PARAMCECIUM, 91 



surface there slopes backwards a shallow cone-shaped 

 ^ ^. depression, the vestibule. It is lined by 

 special cilia, which cause food-currents. At 

 its base or inner end is an opening into the inner fluid 

 protoplasm, which is the permanent mouth or ingestive 

 aperture. Food-particles, usually microscopic algae, are 

 driven by the ingestive cilia down the vestibule and 

 through the mouth into the interior of the body. Here 

 they are then digested and their residua are egested, 

 but they appear to follow a definite course within the 

 organism, first towards the posterior end, then forwards 

 dorsally and backwards along the ventral surface, to 

 be eventually extruded or egested at a special spot just 

 behind the mouth. There appears to be no permanent 

 opening or anus, but the temporary anus is always formed 

 at the same spot. 



Under the dorsal surface and lying towards the anterior 

 and posterior ends of the animal there are two contractile 

 vacuoles which do not differ essentially from the single one 

 found in Aino^ba. About the centre of the animal there lies 

 a large oval macro-nucleus, in close contact with which 

 there is a small micro-nucleus. 



There are no definite organs of respiration, sense-organs, 

 nor nervous system. 



Growth in Pararnoeciiwi is succeeded by binary fission 

 into two equal parts by an oblique division, but sooner or 

 later the process of conjugation must intervene in order 

 that hfe may be maintained. 



We have already dealt with the general phenomenon of 

 conjugation (see Chapter V.). Paramcecium proceeds 

 normally in conjugation. During this pro- 

 cess two individuals are in close contact 

 along their ventral surface, their protoplasm becoming 

 continuous through their mouths. The essential changes 

 are as follows : — 



(i) In each individual the macro-nucleus breaks up 

 and disintegrates, to be thrown out or absorbed, and 

 the micro-nucleus grows rapidly and then divides by 

 two rapid divisions into four, two of these pieces being 

 absorbed. Thus by these processes the macro-nucleus 

 and micro -nucleus are now reduced to two fragnients 



