IT 



PHYLUM PROTOZOA 93 



but do not fuse. The meganucleus (mg. nu.) of each breaks up into 

 small masses, which eventually disappear, being apparently absorbed 

 into the protoplasm. At the same time the micronucleus (mi. nu.) 

 of each divides, each product of division immediately dividing 

 again, so that each conjugating body is provided with four 

 micronuclei (B). Three of these (mi. nu'., mi. nu" .) disappear ; the 

 fourth divides again into two, of which one is distinguished as the 

 stationary pronucleus, 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 conjugating cells (E). The animalcules then separate 

 from one another, and the nucleus of each divides and gives rise to 

 the permanent mega- and micro-nuclei (G, Mg. nu., Mi. nu.), the 

 original nuclear condition becoming completely established only 

 after the two animals have separated and have twice undergone 

 binary fission. 



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 caudatum is one of several species of the genus 

 Paramoecium, belonging to the family Paramoecidce, 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 J 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. 



