THE PROTOZOA THE DAWN OF LIFE 27 



cilia, arranged in longitudinal rows, and these cilia are kept in an 

 incessant to-and-fro vibration by means of which the Infusorians 

 move about and obtain their food. The cilia in the neighbourhood 

 of the buccal groove produce currents in the water, in which 

 minute particles of organic matter are caught and carried down 

 the gullet into the soft protoplasm, where they become surrounded 

 by a globule of water or " f ood-vacuole " ; they circulate through 

 the protoplasm, and the soluble parts become gradually digested 

 and assimilated ; the insoluble and effete matters are ejected at 

 a definite anal spot. 



Several of these " food-vacuoles " may be seen in the body 

 of the Slipper-animalcule, and also two large contractile vacuoles, 

 one at each end of the body, which keep up a regular pulsation 

 of expansion and contraction. There are two nuclei, one a com- 

 paratively large ovoid body, called the meganucleus ; the other, 

 closely applied to the meganucleus, is a small rounded body, and 

 is called the micronucleus. 



Multiplication in the Slipper-animalcule takes place by trans- 

 verse fission, the nuclei dividing prior to the division of the body. 

 This process of multiplication, however, cannot be continued in- 

 definitely, and there comes a time when the Paramcecium must 

 conjugate with another of its species or perish. In this process, 

 two Paramcecia become applied by their ventral surfaces, but do 

 not actually fuse together ; then their meganuclei break up and 

 disappear, while an interchange of the micronuclei of the con- 

 jugating individuals takes place, with the result that each develops 

 a new mega- and micro-nucleus, partly from the substance of its 

 own micronucleus, partly from that of its partner. 



It will be observed that in Paramcecium we have something 

 more complex than a mere sexual cell, for of its whole body it is 

 only a very small part, the micronucleus, which functionally 

 conjugates; we therefore find an advance in the complexity of 

 the individual and towards the differentiation of sex, for the 

 micronucleus is only to be compared with the nucleus of the sexual 

 cells of the, Metazoa. Gradually, step by step, we may trace the 

 evolution of the sexes. In the free-swimming Ciliata, such as 

 the Paramcecium we have just been examining, there appears to 

 be no distinction of sex into definite male and female form, all the 

 individuals in which conjugation is taking place being of exactly 



