CHROMIDIA AND NUCLEI OF A PROTOZOON 213 



structures occur in it and were called chromidia by Schaudinn, 

 but Dobell states that they have nothing in common with 

 true chromidia such as occur in Arcella. 



It has been recorded in Part IV how complicated the 

 life-cycle of a common amoeba is. Calkins has followed the 

 nuclear process in Amoeba proteus. In the case of organisms 

 of indefinite form it is difficult for the student to retain a 

 mental picture of a complicated series of nuclear and chro- 

 midial processes, and of cell- divisions. This is easier in the 

 case of the shelled amoebae. Of one of these, Arcella vulgaris, 

 the life-history has been worked out with some degree of 

 completeness, 1 so we may take this organism as an example. 



Arcella (Ehrenberg) is a common and widely distributed 

 genus. Arcella vulgaris occurs chiefly in bogs and other 

 still waters. Seen in surface view it has the appearance 

 shown in Fig. 60 : 1. Its flattened dome-shaped brown- 

 coloured shell is finely tessellated, and measures from 80/x, to 

 140//, across. In profile view the shell is seen to have a 

 diaphragm with a central perforation, the pylome, less than 

 one-third the width of the shell. A section of the shell 

 shows it to consist of an inner continuous chitinous layer, 

 slightly exaggerated in thickness, 2, b ; the close-set cuboidal 

 elements of the outer layer are cemented together by the 

 same substance as that which forms the inner layer. From 



1 In order to compile the following sketch I consulted various authors : Delage and 

 Herouard, 1896; E. A. Minchin, 1901 ; G. N. Calkins, F. Doflein, and S. J. Hickson, 

 1909. The chromidial phases were worked out by R. Hertwig, the structure of the 

 shell by Awerinzew, the gametes by Elpetiewsky. 



