Physiology and Biology of the Protozoa. 491 



favourable conditions, and it seems desirable, for the preser- 

 vation of the species, to produce a great number of individuals 

 very rapidly. At the close of these hurried divisions a period 

 of conjugation would then occur, and this, as is well known, 

 has always been observed in very small individuals. If the 

 last-mentioned mode of increase were the sole one, we should 

 always observe in every colony of Infusoria a diminution of 

 the individuals combined with an increase of their number, 

 and an equally regular cyclical recurrence of the period of 

 conjugation. But every one who has been long occupied with 

 the Infusoria knows that this is not the case ; and it is par- 

 ticularly well known to those who have for a lono- time 

 observed colonies rich in individuals and constantly increasing, 

 and sought in vain for conjugation-states, which at other times 

 had been present in great numbers. 



I must not, however, develop these ideas any further, for, 

 as already remarked, the empirical facts from which they pro- 

 ceed still stand on too weak a foundation, and 1 will rather 

 wait until time and opportunity may furnish me with suitable 

 material for working further upon them. 



On the Nervous System of the Infusoria. 



In my experiments with Stentors my attention has been 

 called to a question which I would here briefly touch upon, 

 namely, what may be the nature of the nervous elements iu 

 the cell-body of the Infusoria ? Some light is thrown upju 

 this by the behaviour of the Infusoria during conjugation^uid 

 spontaneous division, as I have already indicated in my pre- 

 liminary communication above referred to. Thus if we 

 observe a pair in copula or an Infusorian engaged in division 

 in which the two halves are not yet completely separated, one 

 is struck with the fact that these animals move exactly like a 

 single individual, that both of them make exactly concor- 

 dant movements so long as they are still united by a bridge 

 of protoplasm. I have frequently traced this in different 

 species of Infusoria; but here, again, the Stentors are particu- 

 larly adapted to the purpose, as the movements are so dis- 

 tinctly to be detected under the microscope in the great 

 peristomial cilia. So long as the two daughter-individuals 

 are united by even the thinnest thread of protoplasm (tig. 14) 

 they behave precisely like a single individual ; if the peri- 

 stomial cilia of the anterior half strike forward, so also do 

 those of the posterior, and at the same moment in which the 

 former, from any cause, change the direction of their motion, 

 so also do the latter. The natation is therefore perfectly 



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