ii PHYLUM PROTOZOA 91 



become applied by their ventral faces (Fig. 70, A), but do not fuse. 

 The meganucleus (mg. nu.) of each breaks up into small masses, 

 which disappear, being apparently absorbed into the protoplasm. 

 At the same time the micronucleus (mi. nn.) of each divides, 

 each product of division immediately dividing again, so that each 

 gamete or conjugating body is provided with four micronuclei (B). 

 Two of these (mi. nu.' ', mi. nu.") disappear ; of the remaining two 

 one is distinguished as the stationary pronudeus, the other as the 

 active pronucleus. The active pronucleus of each Infusor now 

 passes into the body of the other and fuses with its stationary 

 pronucleus (D), each individual thus coming to possess a single 

 nuclear body derived in equal proportions from the two conjugat- 

 ing cells (E). The animalcules then separate from one another, 

 and the nucleus of each divides and gives rise to the permanent 

 mega- (G, Mg. nu.) and micronuclei (Mi. nu.). 



2. CLASSIFICATION AND GENERAL ORGANISATION. 



In the majority of the Infusoria the body is ciliated throughout 

 life, but in certain forms cilia are present only in the immature 

 condition, the adult being provided with peculiar organs of 

 prehension or tentacles. We thus get two orders, viz. : 



ORDER 1. CILIATA. 

 Infusoria provided with cilia throughout life. 



ORDER 2. TENTACULIFERA. 



Infusoria possessing cilia in the young condition, tentacles in 

 the adult. 



Systematic position of the Example. 



Paramoecium aurelia is one of several species of the genus 

 Paramcecinm, belonging the family Parmcecidce, of the sub-order 

 Trichostomata, and order Ciliata. The presence of cilia in the 

 adult condition places it among the Ciliata : the presence of a 

 permanently open mouth into which food particles are swept by 

 the movement of the cilia, among the Trichostomata. The Para- 

 moecidse are free-swimming, asymmetrical, uniformly ciliated, with 

 a ventrally placed mouth. P. caudatum is about \ i- mm. in 

 length, its length about four times its breadth, rounded in front, 

 and bluntly pointed behind, and a single micronucleus is present. 



ORDER 1. CILIATA. 



This order presents a wider range of variations some of them 

 of a truly extraordinary character than any other group of 

 Protozoa. 



