146 SOME MINUTE ANIMAL PARASITES 



portant in connexion with pathogenic forms. Much 

 of that importance has now vanished, for it has been 

 shown by Darling that the appearance not only of 

 the nucleus, but of the whole trophozoite, varies 

 with the age of the parasite that is, the nucleus has 

 a cyclical structure. To make a general statement, 

 the nucleus of an Entamceba is not very rich in 

 chromatin, but the chromatin at one time forms 

 somewhat irregular masses on the inside of the 

 nuclear membrane, and is scattered as granules 

 through the nucleus, while at another period in the 

 existence of the organism most of the chromatin 

 will be concentrated to form a central karyosome. 



Excretion is a vital necessity for all living 

 organisms. Two main systems of excretion occur 

 among the amoebae, and these systems can be roughly 

 correlated with the mode of life of the particular 

 amoebae considered. Free-living forms, such as 

 Amoeba proteus, collect their liquid excretions into 

 one part of the cytoplasm, forming a vesicle which 

 has the power of rhythmic contraction. The vesicle 

 gradually fills^ and becomes very obvious, under 

 the microscope, to the observer. Then, suddenly, 

 the contents are expelled, and the vesicle or con- 

 tractile vacuole collapses and disappears from view. 

 Other amoebae have central reservoirs into which 

 a number of subsidiary, radiating, collecting chan- 

 nels discharge. The mechanism of such excretory 

 vacuoles is very suggestive of the closing of the iris 

 diaphragm of a camera or microscope. 



Yet another set of amoebae, including the greater 

 number of pathogenic forms, possess no general 



