PABAMCECIUM 



39 



protoplasmic body. 1 It is surprising what a high degree of 

 organization, as indicated by complexity of structure, may be 

 attained in such a case. 



Differentiation and division of labour aretheresultsof progressive 

 evolution, and at the same time the means by which further 

 progress is effected. Even in Amoeba and Hsematococcus we see 

 clearly enough the operation of these two great principles. Many 

 unicellular organisms, however, exhibit a far higher degree of 

 organization. The Protozoon, Paramceeium (Fig. 8), so common 

 in infusions of decaying vegetable matter, swims actively about by 

 means of innumerable short vibratile cilia which project all over 

 the surface of the body. It has a definite mouth, through which 



PV 



MY 



tN EC FV 



FlG. 8. Paramcecium anrelia, X 300. (From Marshall and Hurst's 

 "Practical Zoology.") 



AV, anterior contractile vacuole (dilated) ; EC, ectoplasm with trichocysts ; EN, endo- 

 plasm ; EP, micronucleus ; FV, food vacuole ; M, mouth ; MY, contractile fibrillee ; 

 N, meganucleus ; OG, groove leading to mouth : PV, posterior contractile vacuole 

 (contracted) ; TR, discharged trichocyst threads ; X, cilia. 



solid food particles are taken in, and a less definite anal spot at 

 which faecal matter is ejected. It has special weapons of offence or 

 defence (trichocysts) which can shoot out from the surface of the 

 body long threads when the animal is irritated. It has two 

 contractile vacuoles, each with a system of radiating canals 

 discharging into it, and it has two nuclei, large and small 

 (meganucleus and micronucleus), which appear to fulfil different 

 functions and each of which doubtless has a complex structure of 

 its own. How complex the structure of the nucleus may be we 

 shall be better able to judge when we come to speak of the 

 phenomena of nuclear division in a subsequent chapter. 



1 The organs into which a single cell may be differentiated are sometimes spoken 

 of as organellae, but if we define an organ as any part of an organism which is 

 specialized for the fulfilment of some particular function, it is quite unnecessary to 

 distinguish the organs of a single cell by a special term. 



