22 ESSENTIALS OF ZOOLOGY 



latter state the Protozoon is subject to rhythms of growth, 

 encystment and reproduction. Apart from the encystment 

 and the desiccation, it is constantly working for a living, and 

 under normal conditions it is found in so many diverse situa- 

 tions, with reference to depth, light, and other factors, that we 

 are led to conclude that it is not merely the victim of circum- 

 stance but is volitional. It is attracted or repelled by heat, 

 cold, and other influences. It moves, however, not only in 

 response to external stimuli, but of its own accord, and even 

 if the feeling be only a dim, incipient one, chemical and 

 physical changes communicated by the cell sap, we might be 

 inclined to say that Vorticella is aware when it is hungry and 

 when it is contracted or expanded, and that all are conscious 

 of their periods of movement and rest. 1 



These considerations apply to the individual. There are 

 other studies which concern the race, the assemblage of the 

 individuals which constitute a more or less complete group. 



Reproduction. It is interesting to find among the 

 Protozoa evidences of sex. After a number of ordinary cell 

 divisions a change takes place which is expressed in the 

 tendency to fuse in pairs. It is still more striking when the 

 conjugating cells are different in size, as in Vorticella. The 

 cell after the conjugation passes through a number of phases 

 of growth and division, and observation goes to show that 

 normally the number is approximately the same for each 

 species, and when this number has been accomplished no 

 further multiplication in this manner can take place until 

 conjugation has been effected. It is evident, for one thing, 

 that the fusion occurs by a chemical attraction between the 

 gametes. If the chemical difference is initiated at the first 

 division after conjugation and becomes more and more intense 

 at the subsequent divisions, and finally becomes sexual, then 

 about an equal number of male and female gametes will occur. 



The phenomena of division and conjugation have been 

 followed for the most part in certain Ciliates and in parasitic 

 Protozoa, and it is evident from the succession of events that 

 the nucleus is the centre of the changes which occur. In 



1 1904. Jennings, Behaviour of Lower Organisms. Carnegie Institution 

 of Washington. 



