions of 

 their nucleus and separate again (partial conjugation). 

 This has been much studied in ciliated Infusorians. 



In having no " body " the Protozoa are to some extent 

 relieved from the necessity of death. Within the com- 

 pass of a single cell they perform a crowd of functions, 

 but wear and tear are often made good again, the units 



._ 



have great power of self -recuperation. They may, in- 

 deed, be crushed to powder, and they lead no charmed 

 life safe from the appetite of higher forms. But these 

 are violent deaths, and the same term maybe applied to 

 the huge mortality among open-sea Protozoa, which 

 seem often to be killed in enormous numbers by sudden 

 changes of temperature and the like. What Weismann 

 and others have insisted on is that the unicellular Protozoa, 

 in natural conditions, need never die a natural death, 

 being in that sense immortal. It is true that a Protozoon 

 may multiply by dividing into two or more parts, but 

 only a sort of metaphysical individual it y is thus lost, 

 and there is nothing left to bury. 



In connection with this " immortality of the Protozoa ' 

 it should be noticed that an isolated family of Infusorians 

 multiplying asexually (by dividing into two) for hundreds 

 of successive generations usually comes to an end after 



14 



